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From "Manoj Govindassamy (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-11402) HDFS Snapshots should capture point-in-time copies of OPEN files
Date Thu, 16 Feb 2017 03:46:41 GMT

    [ https://issues.apache.org/jira/browse/HDFS-11402?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15869091#comment-15869091
] 

Manoj Govindassamy commented on HDFS-11402:
-------------------------------------------

[~yzhangal],

LeaseManager maintains INodeFileIDs of all leased open files in-memory. Moreover, with the
proposed design LeaseManager now exposes a functionality to get all files with leases under
the given directory. So at the time of Snapshot, the SnapshotManager will only run extra operations
for the open files under the snapshot dir and not on all the open files in the system. For
all the open files, creation of diff record is very light weight as it only copies the length
field from the file and not blocks. 

Your proposal of maintaining diff record for each open file at the time of file length update
well before the Snaoshot is _not_ going to make the overall time complexity O(1). The diff
record computation is not the one contributing to the extra time consumption at the Snapshot
time. Its the generation of open files under the snapshot directory and the diff record saving
for each of those open files is making the overall complexity for the snapshot creation an
O(open files count under snap root). And, {{dfs.namenode.snapshot.freeze.openfiles}} config
is {{false}} by default. Users who are interested in this feature can enable this at a marginal
cost and otherwise there aren't any performance dips in the normal snapshot operation. 

> 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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