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From Alexandre Normand <alexandre.norm...@gmail.com>
Subject Re: Seeking advice on skipped/lost data during data migration from and to a hbase table
Date Tue, 07 Feb 2017 18:34:07 GMT
Thanks for the correction, Sean.

I'm thinking of trying to reproduce the problem on a non-production cluster
using the same migration job that I was talking about in my original post
(we have similar data as production on a non-prod cluster) but then, I'm
not sure how to validate that what we're experiencing is related to that
bug. Ideally, we'd have some hint using scanner client or region server
logs but I haven't seen anything from looking at HBASE-13090.

Did I miss something that could be useful?



On Tue, Feb 7, 2017 at 10:09 AM Sean Busbey <busbey@apache.org> wrote:

> HBASE-15378 says that it was caused by HBASE-13090, I think.
>
> That issue is present in CDH5.5.4:
>
>
> http://archive.cloudera.com/cdh5/cdh/5/hbase-1.0.0-cdh5.5.4.releasenotes.html
>
> (Search in page for HBASE-13090)
>
> On Tue, Feb 7, 2017 at 11:51 AM, Alexandre Normand
> <alexandre.normand@gmail.com> wrote:
> > Reporting back with some results.
> >
> > We ran several RowCounters and each one gives us the same count back. It
> > could be because RowCounter is much more lightweight than our migration
> job
> > (which reads every cell and turns back to write an equivalent version in
> > another table) but it's hard to tell.
> >
> > Taking a step back, it looks like the bug described in HBASE-15378 was
> > introduced in 1.1.0 which wouldn't affect us since we're still
> > on 1.0.0-cdh5.5.4.
> >
> > I guess that puts us back to square one. Any other ideas?
> >
> > On Sun, Feb 5, 2017 at 1:10 PM Alexandre Normand <
> > alexandre.normand@gmail.com> wrote:
> >
> >> That's a good suggestion. I'll give that a try.
> >>
> >> Thanks again!
> >>
> >> On Sun, Feb 5, 2017 at 1:07 PM Ted Yu <yuzhihong@gmail.com> wrote:
> >>
> >> You can run rowcounter on the source tables multiple times.
> >>
> >> With region servers under load, you would observe inconsistent results
> from
> >> different runs.
> >>
> >> On Sun, Feb 5, 2017 at 12:54 PM, Alexandre Normand <
> >> alexandre.normand@gmail.com> wrote:
> >>
> >> > Thanks, Ted. We're running HBase 1.0.0-cdh5.5.4 which isn't in the
> fixed
> >> > versions so this might be related. This is somewhat reassuring to
> think
> >> > that this would be missed data on the scan/source side because this
> would
> >> > mean that our other ingest/write workloads wouldn't be affected.
> >> >
> >> > From reading the jira description, it sounds like it would be
> difficult
> >> to
> >> > confirm that we've been affected by this bug. Am I right?
> >> >
> >> > On Sun, Feb 5, 2017 at 12:36 PM Ted Yu <yuzhihong@gmail.com> wrote:
> >> >
> >> > > Which release of hbase are you using ?
> >> > >
> >> > > To be specific, does the release have HBASE-15378 ?
> >> > >
> >> > > Cheers
> >> > >
> >> > > On Sun, Feb 5, 2017 at 11:32 AM, Alexandre Normand <
> >> > > alexandre.normand@gmail.com> wrote:
> >> > >
> >> > > > We're migrating data from a previous iteration of a table to
a new
> >> one
> >> > > and
> >> > > > this process involved a MR job that scans data from the source
> table
> >> > and
> >> > > > writes the equivalent data in the new table. The source table
has
> >> 6000+
> >> > > > regions and it frequently splits because we're still ingesting
> time
> >> > > series
> >> > > > data into it. We used buffered writing on the other end when
> writing
> >> to
> >> > > the
> >> > > > new table and we have a yarn resource pool to limit the concurrent
> >> > > writing.
> >> > > >
> >> > > > First, I should say that this job took a long time but still
> mostly
> >> > > worked.
> >> > > > However, we've built a mechanism to compare requested data fetched
> >> from
> >> > > > each one of the tables and found that some rows (0.02%) are
> missing
> >> > from
> >> > > > the destination. We've ruled out a few things already:
> >> > > >
> >> > > > * Functional bug in the job that would have resulted in skipping
> that
> >> > > 0.02%
> >> > > > of the rows.
> >> > > > * Potential for that data not having existed when the migration
> job
> >> > > > initially ran.
> >> > > >
> >> > > > At a high-level, the suspects could be:
> >> > > >
> >> > > > * The source table splitting could have resulted in some input
> keys
> >> not
> >> > > > being read. However, since a hbase split is comprised of a
> >> > > startKey/endKey,
> >> > > > this seems like this would not be expected unless there was a
bug
> in
> >> > > there
> >> > > > somehow.
> >> > > > * The writing/flushing losing a batch. Since we're buffering
> writes
> >> and
> >> > > > flush everything on the clean up of map tasks, we would expect
> write
> >> > > > failures to cause task failures/retries and therefore to not
be a
> >> > problem
> >> > > > in the end. Given that this flush is synchronous and, according
to
> >> our
> >> > > > understanding, completes when the data is in the WAL and memstore,
> >> this
> >> > > > also seems unlikely unless there's a bug.
> >> > > >
> >> > > > I should add that we've extracted a sample of 1% of the source
> rows
> >> > > (doing
> >> > > > all of them is really time consuming because of the size of data)
> and
> >> > > found
> >> > > > that missing data often appears in clusters of the source hbase
> row
> >> > keys.
> >> > > > This doesn't really help pointing at a problem with the scan
side
> of
> >> > > things
> >> > > > or the write side of things (since a failure in either would
> result
> >> in
> >> > a
> >> > > > similar output) but we thought it was interesting. That said,
we
> do
> >> > have
> >> > > a
> >> > > > few keys that are missing that aren't clustered. This could be
> >> because
> >> > > > we've only ran the comparison for 1% of the data or it could
be
> that
> >> > > > whatever is causing this can affect very isolated cases.
> >> > > >
> >> > > > We're now trying to understand how this could have happened in
> order
> >> to
> >> > > > understand how it could impact other jobs/applications and also
to
> >> > > increase
> >> > > > our confidence that we write a modified version of the migration
> job
> >> to
> >> > > > re-migrate the skipped/missing data.
> >> > > >
> >> > > > Any ideas or advice would be much appreciated.
> >> > > >
> >> > > > Thanks!
> >> > > >
> >> > > > --
> >> > > > Alex
> >> > > >
> >> > >
> >> > --
> >> > Alex
> >> >
> >>
> >> --
> >> Alex
> >>
> > --
> > Alex
>
-- 
Alex

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