223 lines
8.6 KiB
HTML
223 lines
8.6 KiB
HTML
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<!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 file layout and compactions</title>
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</head>
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<body>
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<h1>Files</h1>
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The implementation of leveldb is similar in spirit to the
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representation of a single
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<a href="http://labs.google.com/papers/bigtable.html">
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Bigtable tablet (section 5.3)</a>.
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However the organization of the files that make up the representation
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is somewhat different and is explained below.
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<p>
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Each database is represented by a set of file stored in a directory.
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There are several different types of files as documented below:
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<p>
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<h2>Log files</h2>
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<p>
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A log file (*.log) stores a sequence of recent updates. Each update
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is appended to the current log file. When the log file reaches a
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pre-determined size (approximately 1MB by default), it is converted
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to a sorted table (see below) and a new log file is created for future
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updates.
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<p>
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A copy of the current log file is kept in an in-memory structure (the
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<code>memtable</code>). This copy is consulted on every read so that read
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operations reflect all logged updates.
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<p>
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<h2>Sorted tables</h2>
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<p>
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A sorted table (*.sst) stores a sequence of entries sorted by key.
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Each entry is either a value for the key, or a deletion marker for the
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key. (Deletion markers are kept around to hide obsolete values
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present in older sorted tables).
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<p>
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The set of sorted tables are organized into a sequence of levels. The
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sorted table generated from a log file is placed in a special <code>young</code>
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level (also called level-0). When the number of young files exceeds a
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certain threshold (currently four), all of the young files are merged
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together with all of the overlapping level-1 files to produce a
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sequence of new level-1 files (we create a new level-1 file for every
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2MB of data.)
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<p>
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Files in the young level may contain overlapping keys. However files
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in other levels have distinct non-overlapping key ranges. Consider
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level number L where L >= 1. When the combined size of files in
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level-L exceeds (10^L) MB (i.e., 10MB for level-1, 100MB for level-2,
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...), one file in level-L, and all of the overlapping files in
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level-(L+1) are merged to form a set of new files for level-(L+1).
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These merges have the effect of gradually migrating new updates from
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the young level to the largest level using only bulk reads and writes
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(i.e., minimizing expensive seeks).
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<h2>Large value files</h2>
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<p>
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Each large value (greater than 64KB by default) is placed in a large
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value file (*.val) of its own. An entry is maintained in the log
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and/or sorted tables that maps from the corresponding key to the
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name of this large value file. The name of the large value file
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is derived from a SHA1 hash of the value and its length so that
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identical values share the same file.
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<p>
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<h2>Manifest</h2>
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<p>
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A MANIFEST file lists the set of sorted tables that make up each
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level, the corresponding key ranges, and other important metadata.
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A new MANIFEST file (with a new number embedded in the file name)
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is created whenever the database is reopened. The MANIFEST file is
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formatted as a log, and changes made to the serving state (as files
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are added or removed) are appended to this log.
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<p>
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<h2>Current</h2>
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<p>
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CURRENT is a simple text file that contains the name of the latest
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MANIFEST file.
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<p>
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<h2>Info logs</h2>
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<p>
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Informational messages are printed to files named LOG and LOG.old.
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<p>
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<h2>Others</h2>
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<p>
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Other files used for miscellaneous purposes may also be present
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(LOCK, *.dbtmp).
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<h1>Level 0</h1>
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When the log file grows above a certain size (1MB by default):
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<ul>
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<li>Write the contents of the current memtable to an sstable
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<li>Replace the current memtable by a brand new empty memtable
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<li>Switch to a new log file
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<li>Delete the old log file and the old memtable
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</ul>
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Experimental measurements show that generating an sstable from a 1MB
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log file takes ~12ms, which seems like an acceptable latency hiccup to
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add infrequently to a log write.
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<p>
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The new sstable is added to a special level-0 level. level-0 contains
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a set of files (up to 4 by default). However unlike other levels,
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these files do not cover disjoint ranges, but may overlap each other.
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<h1>Compactions</h1>
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<p>
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When the size of level L exceeds its limit, we compact it in a
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background thread. The compaction picks a file from level L and all
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overlapping files from the next level L+1. Note that if a level-L
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file overlaps only part of a level-(L+1) file, the entire file at
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level-(L+1) is used as an input to the compaction and will be
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discarded after the compaction. Aside: because level-0 is special
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(files in it may overlap each other), we treat compactions from
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level-0 to level-1 specially: a level-0 compaction may pick more than
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one level-0 file in case some of these files overlap each other.
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<p>
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A compaction merges the contents of the picked files to produce a
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sequence of level-(L+1) files. We switch to producing a new
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level-(L+1) file after the current output file has reached the target
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file size (2MB). The old files are discarded and the new files are
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added to the serving state.
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<p>
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Compactions for a particular level rotate through the key space. In
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more detail, for each level L, we remember the ending key of the last
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compaction at level L. The next compaction for level L will pick the
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first file that starts after this key (wrapping around to the
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beginning of the key space if there is no such file).
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<p>
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Compactions drop overwritten values. They also drop deletion markers
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if there are no higher numbered levels that contain a file whose range
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overlaps the current key.
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<h2>Timing</h2>
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Level-0 compactions will read up to four 1MB files from level-0, and
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at worst all the level-1 files (10MB). I.e., we will read 14MB and
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write 14MB.
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<p>
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Other than the special level-0 compactions, we will pick one 2MB file
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from level L. In the worst case, this will overlap ~ 12 files from
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level L+1 (10 because level-(L+1) is ten times the size of level-L,
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and another two at the boundaries since the file ranges at level-L
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will usually not be aligned with the file ranges at level-L+1). The
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compaction will therefore read 26MB and write 26MB. Assuming a disk
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IO rate of 100MB/s (ballpark range for modern drives), the worst
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compaction cost will be approximately 0.5 second.
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<p>
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If we throttle the background writing to something small, say 10% of
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the full 100MB/s speed, a compaction may take up to 5 seconds. If the
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user is writing at 10MB/s, we might build up lots of level-0 files
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(~50 to hold the 5*10MB). This may signficantly increase the cost of
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reads due to the overhead of merging more files together on every
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read.
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<p>
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Solution 1: To reduce this problem, we might want to increase the log
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switching threshold when the number of level-0 files is large. Though
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the downside is that the larger this threshold, the larger the delay
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that we will add to write latency when a write triggers a log switch.
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<p>
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Solution 2: We might want to decrease write rate artificially when the
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number of level-0 files goes up.
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<p>
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Solution 3: We work on reducing the cost of very wide merges.
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Perhaps most of the level-0 files will have their blocks sitting
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uncompressed in the cache and we will only need to worry about the
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O(N) complexity in the merging iterator.
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<h2>Number of files</h2>
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Instead of always making 2MB files, we could make larger files for
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larger levels to reduce the total file count, though at the expense of
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more bursty compactions. Alternatively, we could shard the set of
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files into multiple directories.
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<p>
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An experiment on an <code>ext3</code> filesystem on Feb 04, 2011 shows
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the following timings to do 100K file opens in directories with
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varying number of files:
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<table class="datatable">
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<tr><th>Files in directory</th><th>Microseconds to open a file</th></tr>
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<tr><td>1000</td><td>9</td>
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<tr><td>10000</td><td>10</td>
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<tr><td>100000</td><td>16</td>
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</table>
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So maybe even the sharding is not necessary on modern filesystems?
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<h1>Recovery</h1>
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<ul>
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<li> Read CURRENT to find name of the latest committed MANIFEST
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<li> Read the named MANIFEST file
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<li> Clean up stale files
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<li> We could open all sstables here, but it is probably better to be lazy...
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<li> Convert log chunk to a new level-0 sstable
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<li> Start directing new writes to a new log file with recovered sequence#
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</ul>
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<h1>Garbage collection of files</h1>
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<code>DeleteObsoleteFiles()</code> is called at the end of every
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compaction and at the end of recovery. It finds the names of all
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files in the database. It deletes all log files that are not the
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current log file. It deletes all table files that are not referenced
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from some level and are not the output of an active compaction. It
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deletes all large value files that are not referenced from any live
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table or log file.
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</body>
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</html>
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