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From Andras Szerdahelyi <andras.szerdahe...@ignitionone.com>
Subject Re: bloom filter fp ratio of 0.98 with fp_chance of 0.01
Date Wed, 27 Mar 2013 08:19:06 GMT
Aaron,

What version are you using ?

1.1.9

Have you changed the bf_ chance ? The sstables need to be rebuilt for it to take affect.

I did ( several times ) and I ran upgradesstables after

Not sure what this means.
Are you saying it's in a boat on a river, with tangerine trees and marmalade skies ?

You nailed it. A significant number of reads are done from hundreds of sstables ( I have to
add, compaction is apparently constantly 6000-7000 tasks behind and the vast majority of the
reads access recently written data )

Take a look at the nodetool cfhistograms to get a better idea of the row size and use that
info when consdiering the sstable size.

It's around 1-20K, what should I optimise the LCS sstable size for? I suppose "I want to fit
as many complete rows as possible in to a single sstable to keep file count down while avoiding
compactions of oversized ( double digit gigabytes? ) sstables at higher levels ? "
Do I have to run a major compaction after a change to sstable_size_in_mb ? The larger sstable
size wouldn't really affect sstables on levels above L0 , would it?



Thanks!!
Andras


From: aaron morton <aaron@thelastpickle.com<mailto:aaron@thelastpickle.com>>
Reply-To: "user@cassandra.apache.org<mailto:user@cassandra.apache.org>" <user@cassandra.apache.org<mailto:user@cassandra.apache.org>>
Date: Tuesday 26 March 2013 21:46
To: "user@cassandra.apache.org<mailto:user@cassandra.apache.org>" <user@cassandra.apache.org<mailto:user@cassandra.apache.org>>
Subject: Re: bloom filter fp ratio of 0.98 with fp_chance of 0.01

What version are you using ?
1.2.0 allowed a null bf chance, and I think it returned .1 for LCS and .01 for STS compaction.
Have you changed the bf_ chance ? The sstables need to be rebuilt for it to take affect.

and sstables read is in the skies
Not sure what this means.
Are you saying it's in a boat on a river, with tangerine trees and marmalade skies ?

                SSTable count: 22682
Lots of files there, I imagine this would dilute the effectiveness of the key cache. It's
caching (sstable, key) tuples.
You may want to look at increasing the sstable_size with LCS.

                Compacted row minimum size: 104
                Compacted row maximum size: 263210
                Compacted row mean size: 3041
Take a look at the nodetool cfhistograms to get a better idea of the row size and use that
info when consdiering the sstable size.

Cheers
-----------------
Aaron Morton
Freelance Cassandra Consultant
New Zealand

@aaronmorton
http://www.thelastpickle.com

On 26/03/2013, at 6:16 AM, Andras Szerdahelyi <andras.szerdahelyi@ignitionone.com<mailto:andras.szerdahelyi@ignitionone.com>>
wrote:

Hello list,

Could anyone shed some light on how an FP chance of 0.01 coexist with a measured FP ratio
of .. 0.98 ? Am I reading this wrong or are 98% of the requests hitting the bloom filter create
a false positive while the "target" false ratio is 0.01?
( Also key cache hit ratio is around 0.001 and sstables read is in the skies ( non-exponential
(non-) drop off  for LCS )  but that should be filed under "effect" and not "cause"? )

[default@unknown] use KS;
Authenticated to keyspace: KS
[default@KS] describe CF;
    ColumnFamily: CF
      Key Validation Class: org.apache.cassandra.db.marshal.BytesType
      Default column value validator: org.apache.cassandra.db.marshal.BytesType
      Columns sorted by: org.apache.cassandra.db.marshal.BytesType
      GC grace seconds: 691200
      Compaction min/max thresholds: 4/32
      Read repair chance: 0.1
      DC Local Read repair chance: 0.0
      Replicate on write: true
      Caching: ALL
      Bloom Filter FP chance: 0.01
      Built indexes: []
      Compaction Strategy: org.apache.cassandra.db.compaction.LeveledCompactionStrategy
      Compaction Strategy Options:
        sstable_size_in_mb: 5
      Compression Options:
        sstable_compression: org.apache.cassandra.io.compress.SnappyCompressor

Keyspace: KS
        Read Count: 628950
        Read Latency: 93.19921121869784 ms.
        Write Count: 1219021
        Write Latency: 0.14352380885973254 ms.
        Pending Tasks: 0
                Column Family: CF
                SSTable count: 22682
                Space used (live): 119771434915
                Space used (total): 119771434915
                Number of Keys (estimate): 203837952
                Memtable Columns Count: 13125
                Memtable Data Size: 33212827
                Memtable Switch Count: 15
                Read Count: 629009
                Read Latency: 88.434 ms.
                Write Count: 1219038
                Write Latency: 0.095 ms.
                Pending Tasks: 0
                Bloom Filter False Positives: 37939419
                Bloom Filter False Ratio: 0.97928
                Bloom Filter Space Used: 261572784
                Compacted row minimum size: 104
                Compacted row maximum size: 263210
                Compacted row mean size: 3041

I upgraded sstables after changing the FP chance

Thanks!
Andras


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