[
https://issues.apache.org/jira/browse/HDFS-11187?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16314247#comment-16314247
]
Yongjun Zhang commented on HDFS-11187:
--------------------------------------
[~jojochuang],
Thanks for investigating the issue and patch. The changes make sense to me,
some comments:
1. {{calculateLastPartialChunkChecksum()}} can be renamed to
{{loadLastPartialChunkChecksum}}, and it can be changed to "void" instead of
returning {{byte[]}}, because it calls
{{setLastPartialChunkChecksum(lastChecksum)}}
2. {{loadPartialChunkChecksumForFinalized}} can be renamed to
{{getPartialChunkChecksumForFinalized}}, because we only do the load when
needed inside the implementation
3. Because of 1, {{loadPartialChunkChecksumForFinalized}} can be streamlined
to have a shared return statement. Currently we load checksum when
{code}
if (replicaVisibleLength % CHUNK_SIZE != 0 &&
finalized.getLastPartialChunkChecksum() == null) {
{code}
that means we don't load checksum when replicaVisibleLength is CHUNK_SIZE, in
which case it's possible checksum is not loaded (thus null), suggest to add a
comment how that case is handled.
4. Move the comment
{code}
// The number of bytes per checksum here determines the alignment
// of reads: we always start reading at a checksum chunk boundary,
// even if the checksum type is NULL. So, choosing too big of a value
// would risk sending too much unnecessary data. 512 (1 disk sector)
// is likely to result in minimal extra IO.
{code}
to where CHUNK_SIZE is defined.
5. There is a possible race condition that a finalized replica is moved to RBW
then modified by another writer, and the reader is trying to access the same
replica. We can discuss this situation separately.
Thanks.
> Optimize disk access for last partial chunk checksum of Finalized replica
> -------------------------------------------------------------------------
>
> Key: HDFS-11187
> URL: https://issues.apache.org/jira/browse/HDFS-11187
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode
> Reporter: Wei-Chiu Chuang
> Assignee: Wei-Chiu Chuang
> Attachments: HDFS-11187.001.patch, HDFS-11187.002.patch
>
>
> The patch at HDFS-11160 ensures BlockSender reads the correct version of
> metafile when there are concurrent writers.
> However, the implementation is not optimal, because it must always read the
> last partial chunk checksum from disk while holding FsDatasetImpl lock for
> every reader. It is possible to optimize this by keeping an up-to-date
> version of last partial checksum in-memory and reduce disk access.
> I am separating the optimization into a new jira, because maintaining the
> state of in-memory checksum requires a lot more work.
--
This message was sent by Atlassian JIRA
(v6.4.14#64029)
---------------------------------------------------------------------
To unsubscribe, e-mail: [email protected]
For additional commands, e-mail: [email protected]
