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From "Hanson (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-7549) Heavy Disk Read I/O
Date Wed, 16 Jul 2014 08:34:04 GMT


Hanson commented on CASSANDRA-7549:

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
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:
>             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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