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https://issues.apache.org/jira/browse/HDFS-11402?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16662145#comment-16662145
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Sebastien Barnoud edited comment on HDFS-11402 at 10/24/18 11:32 AM:
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To be more precise, they are 2 usage of snapshots:
1) Retrieve files has they were at the snapshot date =>
dfs.namenode.snapshot.capture.openfiles=true
2) Distcp in snapshot diff mode
=> if dfs.namenode.snapshot.capture.openfiles=true the same open file may be
copied multiple times (for example flume sync target in append mode), which may
not be expected
In that case, we need to exclude open files from the snapshot, else the diff
won't reports any difference between snapshots and leads to data lose on the
target of the distcp.
was (Author: sebastien barnoud):
To more precise, they are 2 usage of snapshots:
1) Retrieve files has they were at the snapshot date =>
dfs.namenode.snapshot.capture.openfiles=true
2) Distcp in snapshot diff mode
=> if dfs.namenode.snapshot.capture.openfiles=true the same open file may be
copied multiple times (for example flume sync target in append mode), which may
not be expected
In that case, we need to exclude open files from the snapshot, else the diff
won't reports any difference between snapshots and leads to data lose on the
target of the distcp.
> HDFS Snapshots should capture point-in-time copies of OPEN files
> ----------------------------------------------------------------
>
> Key: HDFS-11402
> URL: https://issues.apache.org/jira/browse/HDFS-11402
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: hdfs
> Affects Versions: 2.6.0
> Reporter: Manoj Govindassamy
> Assignee: Manoj Govindassamy
> Priority: Major
> Fix For: 2.9.0, 3.0.0-alpha4
>
> Attachments: HDFS-11402-branch-2.01.patch, HDFS-11402.01.patch,
> HDFS-11402.02.patch, HDFS-11402.03.patch, HDFS-11402.04.patch,
> HDFS-11402.05.patch, HDFS-11402.06.patch, HDFS-11402.07.patch,
> HDFS-11402.08.patch
>
>
> *Problem:*
> 1. When there are files being written and when HDFS Snapshots are taken in
> parallel, Snapshots do capture all these files, but these being written files
> in Snapshots do not have the point-in-time file length captured. That is,
> these open files are not frozen in HDFS Snapshots. These open files
> grow/shrink in length, just like the original file, even after the snapshot
> time.
> 2. At the time of File close or any other meta data modification operation on
> these files, HDFS reconciles the file length and records the modification in
> the last taken Snapshot. All the previously taken Snapshots continue to have
> those open Files with no modification recorded. So, all those previous
> snapshots end up using the final modification record in the last snapshot.
> Thus after the file close, file lengths in all those snapshots will end up
> same.
> Assume File1 is opened for write and a total of 1MB written to it. While the
> writes are happening, snapshots are taken in parallel.
> {noformat}
> |---Time---T1-----------T2-------------T3----------------T4------>
> |-----------------------Snap1----------Snap2-------------Snap3--->
> |---File1.open---write---------write-----------close------------->
> {noformat}
> Then at time,
> T2:
> Snap1.File1.length = 0
> T3:
> Snap1.File1.length = 0
> Snap2.File1.length = 0
> <File1 write completed and closed>
> T4:
> Snap1.File1.length = 1MB
> Snap2.File1.length = 1MB
> Snap3.File1.length = 1MB
> *Proposal*
> 1. At the time of taking Snapshot, {{SnapshotManager#createSnapshot}} can
> optionally request {{DirectorySnapshottableFeature#addSnapshot}} to freeze
> open files.
> 2. {{DirectorySnapshottableFeature#addSnapshot}} can consult with
> {{LeaseManager}} and get a list INodesInPath for all open files under the
> snapshot dir.
> 3. {{DirectorySnapshottableFeature#addSnapshot}} after the Snapshot creation,
> Diff creation and updating modification time, can invoke
> {{INodeFile#recordModification}} for each of the open files. This way, the
> Snapshot just taken will have a {{FileDiff}} with {{fileSize}} captured for
> each of the open files.
> 4. Above model follows the current Snapshot and Diff protocols and doesn't
> introduce any any disk formats. So, I don't think we will be needing any new
> FSImage Loader/Saver changes for Snapshots.
> 5. One of the design goals of HDFS Snapshot was ability to take any number of
> snapshots in O(1) time. LeaseManager though has all the open files with
> leases in-memory map, an iteration is still needed to prune the needed open
> files and then run recordModification on each of them. So, it will not be a
> strict O(1) with the above proposal. But, its going be a marginal increase
> only as the new order will be of O(open_files_under_snap_dir). In order to
> avoid HDFS Snapshots change in behavior for open files and avoid change in
> time complexity, this improvement can be made under a new config
> {{"dfs.namenode.snapshot.freeze.openfiles"}} which by default can be
> {{false}}.
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