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From "Eli Collins (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1158) HDFS-457 increases the chances of losing blocks
Date Sun, 29 Aug 2010 18:18:58 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1158?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12904005#action_12904005
] 

Eli Collins commented on HDFS-1158:
-----------------------------------

If we make the DN automatically decommission itself when it finds a bad disk then we might
as well revert HDFS-457 no?

Seems like there are two separate issues here:
# DNs don't fail gracefully if the system disk (eg where /tmp, logs etc are stored) fails
# A DN that has lost some volumes refuses to come up when it's restarted. eg a DN that is
running with 8 disks, and suffers two disks failures will continue to run with 6 disks, however
if you restart the DN w/o first updating the configuration to remove the bad disks from dfs.data.dir
then the DN will refuse to come up (because it checks for failed volumes on startup) and now
the 6 valid disks that were just available are now no longer available. If you hit this scenario
with three DNs any blocks that only resides on these DNs is no longer available.

The solution for the first issue seems straight-forward, DNs should handle system disk failure
gracefully by decommissioning themselves. 

For the second issue, it seems like the desired behavior (assuming we want DNs to tolerate
disk failures) is for the DN to restart successfully as long as there are no more than dfs.datanode.failed.volumes.tolerated
failed disks (otherwise it should decommissions itself). This way when the DN above restarts
it continues to offer service for the 6 good disks even though there are 2 bad disks (which
were presumably re-replicated when they failed originally).




>  HDFS-457 increases the chances of losing blocks
> ------------------------------------------------
>
>                 Key: HDFS-1158
>                 URL: https://issues.apache.org/jira/browse/HDFS-1158
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: data-node
>    Affects Versions: 0.20.2
>            Reporter: Koji Noguchi
>             Fix For: 0.20.3
>
>         Attachments: rev-HDFS-457.patch
>
>
> Whenever we restart a cluster, there's a chance of losing some blocks if more than three
datanodes don't come up.
> HDFS-457 increases this chance by keeping the datanodes up even when 
>    # /tmp disk goes read-only
>    # /disk0 that is used for storing PID goes read-only 
> and probably more.
> In our environment, /tmp and /disk0 are from the same device.
> When trying to restart a datanode, it would fail with
> 1) 
> {noformat}
> 2010-05-15 05:45:45,575 WARN org.mortbay.log: tmpdir
> java.io.IOException: Read-only file system
>         at java.io.UnixFileSystem.createFileExclusively(Native Method)
>         at java.io.File.checkAndCreate(File.java:1704)
>         at java.io.File.createTempFile(File.java:1792)
>         at java.io.File.createTempFile(File.java:1828)
>         at org.mortbay.jetty.webapp.WebAppContext.getTempDirectory(WebAppContext.java:745)
> {noformat}
> or 
> 2) 
> {noformat}
> hadoop-daemon.sh: line 117: /disk/0/hadoop-datanode....com.out: Read-only file system
> hadoop-daemon.sh: line 118: /disk/0/hadoop-datanode.pid: Read-only file system
> {noformat}
> I can recover the missing blocks but it takes some time.
> Also, we are losing track of block movements since log directory can also go to read-only
but datanode would continue running.
> For 0.21 release, can we revert HDFS-457 or make it configurable?

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