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From Alex Baranau <alex.barano...@gmail.com>
Subject Re: HBase: "small" WAL transactions Q
Date Wed, 03 Oct 2012 02:04:03 GMT
> Currently HRegion.mutateRowsWithLocks actually acquires
> locks on all rows first (since the contract here is a transaction),
> so (currently) you would get unnecessarily reduced concurrency
> using that API for changes that do not need to be atomic.

Right, it's about "unnecessarily reduced concurrency" vs "faster writing
edits to WAL". In case the changes you write do not intersect (do not
belong to the same row), which I imagine is the most common case when using
HBase, then it makes sense to choose faster writing to WAL.

> Also note that a Put(List<Put>) operation already writes multiple
> updates to a single WALEdit (doing a best effort batching).

Do you mean HTable.put(List<Put>) operation? Really? Hm.. Oh, you probably
mean that updates *that belong to the same row* are getting written to WAL
as single WALEdit. Yeah, that was a great improvement (esp. w.r.t. to
consistency).

If there are no objections, I'd add this idea of "faster writing edits to
WAL" by putting more updates of multiple rows into single WALEdit (which
essentially is WAL write transaction) into JIRA.

Would be great to hear J-D's thoughts: if I remember correctly, he
mentioned that he tried to do FS sync() on each write to WAL (to ensure
"real durability"). Again, if I remember correctly this brought quite a lot
of overhead... which can be reduced by bigger writes to WAL. Or may be it
wasn't J-D who talked about it on the hackathon after HBaseCon?

Alex

On Tue, Oct 2, 2012 at 8:20 PM, lars hofhansl <lhofhansl@yahoo.com> wrote:

> This is an interesting observation. I have not thought about HBASE-5229 in
> terms of a performance improvement.
> Currently HRegion.mutateRowsWithLocks actually acquires locks on all rows
> first (since the contract here is a transaction), so (currently) you would
> get unnecessarily reduced concurrency using that API for changes that do
> not need to be atomic.
>
>
> Also note that a Put(List<Put>) operation already writes multiple updates
> to a single WALEdit (doing a best effort batching).
>
> -- Lars
>
>
>
> ________________________________
>  From: Alex Baranau <alex.baranov.v@gmail.com>
> To: user@hbase.apache.org
> Sent: Tuesday, October 2, 2012 4:29 PM
> Subject: HBase: "small" WAL transactions Q
>
> Hello,
>
> May be silly question.
>
> Data in WAL is written in small transactions. One transaction is a set of
> KeyValues for specific (single) row. As we want each written transaction to
> be durable we write them into the WAL one-by-one (ideally with FS sync()
> calls, etc. on each write). Which is very costly (doing that for each
> write).
>
> Having bigger WAL transactions (writing changes to several "close" records)
> should be more efficient (would result in increase of write throughput).
> I.e. WALEdit record would contain updates to the multiple different rows.
> As far as I understand smth like that was implemented in HBASE-5229 [1].
> But it is not a default behavior when sending multiple records changes to
> RS (e.g. when flushing client-side buffer). It also cannot be forced. What
> are the major reasons for not using that? Is locking multiple "close" rows
> looks so dangerous? Or is it simply not efficient (there's more to that
> besides what I described above)?
>
> Thank you,
> Alex Baranau
> ------
> Sematext :: http://sematext.com/ :: Hadoop - HBase - ElasticSearch - Solr
>
> [1] https://issues.apache.org/jira/browse/HBASE-5229
>

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