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From Keith Wright <>
Subject Re: SSTable size versus read performance
Date Thu, 16 May 2013 14:54:26 GMT
Does Cassandra need to load the entire SSTable into memory to uncompress it or does it only
load the relevant block?  I ask because if its the latter, that would not explain why I'm
seeing so much higher read MB/s in the 1.2 cluster as the block sizes are the same in both.

From: Edward Capriolo <<>>
Reply-To: "<>" <<>>
Date: Thursday, May 16, 2013 10:47 AM
To: "<>" <<>>
Subject: Re: SSTable size versus read performance

With you use compression you should play with your block size. I believe the default may be
32K but I had more success with 8K, nearly same compression ratio, less young gen memory pressure.

On Thu, May 16, 2013 at 10:42 AM, Keith Wright <<>>
The biggest reason I'm using compression here is that my data lends itself well to it due
to the composite columns.  My current compression ratio is 30.5%.  Not sure it matters but
my BF false positive ration os 0.048.

From: Edward Capriolo <<>>
Reply-To: "<>" <<>>
Date: Thursday, May 16, 2013 10:23 AM
To: "<>" <<>>
Subject: Re: SSTable size versus read performance

I am not sure of the new default is to use compression, but I do not believe compression is
a good default. I find compression is better for larger column families that are sparsely
read. For high throughput CF's I feel that decompressing larger blocks hurts performance more
then compression adds.

On Thu, May 16, 2013 at 10:14 AM, Keith Wright <<>>
Hi all,

    I currently have 2 clusters, one running on 1.1.10 using CQL2 and one running on 1.2.4
using CQL3 and Vnodes.   The machines in the 1.2.4 cluster are expected to have better IO
performance as we are going from 1 SSD data disk per node in the 1.1 cluster to 3 SSD data
disks per node in the 1.2 cluster with higher end drives (commit logs are on their own disk
shared with the OS).  I am doing some stress testing on the 1.2 cluster and have found that
although the reads / sec as seen from iostat are approximately the same (3K / sec) in both
clusters, the MB/s read in the new cluster is MUCH higher (7 MB/s in 1.1 as compared to 30-50
MB/s in 1.2).  As a result, I am seeing excessive iowait in the 1.2 cluster causing high average
read times of 30 ms under the same load (1.1 cluster sees around 5 ms).  They are both using
Leveled compaction but one thing I did change in the new cluster was to increase the sstable
size from the OOTB setting to 32 MB.  Note that my reads are by definition highly random as
we are running memcached in front for various reasons.  Does cassandra need to read the entire
SSTable when fetching a row or only the relevant chunk (I have the OOTB chunk size and BF
settings)?  I just decreased the sstable size to 5 MB and am waiting for compactions to complete
to see if that makes a difference.


Relevant table definition if helpful (note that I also changed to the LZ4 compressor expecting
better read performance and I decreased the crc change again to minimize read latency):

CREATE TABLE global_user (
user_id BIGINT,
app_id INT,
type TEXT,
name TEXT,
values map<TIMESTAMP,FLOAT>,
sku_time map<TEXT,TIMESTAMP>,
extra_param map<TEXT,TEXT>,
PRIMARY KEY (user_id, app_id, type, name)
) with compression={'crc_check_chance':0.1,'sstable_compression':'LZ4Compressor'} and
compaction={'class':'LeveledCompactionStrategy'} and
compaction_strategy_options = {'sstable_size_in_mb':5} and
gc_grace_seconds = 86400;

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