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git-svn-id: https://leveldb.googlecode.com/svn/trunk@28 62dab493-f737-651d-591e-8d6aee1b9529
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dgrogan@chromium.org
2011-05-21 02:17:43 +00:00
parent 3c111335a7
commit da79909507
34 changed files with 953 additions and 406 deletions
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@@ -17,14 +17,14 @@ 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 file stored in a directory.
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 1MB by default), it is converted
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>
@@ -83,19 +83,15 @@ Other files used for miscellaneous purposes may also be present
<h1>Level 0</h1>
When the log file grows above a certain size (1MB by default):
<ul>
<li>Write the contents of the current memtable to an sstable
<li>Replace the current memtable by a brand new empty memtable
<li>Switch to a new log file
<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>
Experimental measurements show that generating an sstable from a 1MB
log file takes ~12ms, which seems like an acceptable latency hiccup to
add infrequently to a log write.
<p>
The new sstable is added to a special level-0 level. level-0 contains
a set of files (up to 4 by default). However unlike other levels,
these files do not cover disjoint ranges, but may overlap each other.
<h1>Compactions</h1>
@@ -162,8 +158,8 @@ 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 larger the delay
that we will add to write latency when a write triggers a log switch.
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