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https://issues.apache.org/jira/browse/HDFS-4190?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13498448#comment-13498448
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Tsz Wo (Nicholas), SZE commented on HDFS-4190:
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{quote}
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.
{quote}
Uma, this sounds like a very good idea! Since some setup may use a large block
size, say 512MB or 1GB, it is probably good to limit the buffer to some value,
say 64MB.
> 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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