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From: Amir <amir74tt@yahoo.com>
Subject: Problem with Deletes
Date: Thu, 17 Jun 2010 16:10:52 +0000 (UTC)
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Hi All,

I'm running a benchmark on Cassandra while using a benchmark client which I've 
written myself.

I'm running the following scenario:
One Cassandra node on the same machine as the client.
The client writes a new key every 1 second and deletes it after 10 seconds, so 
at any given time there should be only 10 keys.
The key value size is 2K.

When I ran this scenario I looked at the data folder and saw that initially 
cassandra created 4 files (SSTables) of ~130K and then compacted them to 20K 
(which is exactly what I expected 10 keys * 2K = 20K).
And afterwards there where another 3 130K files which were compacted together 
with the first 20K file to a new 20K file, and so on... 

This scenario ran exactly as I excepted.

Now I ran the same scenario but this time with key value size=2M.

When I ran this scenario initially cassandra created 4 files (SSTables) of ~64M 
and then compacted them to 20M (which is exactly what I expected 10 keys * 2M = 
20M). But after creating another 3 64M files - the problem started!
It didn't compact them with the first 20M - instead it created another 64M file 
and compacted all 4 of them to 260M file (!), and after creating another 4 64M 
files it compacted them to another 260M file and so on...

It looks to me like in this scenario for some reason the compaction doesn't 
make any deletes. I don't have any idea why :-(

Additional info which I should mention:
In the storage-conf.xml the following are not default:
GCGraceSeconds = 0
MemtableFlushAfterMinutes = 1
<ColumnFamily Name="Standard2"
                    CompareWith="UTF8Type"
                    KeysCached="99%"/>


Thanks a lot for your help,

Amir