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From karthik tunga <karthik.tu...@gmail.com>
Subject Re: Multiple WALs
Date Sun, 02 Oct 2011 00:34:31 GMT
Hey Stack,

Along with the log replaying part, logic is also needed for log roll over.
This, I think, easier compared to the merging of the logs. Any edits less
than the last sequence number  on the file system can be removed from all
the WALs.

Cheers,
Karthik

On 1 October 2011 18:05, Jesse Yates <jesse.k.yates@gmail.com> wrote:

> I think adding the abstraction layer and making it not only pluggable, but
> configurable would be great.
>
>  It would be nice to be able to tie into a service that logs directly to
> disk, rather than go through HDFS giving some potentially awesome speedup
> at
> the cost of having to write a logging service that handles replication,
> etc.
> Side note, Accumulo is using their own service to storing the WAL, rather
> than HDFS and I suspect that plays a big role in people's claim of its
> ability to do 'outperform' HBase.
>
> -Jesse Yates
>
> On Sat, Oct 1, 2011 at 2:04 PM, Stack <stack@duboce.net> wrote:
>
> > Yes.  For sure.  Would need to check that the split can deal w/
> > multiple logs written by the one server concurrently (sort by sequence
> > edit id after sorting on all the rest that makes up a wal log key).
> >
> > St.Ack
> >
> > On Sat, Oct 1, 2011 at 1:36 PM, karthik tunga <karthik.tunga@gmail.com>
> > wrote:
> > > Hey,
> > >
> > > Doesn't multiple WALs need some kind of merging when recovering from a
> > crash
> > > ?
> > >
> > > Cheers,
> > > Karthik
> > >
> > >
> > > On 1 October 2011 15:17, Stack <stack@duboce.net> wrote:
> > >
> > >> +1 on making WAL pluggable so we can experiment.  Being able to write
> > >> multiple WALs at once should be easy enough to do (the WAL split code
> > >> should be able to handle it). Also a suggestion made a while back was
> > >> making it so hbase could be configured to write two filesystems --
> > >> there'd be hbase.rootdir as now -- and then we'd allow specifying
> > >> another fs to use for writing WALs (If not specified, we'd just use
> > >> hbase.rootdir for all filesystem interactions as now).
> > >>
> > >> St.Ack
> > >>
> > >> On Sat, Oct 1, 2011 at 10:56 AM, Dhruba Borthakur <dhruba@gmail.com>
> > >> wrote:
> > >> > I have been experimenting with the WAL settings too. It is obvious
> > that
> > >> > turning off the wal makes ur transactions go faster, HDFS write/sync
> > are
> > >> not
> > >> > yet very optimized for high throughput small writes.
> > >> >
> > >> > However, irrespective of whether I have one wal or two, I have
> seeing
> > the
> > >> > same throughput. I have experimented with an HDFS setting that
> allows
> > >> > writing/sync to multiple replicas in parallel, and that has
> increased
> > >> > performance for my test workload, see
> > >> > https://issues.apache.org/jira/browse/HDFS-1783.
> > >> >
> > >> > About using one wal or two, it will be nice if we can separate out
> the
> > >> wal
> > >> > API elegantly and make it pluggable. In that case, we can experiment
> > >> HBase
> > >> > with multiple systems. Once we have it pluggable, we can make the
> > habse
> > >> wal
> > >> > go to a separate HDFS (pure SSD based maybe?).
> > >> >
> > >> > -dhruba
> > >> >
> > >> >
> > >> > On Sat, Oct 1, 2011 at 8:09 AM, Akash Ashok <thehellmaker@gmail.com
> >
> > >> wrote:
> > >> >
> > >> >> Hey,
> > >> >> I've see that setting writeToWAL(false) boosts up the writes like
> > crazy.
> > >> I
> > >> >> was just thinking having MuiltipleWAL on HBase. I understand that
> > this
> > >> is a
> > >> >> consideration in BigTable paper that a WAL per region is not used
> > >> because
> > >> >> it
> > >> >> might result in a lot of disk seeks when there are large number
of
> > >> reasons.
> > >> >> But how about having as many WALs as the number of HardDrives
in
> the
> > >> >> system.
> > >> >> I see that the recommended configs for HBase are 4 - 12 hard drives
> > per
> > >> >> node. This might kick the writes up a notch.
> > >> >>
> > >> >> Would like to know the general opinion on this one?
> > >> >>
> > >> >> Cheers,
> > >> >> Akash A
> > >> >>
> > >> >
> > >> >
> > >> >
> > >> > --
> > >> > Connect to me at http://www.facebook.com/dhruba
> > >> >
> > >>
> > >
> >
>

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