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https://issues.apache.org/jira/browse/HBASE-13389?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14395994#comment-14395994
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Jeffrey Zhong commented on HBASE-13389:
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Thanks [~lhofhansl] for looking this. I think your patch can help a bit.
{quote}
Do we need valid (non 0) mvcc readpoints for committed data (i.e. data that was
flushed to an HFile and hence we'll never need to replay any HLogs for those)?
Do we need these anywhere but in the memstore?
{quote}
There are three cases(I could think of and maybe more) that we need the
logSeqId(mvcc) around to help us keep the put order.
Assuming all put/deletes are of same row & timestamp(version)
case 1) region server recovery case
We need mvcc(logSeqId) only when region is in recovery mode but not after
recovery.
case 2) replication receiving side, we need logSeqId to maintain the order
because region move or recovery in replication playing side cause puts out of
order
We need mvcc for couple of days(to be safe) so that at least the data
eventually in receiving side are correct.
case 3) put , delete, put. Currently delete overshadows the later put but with
logSeqId we can easily solve the issue because logSeqId is the real version of
a put.
Seems to me not needed(before I thought we need to keep mvcc around till a
major compaction)
> [REGRESSION] HBASE-12600 undoes skip-mvcc parse optimizations
> -------------------------------------------------------------
>
> Key: HBASE-13389
> URL: https://issues.apache.org/jira/browse/HBASE-13389
> Project: HBase
> Issue Type: Sub-task
> Components: Performance
> Reporter: stack
> Attachments: 13389.txt
>
>
> HBASE-12600 moved the edit sequenceid from tags to instead exploit the
> mvcc/sequenceid slot in a key. Now Cells near-always have an associated
> mvcc/sequenceid where previous it was rare or the mvcc was kept up at the
> file level. This is sort of how it should be many of us would argue but as a
> side-effect of this change, read-time optimizations that helped speed scans
> were undone by this change.
> In this issue, lets see if we can get the optimizations back -- or just
> remove the optimizations altogether.
> The parse of mvcc/sequenceid is expensive. It was noticed over in HBASE-13291.
> The optimizations undone by this changes are (to quote the optimizer himself,
> Mr [~lhofhansl]):
> {quote}
> Looks like this undoes all of HBASE-9751, HBASE-8151, and HBASE-8166.
> We're always storing the mvcc readpoints, and we never compare them against
> the actual smallestReadpoint, and hence we're always performing all the
> checks, tests, and comparisons that these jiras removed in addition to
> actually storing the data - which with up to 8 bytes per Cell is not trivial.
> {quote}
> This is the 'breaking' change:
> https://github.com/apache/hbase/commit/2c280e62530777ee43e6148fd6fcf6dac62881c0#diff-07c7ac0a9179cedff02112489a20157fR96
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