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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 18:08:12 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4190?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13498183#comment-13498183

Todd Lipcon commented on HDFS-4190:

I bet we could get the same improvement by either:
a) increasing the "packet size" when block scanning to something like 1MB
b) using direct byte buffers in BlockSender, so that we can use the faster CRC verification.

There's already a patch floating around which starts to do (b) for the read path in general,
and it would probably help in this case as well.
> 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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