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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4190) Read complete block into memory once in BlockScanning and reduce concurrent disk access
Date Thu, 15 Nov 2012 19:04:12 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4190?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13498248#comment-13498248

Todd Lipcon commented on HDFS-4190:

bq. Could you point me the JIRA for #b ? Thanks Todd. So, that I will try to make use that
in this case.

Turns out I was remembering wrong - that's what I get for going on JIRA before I've had any
coffee! I was thinking of HDFS-3529, but that's for the write path. A similar improvement
could be made on the read path to avoid a memcpy for the non-transferTo case.
> Read complete block into memory once in BlockScanning and reduce concurrent disk access
> ---------------------------------------------------------------------------------------
>                 Key: HDFS-4190
>                 URL: https://issues.apache.org/jira/browse/HDFS-4190
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: data-node
>    Affects Versions: 3.0.0
>            Reporter: Uma Maheswara Rao G
> When we perform bulk write operations to DFS we observed that block scan is one bottleneck
for concurrent disk access.
> To see real load on disks, keep single data node and local client flushing data to DFS.
> When we switch off block scanning we have seen >10% improvement. I will update real
figures in comment.
> Even though I am doing only write operation, implicitly there will be a read operation
for each block due to block scanning. Next scan will happen only after 21 days, but once scan
will happen after adding the block. This will be the concurrent access to disks.
> Other point to note is that, we will read the block, packet by packet in block scanning
as well. We know that, we have to read&scan complete block, so, it may be correct to load
complete block once and do checksums verification for that data?
> I tried with MemoryMappedBuffers:
> mapped the complete block once in blockScanning and does the checksum verification with
that. Seen good improvement in that bulk write scenario.
> But we don't have any API to clean the mapped buffer immediately. With my experiment
I just used, Cleaner class from sun package. That will not be correct to use in production.
So, we have to write JNI call to clean that mmapped buffer.
> I am not sure I missed something here. please correct me If i missed some points.
> Thoughts?

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