299ccedfec
- Added DB::CompactRange() method. Changed manual compaction code so it breaks up compactions of big ranges into smaller compactions. Changed the code that pushes the output of memtable compactions to higher levels to obey the grandparent constraint: i.e., we must never have a single file in level L that overlaps too much data in level L+1 (to avoid very expensive L-1 compactions). Added code to pretty-print internal keys. - Fixed bug where we would not detect overlap with files in level-0 because we were incorrectly using binary search on an array of files with overlapping ranges. Added "leveldb.sstables" property that can be used to dump all of the sstables and ranges that make up the db state. - Removing post_write_snapshot support. Email to leveldb mailing list brought up no users, just confusion from one person about what it meant. - Fixing static_cast char to unsigned on BIG_ENDIAN platforms. Fixes Issue 35 and Issue 36. - Comment clarification to address leveldb Issue 37. - Change license in posix_logger.h to match other files. - A build problem where uint32 was used instead of uint32_t. Sync with upstream @24408625
487 lines
18 KiB
HTML
487 lines
18 KiB
HTML
<!DOCTYPE html>
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<html>
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<head>
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<link rel="stylesheet" type="text/css" href="doc.css" />
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<title>Leveldb</title>
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</head>
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<body>
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<h1>Leveldb</h1>
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<address>Jeff Dean, Sanjay Ghemawat</address>
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<p>
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The <code>leveldb</code> library provides a persistent key value store. Keys and
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values are arbitrary byte arrays. The keys are ordered within the key
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value store according to a user-specified comparator function.
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<p>
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<h1>Opening A Database</h1>
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<p>
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A <code>leveldb</code> database has a name which corresponds to a file system
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directory. All of the contents of database are stored in this
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directory. The following example shows how to open a database,
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creating it if necessary:
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<p>
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<pre>
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#include <assert>
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#include "leveldb/db.h"
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leveldb::DB* db;
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leveldb::Options options;
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options.create_if_missing = true;
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leveldb::Status status = leveldb::DB::Open(options, "/tmp/testdb", &db);
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assert(status.ok());
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...
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</pre>
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If you want to raise an error if the database already exists, add
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the following line before the <code>leveldb::DB::Open</code> call:
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<pre>
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options.error_if_exists = true;
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</pre>
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<h1>Status</h1>
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<p>
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You may have noticed the <code>leveldb::Status</code> type above. Values of this
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type are returned by most functions in <code>leveldb</code> that may encounter an
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error. You can check if such a result is ok, and also print an
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associated error message:
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<p>
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<pre>
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leveldb::Status s = ...;
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if (!s.ok()) cerr << s.ToString() << endl;
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</pre>
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<h1>Closing A Database</h1>
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<p>
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When you are done with a database, just delete the database object.
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Example:
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<p>
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<pre>
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... open the db as described above ...
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... do something with db ...
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delete db;
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</pre>
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<h1>Reads And Writes</h1>
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<p>
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The database provides <code>Put</code>, <code>Delete</code>, and <code>Get</code> methods to
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modify/query the database. For example, the following code
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moves the value stored under key1 to key2.
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<pre>
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std::string value;
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leveldb::Status s = db->Get(leveldb::ReadOptions(), key1, &value);
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if (s.ok()) s = db->Put(leveldb::WriteOptions(), key2, value);
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if (s.ok()) s = db->Delete(leveldb::WriteOptions(), key1);
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</pre>
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<h1>Atomic Updates</h1>
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<p>
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Note that if the process dies after the Put of key2 but before the
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delete of key1, the same value may be left stored under multiple keys.
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Such problems can be avoided by using the <code>WriteBatch</code> class to
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atomically apply a set of updates:
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<p>
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<pre>
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#include "leveldb/write_batch.h"
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...
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std::string value;
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leveldb::Status s = db->Get(leveldb::ReadOptions(), key1, &value);
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if (s.ok()) {
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leveldb::WriteBatch batch;
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batch.Delete(key1);
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batch.Put(key2, value);
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s = db->Write(leveldb::WriteOptions(), &batch);
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}
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</pre>
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The <code>WriteBatch</code> holds a sequence of edits to be made to the database,
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and these edits within the batch are applied in order. Note that we
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called <code>Delete</code> before <code>Put</code> so that if <code>key1</code> is identical to <code>key2</code>,
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we do not end up erroneously dropping the value entirely.
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<p>
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Apart from its atomicity benefits, <code>WriteBatch</code> may also be used to
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speed up bulk updates by placing lots of individual mutations into the
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same batch.
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<h1>Synchronous Writes</h1>
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By default, each write to <code>leveldb</code> is asynchronous: it
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returns after pushing the write from the process into the operating
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system. The transfer from operating system memory to the underlying
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persistent storage happens asynchronously. The <code>sync</code> flag
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can be turned on for a particular write to make the write operation
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not return until the data being written has been pushed all the way to
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persistent storage. (On Posix systems, this is implemented by calling
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either <code>fsync(...)</code> or <code>fdatasync(...)</code> or
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<code>msync(..., MS_SYNC)</code> before the write operation returns.)
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<pre>
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leveldb::WriteOptions write_options;
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write_options.sync = true;
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db->Put(write_options, ...);
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</pre>
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Asynchronous writes are often more than a thousand times as fast as
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synchronous writes. The downside of asynchronous writes is that a
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crash of the machine may cause the last few updates to be lost. Note
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that a crash of just the writing process (i.e., not a reboot) will not
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cause any loss since even when <code>sync</code> is false, an update
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is pushed from the process memory into the operating system before it
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is considered done.
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<p>
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Asynchronous writes can often be used safely. For example, when
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loading a large amount of data into the database you can handle lost
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updates by restarting the bulk load after a crash. A hybrid scheme is
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also possible where every Nth write is synchronous, and in the event
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of a crash, the bulk load is restarted just after the last synchronous
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write finished by the previous run. (The synchronous write can update
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a marker that describes where to restart on a crash.)
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<p>
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<code>WriteBatch</code> provides an alternative to asynchronous writes.
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Multiple updates may be placed in the same <code>WriteBatch</code> and
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applied together using a synchronous write (i.e.,
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<code>write_options.sync</code> is set to true). The extra cost of
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the synchronous write will be amortized across all of the writes in
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the batch.
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<p>
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<h1>Concurrency</h1>
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<p>
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A database may only be opened by one process at a time.
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The <code>leveldb</code> implementation acquires a lock from the
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operating system to prevent misuse. Within a single process, the
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same <code>leveldb::DB</code> object may be safely shared by multiple
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concurrent threads. I.e., different threads may write into or fetch
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iterators or call <code>Get</code> on the same database without any
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external synchronization (the leveldb implementation will
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automatically do the required synchronization). However other objects
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(like Iterator and WriteBatch) may require external synchronization.
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If two threads share such an object, they must protect access to it
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using their own locking protocol. More details are available in
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the public header files.
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<p>
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<h1>Iteration</h1>
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<p>
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The following example demonstrates how to print all key,value pairs
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in a database.
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<p>
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<pre>
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leveldb::Iterator* it = db->NewIterator(leveldb::ReadOptions());
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for (it->SeekToFirst(); it->Valid(); it->Next()) {
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cout << it->key().ToString() << ": " << it->value().ToString() << endl;
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}
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assert(it->status().ok()); // Check for any errors found during the scan
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delete it;
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</pre>
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The following variation shows how to process just the keys in the
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range <code>[start,limit)</code>:
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<p>
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<pre>
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for (it->Seek(start);
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it->Valid() && it->key().ToString() < limit;
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it->Next()) {
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...
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}
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</pre>
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You can also process entries in reverse order. (Caveat: reverse
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iteration may be somewhat slower than forward iteration.)
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<p>
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<pre>
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for (it->SeekToLast(); it->Valid(); it->Prev()) {
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...
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}
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</pre>
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<h1>Snapshots</h1>
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<p>
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Snapshots provide consistent read-only views over the entire state of
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the key-value store. <code>ReadOptions::snapshot</code> may be non-NULL to indicate
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that a read should operate on a particular version of the DB state.
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If <code>ReadOptions::snapshot</code> is NULL, the read will operate on an
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implicit snapshot of the current state.
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<p>
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Snapshots are created by the DB::GetSnapshot() method:
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<p>
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<pre>
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leveldb::ReadOptions options;
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options.snapshot = db->GetSnapshot();
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... apply some updates to db ...
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leveldb::Iterator* iter = db->NewIterator(options);
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... read using iter to view the state when the snapshot was created ...
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delete iter;
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db->ReleaseSnapshot(options.snapshot);
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</pre>
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Note that when a snapshot is no longer needed, it should be released
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using the DB::ReleaseSnapshot interface. This allows the
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implementation to get rid of state that was being maintained just to
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support reading as of that snapshot.
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<h1>Slice</h1>
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<p>
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The return value of the <code>it->key()</code> and <code>it->value()</code> calls above
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are instances of the <code>leveldb::Slice</code> type. <code>Slice</code> is a simple
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structure that contains a length and a pointer to an external byte
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array. Returning a <code>Slice</code> is a cheaper alternative to returning a
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<code>std::string</code> since we do not need to copy potentially large keys and
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values. In addition, <code>leveldb</code> methods do not return null-terminated
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C-style strings since <code>leveldb</code> keys and values are allowed to
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contain '\0' bytes.
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<p>
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C++ strings and null-terminated C-style strings can be easily converted
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to a Slice:
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<p>
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<pre>
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leveldb::Slice s1 = "hello";
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std::string str("world");
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leveldb::Slice s2 = str;
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</pre>
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A Slice can be easily converted back to a C++ string:
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<pre>
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std::string str = s1.ToString();
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assert(str == std::string("hello"));
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</pre>
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Be careful when using Slices since it is up to the caller to ensure that
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the external byte array into which the Slice points remains live while
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the Slice is in use. For example, the following is buggy:
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<p>
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<pre>
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leveldb::Slice slice;
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if (...) {
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std::string str = ...;
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slice = str;
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}
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Use(slice);
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</pre>
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When the <code>if</code> statement goes out of scope, <code>str</code> will be destroyed and the
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backing storage for <code>slice</code> will disappear.
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<p>
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<h1>Comparators</h1>
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<p>
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The preceding examples used the default ordering function for key,
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which orders bytes lexicographically. You can however supply a custom
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comparator when opening a database. For example, suppose each
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database key consists of two numbers and we should sort by the first
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number, breaking ties by the second number. First, define a proper
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subclass of <code>leveldb::Comparator</code> that expresses these rules:
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<p>
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<pre>
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class TwoPartComparator : public leveldb::Comparator {
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public:
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// Three-way comparison function:
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// if a < b: negative result
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// if a > b: positive result
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// else: zero result
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int Compare(const leveldb::Slice& a, const leveldb::Slice& b) const {
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int a1, a2, b1, b2;
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ParseKey(a, &a1, &a2);
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ParseKey(b, &b1, &b2);
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if (a1 < b1) return -1;
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if (a1 > b1) return +1;
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if (a2 < b2) return -1;
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if (a2 > b2) return +1;
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return 0;
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}
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// Ignore the following methods for now:
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const char* Name() const { return "TwoPartComparator"; }
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void FindShortestSeparator(std::string*, const leveldb::Slice&) const { }
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void FindShortSuccessor(std::string*) const { }
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};
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</pre>
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Now create a database using this custom comparator:
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<p>
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<pre>
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TwoPartComparator cmp;
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leveldb::DB* db;
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leveldb::Options options;
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options.create_if_missing = true;
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options.comparator = &cmp;
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leveldb::Status status = leveldb::DB::Open(options, "/tmp/testdb", &db);
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...
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</pre>
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<h2>Backwards compatibility</h2>
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<p>
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The result of the comparator's <code>Name</code> method is attached to the
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database when it is created, and is checked on every subsequent
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database open. If the name changes, the <code>leveldb::DB::Open</code> call will
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fail. Therefore, change the name if and only if the new key format
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and comparison function are incompatible with existing databases, and
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it is ok to discard the contents of all existing databases.
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<p>
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You can however still gradually evolve your key format over time with
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a little bit of pre-planning. For example, you could store a version
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number at the end of each key (one byte should suffice for most uses).
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When you wish to switch to a new key format (e.g., adding an optional
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third part to the keys processed by <code>TwoPartComparator</code>),
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(a) keep the same comparator name (b) increment the version number
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for new keys (c) change the comparator function so it uses the
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version numbers found in the keys to decide how to interpret them.
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<p>
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<h1>Performance</h1>
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<p>
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Performance can be tuned by changing the default values of the
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types defined in <code>include/leveldb/options.h</code>.
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<p>
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<h2>Block size</h2>
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<p>
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<code>leveldb</code> groups adjacent keys together into the same block and such a
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block is the unit of transfer to and from persistent storage. The
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default block size is approximately 4096 uncompressed bytes.
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Applications that mostly do bulk scans over the contents of the
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database may wish to increase this size. Applications that do a lot
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of point reads of small values may wish to switch to a smaller block
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size if performance measurements indicate an improvement. There isn't
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much benefit in using blocks smaller than one kilobyte, or larger than
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a few megabytes. Also note that compression will be more effective
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with larger block sizes.
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<p>
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<h2>Compression</h2>
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<p>
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Each block is individually compressed before being written to
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persistent storage. Compression is on by default since the default
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compression method is very fast, and is automatically disabled for
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uncompressible data. In rare cases, applications may want to disable
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compression entirely, but should only do so if benchmarks show a
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performance improvement:
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<p>
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<pre>
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leveldb::Options options;
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options.compression = leveldb::kNoCompression;
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... leveldb::DB::Open(options, name, ...) ....
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</pre>
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<h2>Cache</h2>
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<p>
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The contents of the database are stored in a set of files in the
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filesystem and each file stores a sequence of compressed blocks. If
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<code>options.cache</code> is non-NULL, it is used to cache frequently used
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uncompressed block contents.
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<p>
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<pre>
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#include "leveldb/cache.h"
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leveldb::Options options;
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options.cache = leveldb::NewLRUCache(100 * 1048576); // 100MB cache
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leveldb::DB* db;
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leveldb::DB::Open(options, name, &db);
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... use the db ...
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delete db
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delete options.cache;
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</pre>
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Note that the cache holds uncompressed data, and therefore it should
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be sized according to application level data sizes, without any
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reduction from compression. (Caching of compressed blocks is left to
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the operating system buffer cache, or any custom <code>Env</code>
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implementation provided by the client.)
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<p>
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When performing a bulk read, the application may wish to disable
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caching so that the data processed by the bulk read does not end up
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displacing most of the cached contents. A per-iterator option can be
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used to achieve this:
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<p>
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<pre>
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leveldb::ReadOptions options;
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options.fill_cache = false;
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leveldb::Iterator* it = db->NewIterator(options);
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for (it->SeekToFirst(); it->Valid(); it->Next()) {
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...
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}
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</pre>
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<h2>Key Layout</h2>
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<p>
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Note that the unit of disk transfer and caching is a block. Adjacent
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keys (according to the database sort order) will usually be placed in
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the same block. Therefore the application can improve its performance
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by placing keys that are accessed together near each other and placing
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infrequently used keys in a separate region of the key space.
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<p>
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For example, suppose we are implementing a simple file system on top
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of <code>leveldb</code>. The types of entries we might wish to store are:
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<p>
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<pre>
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filename -> permission-bits, length, list of file_block_ids
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file_block_id -> data
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</pre>
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We might want to prefix <code>filename</code> keys with one letter (say '/') and the
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<code>file_block_id</code> keys with a different letter (say '0') so that scans
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over just the metadata do not force us to fetch and cache bulky file
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contents.
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<p>
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<h1>Checksums</h1>
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<p>
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<code>leveldb</code> associates checksums with all data it stores in the file system.
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There are two separate controls provided over how aggressively these
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checksums are verified:
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<p>
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<ul>
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<li> <code>ReadOptions::verify_checksums</code> may be set to true to force
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checksum verification of all data that is read from the file system on
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behalf of a particular read. By default, no such verification is
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done.
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<p>
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<li> <code>Options::paranoid_checks</code> may be set to true before opening a
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database to make the database implementation raise an error as soon as
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it detects an internal corruption. Depending on which portion of the
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database has been corrupted, the error may be raised when the database
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is opened, or later by another database operation. By default,
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paranoid checking is off so that the database can be used even if
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parts of its persistent storage have been corrupted.
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<p>
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If a database is corrupted (perhaps it cannot be opened when
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paranoid checking is turned on), the <code>leveldb::RepairDB</code> function
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|
may be used to recover as much of the data as possible
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<p>
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</ul>
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<h1>Approximate Sizes</h1>
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|
<p>
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The <code>GetApproximateSizes</code> method can used to get the approximate
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|
number of bytes of file system space used by one or more key ranges.
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<p>
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<pre>
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leveldb::Range ranges[2];
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ranges[0] = leveldb::Range("a", "c");
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ranges[1] = leveldb::Range("x", "z");
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uint64_t sizes[2];
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leveldb::Status s = db->GetApproximateSizes(ranges, 2, sizes);
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</pre>
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The preceding call will set <code>sizes[0]</code> to the approximate number of
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bytes of file system space used by the key range <code>[a..c)</code> and
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<code>sizes[1]</code> to the approximate number of bytes used by the key range
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<code>[x..z)</code>.
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<p>
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<h1>Environment</h1>
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<p>
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|
All file operations (and other operating system calls) issued by the
|
|
<code>leveldb</code> implementation are routed through a <code>leveldb::Env</code> object.
|
|
Sophisticated clients may wish to provide their own <code>Env</code>
|
|
implementation to get better control. For example, an application may
|
|
introduce artificial delays in the file IO paths to limit the impact
|
|
of <code>leveldb</code> on other activities in the system.
|
|
<p>
|
|
<pre>
|
|
class SlowEnv : public leveldb::Env {
|
|
.. implementation of the Env interface ...
|
|
};
|
|
|
|
SlowEnv env;
|
|
leveldb::Options options;
|
|
options.env = &env;
|
|
Status s = leveldb::DB::Open(options, ...);
|
|
</pre>
|
|
<h1>Porting</h1>
|
|
<p>
|
|
<code>leveldb</code> may be ported to a new platform by providing platform
|
|
specific implementations of the types/methods/functions exported by
|
|
<code>leveldb/port/port.h</code>. See <code>leveldb/port/port_example.h</code> for more
|
|
details.
|
|
<p>
|
|
In addition, the new platform may need a new default <code>leveldb::Env</code>
|
|
implementation. See <code>leveldb/util/env_posix.h</code> for an example.
|
|
|
|
<h1>Other Information</h1>
|
|
|
|
<p>
|
|
Details about the <code>leveldb</code> implementation may be found in
|
|
the following documents:
|
|
<ul>
|
|
<li> <a href="impl.html">Implementation notes</a>
|
|
<li> <a href="table_format.txt">Format of an immutable Table file</a>
|
|
<li> <a href="log_format.txt">Format of a log file</a>
|
|
</ul>
|
|
|
|
</body>
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|
</html>
|