[jira] [Commented] (CASSANDRA-7549) Heavy Disk Read I/O

["Benedict (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-7549?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14061991#comment-14061991 ] 

Benedict commented on CASSANDRA-7549:
-------------------------------------

Can you clarify what this ticket is hoping to achieve? "Too small" compared to what? The default Cassandra block size is 64Kb, and since currently reading data in a block requires a sequential scan of data preceding the target record, reading ~40Kb is about correct for reading a very small row with an 8Kb read ahead, and zero page cache utilisation (i.e. completely random reads for a substantially larger than memory dataset). Increasing block size is likely to reduce performance for this kind of workload, not improve it.

> 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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