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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:
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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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