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https://issues.apache.org/jira/browse/HDFS-4213?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13505179#comment-13505179
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Aaron T. Myers commented on HDFS-4213:
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bq. Why don't you change the enum to SyncFlag?
Good thinking, Nicholas. I agree that SyncFlag is better.
bq. Aaron, the sync with UPDATE_LENGTH feature is quite specific to HDFS. I am
not sure if it makes sense to other file systems. Do you have examples that it
does make sense? A follow up question is that how should the default
implementation be?
I agree with you 100% that the UPDATE_LENGTH flag in particular seems highly
HDFS-specific. What doesn't seem so HDFS-specific to me, however, is an
{{hsync(...)}} method which takes an EnumSet as an argument of extra flags.
Given that we currently have {{hsync()}} (no args) as a method in
FSDataOutputStream, I think we should consider also adding
{{hsync(EnumSet<SyncFlag>)}} to FSDataOutputStream. FS implementations which
don't do anything with a particular SyncFlag can just ignore it. This is
similar to how {{create(... EnumSet<CreateFlag> ...)}} works in the existing
FileSystem interface. Not every FS may implement every CreateFlag, but that's
fine.
Note that I'm not wedded to this suggestion. I agree that this isn't very
pertinent right now since SyncFlag contains just UPDATE_LENGTH, but if we ever
add a SyncFlag that is relevant to multiple FileSystem implementations then it
will make sense to have SyncFlag in Common and the {{hsync(EnumSet<SyncFlag>)}}
method as part of FSDataOutputStream.
> When the client calls hsync, allows the client to update the file length in
> the NameNode
> ----------------------------------------------------------------------------------------
>
> Key: HDFS-4213
> URL: https://issues.apache.org/jira/browse/HDFS-4213
> Project: Hadoop HDFS
> Issue Type: New Feature
> Affects Versions: 3.0.0
> Reporter: Jing Zhao
> Assignee: Jing Zhao
> Attachments: HDFS-4213.001.patch, HDFS-4213.002.patch,
> HDFS-4213.003.patch, HDFS-4213.004.patch, HDFS-4213.005.patch,
> HDFS-4213.006.patch
>
>
> As per discussion in HDFS-3960 and HDFS-2802, when clients that need strong
> consistency update the file length at the NameNode, a special sync/flush is
> required for getting the length of the being written files when snapshots are
> taken for these files. This jira implements this sync-with-updating-length by
> 1) calling ClientProtocol#fsync(), and 2) adding a new field to
> ClientProtocol#fsync() to indicate the length information.
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