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Posted to hdfs-issues@hadoop.apache.org by "dhruba borthakur (JIRA)" <ji...@apache.org> on 2010/03/11 19:34:27 UTC
[jira] Created: (HDFS-1034) Enhance datanode to read data and
checksum file in parallel
Enhance datanode to read data and checksum file in parallel
-----------------------------------------------------------
Key: HDFS-1034
URL: https://issues.apache.org/jira/browse/HDFS-1034
Project: Hadoop HDFS
Issue Type: Improvement
Reporter: dhruba borthakur
Assignee: dhruba borthakur
In the current HDFS implementation, a read of a block issued to the datanode results in a disk access to the checksum file followed by a disk access to the checksum file. It would be nice to be able to do these two IOs in parallel to reduce read latency.
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[jira] Commented: (HDFS-1034) Enhance datanode to read data and
checksum file in parallel
Posted by "Todd Lipcon (JIRA)" <ji...@apache.org>.
[ https://issues.apache.org/jira/browse/HDFS-1034?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12844285#action_12844285 ]
Todd Lipcon commented on HDFS-1034:
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In practice I don't imagine the extra disk seek for checksums is a problem for HBase - since the checksum file is relatively small, my guess is that it stays hot in the linux buffer cache and therefore doesn't represent any disk access. Would certainly be interesting to run blktrace on a heavily loaded hbase datanode to see if this is true, though!
> Enhance datanode to read data and checksum file in parallel
> -----------------------------------------------------------
>
> Key: HDFS-1034
> URL: https://issues.apache.org/jira/browse/HDFS-1034
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: dhruba borthakur
> Assignee: dhruba borthakur
>
> In the current HDFS implementation, a read of a block issued to the datanode results in a disk access to the checksum file followed by a disk access to the checksum file. It would be nice to be able to do these two IOs in parallel to reduce read latency.
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[jira] Commented: (HDFS-1034) Enhance datanode to read data and
checksum file in parallel
Posted by "Zlatin Balevsky (JIRA)" <ji...@apache.org>.
[ https://issues.apache.org/jira/browse/HDFS-1034?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12844269#action_12844269 ]
Zlatin Balevsky commented on HDFS-1034:
---------------------------------------
How complicated would it be to store the checksum file on a separate mount point? In JBOD configurations this will enable both reads to happen simultaneously.
> Enhance datanode to read data and checksum file in parallel
> -----------------------------------------------------------
>
> Key: HDFS-1034
> URL: https://issues.apache.org/jira/browse/HDFS-1034
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: dhruba borthakur
> Assignee: dhruba borthakur
>
> In the current HDFS implementation, a read of a block issued to the datanode results in a disk access to the checksum file followed by a disk access to the checksum file. It would be nice to be able to do these two IOs in parallel to reduce read latency.
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[jira] Commented: (HDFS-1034) Enhance datanode to read data and
checksum file in parallel
Posted by "Jay Booth (JIRA)" <ji...@apache.org>.
[ https://issues.apache.org/jira/browse/HDFS-1034?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12844186#action_12844186 ]
Jay Booth commented on HDFS-1034:
---------------------------------
Wouldn't this preclude the use of transferTo for transfer from the main block file? The packet header and sums need to be sent before transferTo is invoked, otherwise things would all be jumbled up together.
> Enhance datanode to read data and checksum file in parallel
> -----------------------------------------------------------
>
> Key: HDFS-1034
> URL: https://issues.apache.org/jira/browse/HDFS-1034
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: dhruba borthakur
> Assignee: dhruba borthakur
>
> In the current HDFS implementation, a read of a block issued to the datanode results in a disk access to the checksum file followed by a disk access to the checksum file. It would be nice to be able to do these two IOs in parallel to reduce read latency.
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[jira] Commented: (HDFS-1034) Enhance datanode to read data and
checksum file in parallel
Posted by "Zlatin Balevsky (JIRA)" <ji...@apache.org>.
[ https://issues.apache.org/jira/browse/HDFS-1034?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12844272#action_12844272 ]
Zlatin Balevsky commented on HDFS-1034:
---------------------------------------
The only possible bottleneck is the extra disk seek which may or may not be a big deal. Probably for HBase-type workloads. There are many ways around that including but not limited to:
a) prepending a copy of the checksum file to the block file while keeping the separate copy intact for off-thread verification after the transfer starts
b) using some ext4-extents jni magic
... ?
> Enhance datanode to read data and checksum file in parallel
> -----------------------------------------------------------
>
> Key: HDFS-1034
> URL: https://issues.apache.org/jira/browse/HDFS-1034
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: dhruba borthakur
> Assignee: dhruba borthakur
>
> In the current HDFS implementation, a read of a block issued to the datanode results in a disk access to the checksum file followed by a disk access to the checksum file. It would be nice to be able to do these two IOs in parallel to reduce read latency.
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[jira] Commented: (HDFS-1034) Enhance datanode to read data and
checksum file in parallel
Posted by "Todd Lipcon (JIRA)" <ji...@apache.org>.
[ https://issues.apache.org/jira/browse/HDFS-1034?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12844232#action_12844232 ]
Todd Lipcon commented on HDFS-1034:
-----------------------------------
A bit of manual readahead on the checksum file could theoretically have the same benefit without the problem Jay mentioned. The Checksum file is opened with a buffer, right? In that case, do we expect to see a big benefit?
> Enhance datanode to read data and checksum file in parallel
> -----------------------------------------------------------
>
> Key: HDFS-1034
> URL: https://issues.apache.org/jira/browse/HDFS-1034
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: dhruba borthakur
> Assignee: dhruba borthakur
>
> In the current HDFS implementation, a read of a block issued to the datanode results in a disk access to the checksum file followed by a disk access to the checksum file. It would be nice to be able to do these two IOs in parallel to reduce read latency.
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[jira] Commented: (HDFS-1034) Enhance datanode to read data and
checksum file in parallel
Posted by "Todd Lipcon (JIRA)" <ji...@apache.org>.
[ https://issues.apache.org/jira/browse/HDFS-1034?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12844270#action_12844270 ]
Todd Lipcon commented on HDFS-1034:
-----------------------------------
Only scary thing about moving checksums to a different mountpoint is that the checksum file's metadata will be on a different journal than the data files. This might end up fine, but it's a little nervewracking in terms of what kind of consistency we get out of the FS - could cause some very subtle bugs.
Have we seen these issues to be a significant bottleneck?
> Enhance datanode to read data and checksum file in parallel
> -----------------------------------------------------------
>
> Key: HDFS-1034
> URL: https://issues.apache.org/jira/browse/HDFS-1034
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: dhruba borthakur
> Assignee: dhruba borthakur
>
> In the current HDFS implementation, a read of a block issued to the datanode results in a disk access to the checksum file followed by a disk access to the checksum file. It would be nice to be able to do these two IOs in parallel to reduce read latency.
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