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https://issues.apache.org/jira/browse/HDFS-3107?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14190942#comment-14190942
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Colin Patrick McCabe commented on HDFS-3107:
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bq. So, but the new patch from Plamen Jeliazkov already have snapshot support
Am I looking at the wrong version? The patch posted on 17 Oct does not seem to
have snapshot support. The relevant code is here:
{code}
2242 boolean truncateInternal(String src, long newLength,
2243 String clientName, String clientMachine,
2244 long mtime, FSPermissionChecker pc)
2245 throws IOException, UnresolvedLinkException {
2246 assert hasWriteLock();
2247 if (isPermissionEnabled) {
2248 checkPathAccess(pc, src, FsAction.WRITE);
2249 }
2250 INodesInPath iip = dir.getINodesInPath4Write(src, true);
2251 final int latestSnapshotId = iip.getLatestSnapshotId();
2252 INodeFile inodeFile = INodeFile.valueOf(iip.getLastINode(), src);
2253 // Data will be lost after truncate occurs so it cannot support
snapshots.
2254 if(inodeFile.isInLatestSnapshot(latestSnapshotId))
2255 throw new HadoopIllegalArgumentException(
2256 "Cannot truncate file with snapshot.");
{code}
I think it's misleading to refer to throwing an exception when there are
snapshots present as "support." But whatever we call it, my patch doesn't
throw an exception here-- it always allows truncation even when the admin is
using snapshots.
I was trying to be helpful and point out an approach that would let you respond
to the comments here. For example, [~sureshms] 's comment here:
https://issues.apache.org/jira/browse/HDFS-3107?focusedCommentId=14155537&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14155537
Or [~szetszwo]'s comment about the need to support rollback. You need to
address the comments that people have made.
P.S. Can we put numbers on patch files? I find it difficult to keep track of
them when they all have the same file name.
> HDFS truncate
> -------------
>
> Key: HDFS-3107
> URL: https://issues.apache.org/jira/browse/HDFS-3107
> Project: Hadoop HDFS
> Issue Type: New Feature
> Components: datanode, namenode
> Reporter: Lei Chang
> Assignee: Plamen Jeliazkov
> Attachments: HDFS-3107.008.patch, HDFS-3107.patch, HDFS-3107.patch,
> HDFS-3107.patch, HDFS-3107.patch, HDFS-3107.patch, HDFS-3107.patch,
> HDFS-3107.patch, HDFS_truncate.pdf, HDFS_truncate.pdf,
> HDFS_truncate_semantics_Mar15.pdf, HDFS_truncate_semantics_Mar21.pdf,
> editsStored, editsStored, editsStored.xml
>
> Original Estimate: 1,344h
> Remaining Estimate: 1,344h
>
> Systems with transaction support often need to undo changes made to the
> underlying storage when a transaction is aborted. Currently HDFS does not
> support truncate (a standard Posix operation) which is a reverse operation of
> append, which makes upper layer applications use ugly workarounds (such as
> keeping track of the discarded byte range per file in a separate metadata
> store, and periodically running a vacuum process to rewrite compacted files)
> to overcome this limitation of HDFS.
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