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https://issues.apache.org/jira/browse/HDFS-12911?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16308948#comment-16308948
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Chris Douglas commented on HDFS-12911:
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A few questions about the Context API in [^HDFS-12911-HDFS-10285-02.patch]. In
general, a few methods might not be cleanly implementable outside the NN:
* {{readLockNS}}/{{readUnlockNS}} are sensitive APIs to expose to an external
service. If the SPS dies, stalls, or loses connectivity to the NN without
releasing the lock: what happens? Instead, would it be possible to break up
{{analyseBlocksStorageMovementsAndAssignToDN}} into operations that- if any
fail due to concurrent modification- could abort and retry? Alternatively (and
as in the naive [^HDFS-12911.00.patch]), the server side of the RPC could be
"heavier", by taking the lock and running this code server-side. This would
counter many of the advantages to running it externally, unfortunately.
* {{getSPSEngine}} may leak a heavier abstraction than is necessary. As used,
it appears to check for liveness of the remote service, which can be a single
call (i.e., instead of checking that both the SPS and namesystem are running,
create a call that checks both in the context object), or it's used to get a
reference to another component. The context could return those components
directly, rather than returning the SPS. It would be cleaner still if the
underlying action could be behind the context API, so the caller isn't managing
multiple abstractions.
* The {{BlockMoveTaskHandler}} and {{FileIDCollector}} abstract the "write"
(scheduling moves) and "read" (scan namespace) interfaces?
* {{addSPSHint}} has no callers in this patch? Is this future functionality, or
only here for symmetry with {{removeSPSHint}}?
* Are these calls idempotent? Or would an external provider need to resync with
the NN on RPC timeouts?
* Since they're stubs in this patch, could we move the external implementation
to a separate JIRA? Just to keep the patch smaller.
> [SPS]: Fix review comments from discussions in HDFS-10285
> ---------------------------------------------------------
>
> Key: HDFS-12911
> URL: https://issues.apache.org/jira/browse/HDFS-12911
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: datanode, namenode
> Reporter: Uma Maheswara Rao G
> Assignee: Rakesh R
> Attachments: HDFS-12911-HDFS-10285-01.patch,
> HDFS-12911-HDFS-10285-02.patch, HDFS-12911.00.patch
>
>
> This is the JIRA for tracking the possible improvements or issues discussed
> in main JIRA
> So far comments to handle
> Daryn:
> # Lock should not kept while executing placement policy.
> # While starting up the NN, SPS Xattrs checks happen even if feature
> disabled. This could potentially impact the startup speed.
> UMA:
> # I am adding one more possible improvement to reduce Xattr objects
> significantly.
> SPS Xattr is constant object. So, we create one Xattr deduplication object
> once statically and use the same object reference when required to add SPS
> Xattr to Inode. So, here additional bytes required for storing SPS Xattr
> would turn to same as single object ref ( i.e 4 bytes in 32 bit). So Xattr
> overhead should come down significantly IMO. Lets explore the feasibility on
> this option.
> Xattr list Future will not be specially created for SPS, that list would have
> been created by SetStoragePolicy already on the same directory. So, no extra
> Feature creation because of SPS alone.
> # Currently SPS putting long id objects in Q for tracking SPS called Inodes.
> So, it is additional created and size of it would be (obj ref + value) = (8 +
> 8) bytes [ ignoring alignment for time being]
> So, the possible improvement here is, instead of creating new Long obj, we
> can keep existing inode object for tracking. Advantage is, Inode object
> already maintained in NN, so no new object creation is needed. So, we just
> need to maintain one obj ref. Above two points should significantly reduce
> the memory requirements of SPS. So, for SPS call: 8bytes for called inode
> tracking + 8 bytes for Xattr ref.
> # Use LightWeightLinkedSet instead of using LinkedList for from Q. This will
> reduce unnecessary Node creations inside LinkedList.
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