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From "Manoj Govindassamy (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-11402) HDFS Snapshots should capture point-in-time copies of OPEN files
Date Thu, 09 Feb 2017 21:53:41 GMT
Manoj Govindassamy created HDFS-11402:
-----------------------------------------

             Summary: 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


*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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