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Diffstat (limited to 'doc/impl.html')
| -rw-r--r-- | doc/impl.html | 26 |
1 files changed, 11 insertions, 15 deletions
diff --git a/doc/impl.html b/doc/impl.html index dd09fea..e870795 100644 --- a/doc/impl.html +++ b/doc/impl.html @@ -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 |