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XOY-Leveldb


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								<title>Leveldb file layout and compactions</title>

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								<h1>Files</h1>


								The implementation of leveldb is similar in spirit to the

								representation of a single

								<a href="http://labs.google.com/papers/bigtable.html">

								Bigtable tablet (section 5.3)</a>.

								However the organization of the files that make up the representation

								is somewhat different and is explained below.


								<p>

								Each database is represented by a set of files stored in a directory.

								There are several different types of files as documented below:

								<p>

								<h2>Log files</h2>

								<p>

								A log file (*.log) stores a sequence of recent updates.  Each update

								is appended to the current log file.  When the log file reaches a

								pre-determined size (approximately 4MB by default), it is converted

								to a sorted table (see below) and a new log file is created for future

								updates.

								<p>

								A copy of the current log file is kept in an in-memory structure (the

								<code>memtable</code>).  This copy is consulted on every read so that read

								operations reflect all logged updates.

								<p>

								<h2>Sorted tables</h2>

								<p>

								A sorted table (*.sst) stores a sequence of entries sorted by key.

								Each entry is either a value for the key, or a deletion marker for the

								key.  (Deletion markers are kept around to hide obsolete values

								present in older sorted tables).

								<p>

								The set of sorted tables are organized into a sequence of levels.  The

								sorted table generated from a log file is placed in a special <code>young</code>

								level (also called level-0).  When the number of young files exceeds a

								certain threshold (currently four), all of the young files are merged

								together with all of the overlapping level-1 files to produce a

								sequence of new level-1 files (we create a new level-1 file for every

								2MB of data.)

								<p>

								Files in the young level may contain overlapping keys.  However files

								in other levels have distinct non-overlapping key ranges.  Consider

								level number L where L >= 1.  When the combined size of files in

								level-L exceeds (10^L) MB (i.e., 10MB for level-1, 100MB for level-2,

								...), one file in level-L, and all of the overlapping files in

								level-(L+1) are merged to form a set of new files for level-(L+1).

								These merges have the effect of gradually migrating new updates from

								the young level to the largest level using only bulk reads and writes

								(i.e., minimizing expensive seeks).


								<h2>Manifest</h2>

								<p>

								A MANIFEST file lists the set of sorted tables that make up each

								level, the corresponding key ranges, and other important metadata.

								A new MANIFEST file (with a new number embedded in the file name)

								is created whenever the database is reopened.  The MANIFEST file is

								formatted as a log, and changes made to the serving state (as files

								are added or removed) are appended to this log.

								<p>

								<h2>Current</h2>

								<p>

								CURRENT is a simple text file that contains the name of the latest

								MANIFEST file.

								<p>

								<h2>Info logs</h2>

								<p>

								Informational messages are printed to files named LOG and LOG.old.

								<p>

								<h2>Others</h2>

								<p>

								Other files used for miscellaneous purposes may also be present

								(LOCK, *.dbtmp).


								<h1>Level 0</h1>

								When the log file grows above a certain size (1MB by default):

								<ul>

								<li>Create a brand new memtable and log file and direct future updates here

								<li>In the background:

								<ul>

								<li>Write the contents of the previous memtable to an sstable

								<li>Discard the memtable

								<li>Delete the old log file and the old memtable

								<li>Add the new sstable to the young (level-0) level.

								</ul>

								</ul>


								<h1>Compactions</h1>


								<p>

								When the size of level L exceeds its limit, we compact it in a

								background thread.  The compaction picks a file from level L and all

								overlapping files from the next level L+1.  Note that if a level-L

								file overlaps only part of a level-(L+1) file, the entire file at

								level-(L+1) is used as an input to the compaction and will be

								discarded after the compaction.  Aside: because level-0 is special

								(files in it may overlap each other), we treat compactions from

								level-0 to level-1 specially: a level-0 compaction may pick more than

								one level-0 file in case some of these files overlap each other.


								<p>

								A compaction merges the contents of the picked files to produce a

								sequence of level-(L+1) files.  We switch to producing a new

								level-(L+1) file after the current output file has reached the target

								file size (2MB).  We also switch to a new output file when the key

								range of the current output file has grown enough to overlap more then

								ten level-(L+2) files.  This last rule ensures that a later compaction

								of a level-(L+1) file will not pick up too much data from level-(L+2).


								<p>

								The old files are discarded and the new files are added to the serving

								state.


								<p>

								Compactions for a particular level rotate through the key space.  In

								more detail, for each level L, we remember the ending key of the last

								compaction at level L.  The next compaction for level L will pick the

								first file that starts after this key (wrapping around to the

								beginning of the key space if there is no such file).


								<p>

								Compactions drop overwritten values.  They also drop deletion markers

								if there are no higher numbered levels that contain a file whose range

								overlaps the current key.


								<h2>Timing</h2>


								Level-0 compactions will read up to four 1MB files from level-0, and

								at worst all the level-1 files (10MB).  I.e., we will read 14MB and

								write 14MB.


								<p>

								Other than the special level-0 compactions, we will pick one 2MB file

								from level L.  In the worst case, this will overlap ~ 12 files from

								level L+1 (10 because level-(L+1) is ten times the size of level-L,

								and another two at the boundaries since the file ranges at level-L

								will usually not be aligned with the file ranges at level-L+1).  The

								compaction will therefore read 26MB and write 26MB.  Assuming a disk

								IO rate of 100MB/s (ballpark range for modern drives), the worst

								compaction cost will be approximately 0.5 second.


								<p>

								If we throttle the background writing to something small, say 10% of

								the full 100MB/s speed, a compaction may take up to 5 seconds.  If the

								user is writing at 10MB/s, we might build up lots of level-0 files

								(~50 to hold the 5*10MB).  This may signficantly increase the cost of

								reads due to the overhead of merging more files together on every

								read.


								<p>

								Solution 1: To reduce this problem, we might want to increase the log

								switching threshold when the number of level-0 files is large.  Though

								the downside is that the larger this threshold, the more memory we will

								need to hold the corresponding memtable.


								<p>

								Solution 2: We might want to decrease write rate artificially when the

								number of level-0 files goes up.


								<p>

								Solution 3: We work on reducing the cost of very wide merges.

								Perhaps most of the level-0 files will have their blocks sitting

								uncompressed in the cache and we will only need to worry about the

								O(N) complexity in the merging iterator.


								<h2>Number of files</h2>


								Instead of always making 2MB files, we could make larger files for

								larger levels to reduce the total file count, though at the expense of

								more bursty compactions.  Alternatively, we could shard the set of

								files into multiple directories.


								<p>

								An experiment on an <code>ext3</code> filesystem on Feb 04, 2011 shows

								the following timings to do 100K file opens in directories with

								varying number of files:

								<table class="datatable">

								<tr><th>Files in directory</th><th>Microseconds to open a file</th></tr>

								<tr><td>1000</td><td>9</td>

								<tr><td>10000</td><td>10</td>

								<tr><td>100000</td><td>16</td>

								</table>

								So maybe even the sharding is not necessary on modern filesystems?


								<h1>Recovery</h1>


								<ul>

								<li> Read CURRENT to find name of the latest committed MANIFEST

								<li> Read the named MANIFEST file

								<li> Clean up stale files

								<li> We could open all sstables here, but it is probably better to be lazy...

								<li> Convert log chunk to a new level-0 sstable

								<li> Start directing new writes to a new log file with recovered sequence#

								</ul>


								<h1>Garbage collection of files</h1>


								<code>DeleteObsoleteFiles()</code> is called at the end of every

								compaction and at the end of recovery.  It finds the names of all

								files in the database.  It deletes all log files that are not the

								current log file.  It deletes all table files that are not referenced

								from some level and are not the output of an active compaction.


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