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https://issues.apache.org/jira/browse/HDFS-12136?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16086282#comment-16086282
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Kihwal Lee edited comment on HDFS-12136 at 7/14/17 9:21 PM:
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Normal serving has been greatly impaired. Appending to a block while it's
scanned is exceeding rare compared to the normal block sending rate, yet the
fix impacted all serving. There's a better way accomplished via:
* Entirely remove (revert) fetching of checksums for finalized blocks in the
BlockSender ctor. Reduces lock hold time by eliminating i/o in the dataset
lock.
* If a checksum exception occurs during the scan, and the genstamp changed,
mark the block as suspect for rescan. This is the edge case.
* Recent suspect blocks considers genstamps. Suspect blocks with a newer
genstamp than last recorded are not skipped.
* Recent suspects expire 10 min after being added to the cache. Prior behavior
was 10 mins after last access - which could lead to indefinite postponement.
No test changes needed. {{TestBlockScanner#testAppendWhileScanning}} proves
this approach continues to work.
Only difference in trunk/branch-2 is context and a few log lines in code copied
into a getStoredBlock method.
was (Author: daryn):
Normal serving has been greatly impaired. Appending to a block while it's
scanned is exceeding rare compared to the normal block sending rate, yet the
fix impacted all serving. There's a bettery way accomplished via:
* Entirely remove (revert) fetching of checksums for finalized blocks in the
BlockSender ctor. Reduces lock hold time by eliminating i/o in the dataset
lock.
* If a checksum exception occurs during the scan, and the genstamp changed,
mark the block as suspect for rescan. This is the edge case.
* Recent suspect blocks considers genstamps. Suspect blocks with a newer
genstamp than last recorded are not skipped.
* Recent suspects expire 10 min after being added to the cache. Prior behavior
was 10 mins after last access - which could lead to indefinite postponement.
No test changes needed. {{TestBlockScanner#testAppendWhileScanning}} proves
this approach continues to work.
Only difference in trunk/branch-2 is context and a few log lines in code copied
into a getStoredBlock method.
> BlockSender performance regression due to volume scanner edge case
> ------------------------------------------------------------------
>
> Key: HDFS-12136
> URL: https://issues.apache.org/jira/browse/HDFS-12136
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode
> Affects Versions: 2.8.0
> Reporter: Daryn Sharp
> Assignee: Daryn Sharp
> Priority: Critical
> Attachments: HDFS-12136.branch-2.patch, HDFS-12136.trunk.patch
>
>
> HDFS-11160 attempted to fix a volume scan race for a file appended mid-scan
> by reading the last checksum of finalized blocks within the {{BlockSender}}
> ctor. Unfortunately it's holding the exclusive dataset lock to open and read
> the metafile multiple times Block sender instantiation becomes serialized.
> Performance completely collapses under heavy disk i/o utilization or high
> xceiver activity. Ex. lost node replication, balancing, or decommissioning.
> The xceiver threads congest creating block senders and impair the heartbeat
> processing that is contending for the same lock. Combined with other lock
> contention issues, pipelines break and nodes sporadically go dead.
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