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From "Uma Maheswara Rao G (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 18:56:12 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4190?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13498242#comment-13498242

Uma Maheswara Rao G commented on HDFS-4190:

Yes Todd, I have already tried #a, but complete block as single packet. Using memory mapped
buffers gave better results than that.
But in both cases there is an improvement. I think we can do like local read with using direct
byte bytebuffer with bigger size(may be equal to block size). So, here we can even reduce
the buffer cleanup step when compared to memmapped buffer usage, we can reuse same buffer
as we will do scanning sequentially with single thread.

faster CRC verification
doing fast CRC may improve scanning speed. But what I am looking the overhead here is seeks
because currently it is loading 4096 size I think. correct me if I misunderstood your point.

> 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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