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From Apache Wiki <wikidi...@apache.org>
Subject [Cassandra Wiki] Trivial Update of "MemtableSSTable" by JonathanEllis
Date Tue, 13 Apr 2010 18:59:27 GMT
Dear Wiki user,

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The "MemtableSSTable" page has been changed by JonathanEllis.
The comment on this change is: spelling fix.
http://wiki.apache.org/cassandra/MemtableSSTable?action=diff&rev1=4&rev2=5

--------------------------------------------------

  
  Once flushed, SSTable files are immutable; no further writes may be done.  So, on the read
path, the server must (potentially, although it uses tricks like bloom filters to avoid doing
so unnecessarily) combine row fragments from all the SSTables on disk, as well as any unflushed
Memtables, to produce the requested data.
  
- To bound the number of SSTable files that must be consulted on reads, and to reclaim [[DistributedDeletes|space
taken by unused data]], Cassandra performs compactions: merging multiple old SSTable files
into a single new one. Compactations are triggered when at least 4 SStables has been flushed
to disk. Since the input SSTables are all sorted by key, merging can be done efficiently,
still requiring no random i/o.  Once compaction is finished, the old SSTable files may be
deleted: note that in the worst case (a workload consisting of no overwrites or deletes) this
will temporarily require 2x your existing on-disk space used.  In today's world of multi-TB
disks this is usually not a problem but it is good to keep in mind when you are setting alert
thresholds.
+ To bound the number of SSTable files that must be consulted on reads, and to reclaim [[DistributedDeletes|space
taken by unused data]], Cassandra performs compactions: merging multiple old SSTable files
into a single new one. Compactions are triggered when at least 4 SStables has been flushed
to disk. Since the input SSTables are all sorted by key, merging can be done efficiently,
still requiring no random i/o.  Once compaction is finished, the old SSTable files may be
deleted: note that in the worst case (a workload consisting of no overwrites or deletes) this
will temporarily require 2x your existing on-disk space used.  In today's world of multi-TB
disks this is usually not a problem but it is good to keep in mind when you are setting alert
thresholds.
  
  (The high-level memtable/sstable design as well as the "Memtable" and "SSTable" names come
from Cassandra's sections 5.3 and 5.4 of [[http://labs.google.com/papers/bigtable.html|Google's
Bigtable paper]], although some of the terminology around compaction differs.)
  

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