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From Julie <julie.su...@nextcentury.com>
Subject Cassandra disk space utilization
Date Wed, 07 Jul 2010 15:56:39 GMT
Hi guys,
I have what may be a dumb question but I am confused by how much disk space is 
being used by my Cassandra nodes.  I have 10 nodes in my cluster with a 
replication factor of 3.  After I write 1,000,000 rows to the database (100kB 
each), I see that they have been distributed very evenly, about 100,000 rows 
per node but because of the replication factor of 3, each node contains about 
300,000 rows.  This is all good.  Since my rows are 100kB each, I expect each 
node to store about 30GB of data, however that is not what I am seeing.  
Instead, I am seeing some nodes that do not experience any compaction 
exceptions but report their space used as MUCH more.  Here's one using 106 GB 
of disk.  My disks are only 160 GB so this is at the bleeding edge and I 
thought my node would be able to store more data.

I only use a single column family so here is the cfstats output from one of my 
nodes (server5):

		Column Family: Standard1
		SSTable count: 12
		Space used (live): 113946099884
		Space used (total): 113946099884
		Memtable Columns Count: 0
		Memtable Data Size: 0
		Memtable Switch Count: 451
		Read Count: 31786
		Read Latency: 161.429 ms.
		Write Count: 300633
		Write Latency: 0.124 ms.
		Pending Tasks: 0
		Key cache: disabled
		Row cache capacity: 3000
		Row cache size: 3000
		Row cache hit rate: 0.38331340841880074
		Compacted row minimum size: 100220
		Compacted row maximum size: 100225
		Compacted row mean size: 100224

Note that I wrote these 1M rows of data yesterday and the system has had 24 
hours to digest it. There are no exceptions in the system.log file.  Here's 
the tail end of it:

...
INFO [SSTABLE-CLEANUP-TIMER] 2010-07-06 16:13:43,162
SSTableDeletingReference.java (line 104) Deleted
/var/lib/cassandra/data/Keyspace1/Standard1-430-Data.db
 INFO [SSTABLE-CLEANUP-TIMER] 2010-07-06 16:13:43,269
SSTableDeletingReference.java (line 104) Deleted
/var/lib/cassandra/data/Keyspace1/Standard1-445-Data.db
 INFO [COMPACTION-POOL:1] 2010-07-06 16:35:21,718 CompactionManager.java (line 
246) Compacting []
 INFO [Timer-1] 2010-07-06 17:01:01,907 Gossiper.java (line 179) InetAddress 
/10.248.107.19 is now dead.
 INFO [GMFD:1] 2010-07-06 17:01:42,039 Gossiper.java (line 568) InetAddress 
/10.248.107.19 is now UP
 INFO [COMPACTION-POOL:1] 2010-07-06 17:35:21,306 CompactionManager.java (line 
246) Compacting []
 INFO [COMPACTION-POOL:1] 2010-07-06 18:35:20,802 CompactionManager.java (line 
246) Compacting []
 INFO [COMPACTION-POOL:1] 2010-07-06 19:35:20,389 CompactionManager.java (line 
246) Compacting []
 INFO [COMPACTION-POOL:1] 2010-07-06 20:35:19,934 CompactionManager.java (line 
246) Compacting []
 INFO [COMPACTION-POOL:1] 2010-07-06 21:35:19,582 CompactionManager.java (line 
246) Compacting []
 INFO [COMPACTION-POOL:1] 2010-07-06 22:35:19,233 CompactionManager.java (line 
246) Compacting []
 INFO [COMPACTION-POOL:1] 2010-07-06 23:35:18,593 CompactionManager.java (line 
246) Compacting []
 INFO [COMPACTION-POOL:1] 2010-07-07 00:35:18,076 CompactionManager.java (line 
246) Compacting []
 INFO [COMPACTION-POOL:1] 2010-07-07 01:35:17,673 CompactionManager.java (line 
246) Compacting []
 INFO [COMPACTION-POOL:1] 2010-07-07 02:35:17,172 CompactionManager.java (line 
246) Compacting []
 INFO [COMPACTION-POOL:1] 2010-07-07 03:35:16,784 CompactionManager.java (line 
246) Compacting []
 INFO [COMPACTION-POOL:1] 2010-07-07 04:35:16,383 CompactionManager.java (line 
246) Compacting []

Thank you for your help!!
Julie



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