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From "Hanson (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (CASSANDRA-7549) Heavy Disk Read I/O
Date Wed, 16 Jul 2014 08:38:04 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-7549?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14063282#comment-14063282
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Hanson edited comment on CASSANDRA-7549 at 7/16/14 8:37 AM:
------------------------------------------------------------

When the Query needs to access an SSTable, the OS will cache this SSTable. But the Read block
size is too small (~40KB observed by “iostat”), so it spends a long time to cache it into
memory (my machine has plenty of memory). During the caching period, the Query is very slow
(only hundreds TPS); after the caching done, the Query is very fast (several thousands TPS).

You mentioned “8Kb read ahead”, is it handled by Cassandra or OS disk driver parameter?
I have tried:
cat /sys/block/sdb/queue/read_ahead_kb
128
echo 1024 > /sys/block/sdb/queue/read_ahead_kb
But no impact to the Read disk I/O per "iostat".



was (Author: he902):
When the Query needs to access an SSTable, the OS will cache this SSTable. But the Read block
size is too small (~40KB observed by “iostat”), so it spends a long time to cache it into
memory (my machine has plenty of memory). During the caching period, the Query is very slow
(only hundreds TPS); after the caching done, the Query is very fast (thousand TPS).

You mentioned “8Kb read ahead”, is it handled by Cassandra or OS disk driver parameter?
I have tried:
cat /sys/block/sdb/queue/read_ahead_kb
128
echo 1024 > /sys/block/sdb/queue/read_ahead_kb
But no impact to the Read disk I/O


> Heavy Disk Read I/O
> -------------------
>
>                 Key: CASSANDRA-7549
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7549
>             Project: Cassandra
>          Issue Type: Improvement
>         Environment: Cassandra 2.0.6
>            Reporter: Hanson
>
> We observed heavy disk Read I/O, sometimes almost ~100% disk I/O %util. The block size
for Disk Read seems too small per “iostat”: 
> 	- DB Query: ~40KB per read
> 	- SSTables Compaction : ~120KB per read
> Could it use larger block size for Disk Read? (from Cassandra or OS disk driver tuning)



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