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From Mohammad Tariq <donta...@gmail.com>
Subject Re: Slow full-table scans
Date Sun, 12 Aug 2012 23:34:31 GMT
Also, give it a shot using  HTablePools and see if it makes any significant
difference.

Regards,
    Mohammad Tariq



On Mon, Aug 13, 2012 at 4:43 AM, Jacques <whshub@gmail.com> wrote:

> I think the first question is where is the time spent.  Does your analysis
> show that all the time spent is on the regionservers or is a portion of the
> bottleneck on the client side?
>
> Jacques
>
>
>
> On Sun, Aug 12, 2012 at 4:00 PM, Mohammad Tariq <dontariq@gmail.com>
> wrote:
>
> > Methods getStartKey and getEndKey provided by  HRegionInfo class can used
> > for that purpose.
> > Also, please make sure, any HTable instance is not left opened once you
> are
> > are done with reads.
> > Regards,
> >     Mohammad Tariq
> >
> >
> >
> > On Mon, Aug 13, 2012 at 4:22 AM, Gurjeet Singh <gurjeet@gmail.com>
> wrote:
> >
> > > Hi Mohammad,
> > >
> > > This is a great idea. Is there a API call to determine the start/end
> > > key for each region ?
> > >
> > > Thanks,
> > > Gurjeet
> > >
> > > On Sun, Aug 12, 2012 at 3:49 PM, Mohammad Tariq <dontariq@gmail.com>
> > > wrote:
> > > > Hello experts,
> > > >
> > > >        Would it be feasible to create a separate thread for each
> > > region??I
> > > > mean we can determine start and end key of each region and issue a
> scan
> > > for
> > > > each region in parallel.
> > > >
> > > > Regards,
> > > >     Mohammad Tariq
> > > >
> > > >
> > > >
> > > > On Mon, Aug 13, 2012 at 3:54 AM, lars hofhansl <lhofhansl@yahoo.com>
> > > wrote:
> > > >
> > > >> Do you really have to retrieve all 200.000 each time?
> > > >> Scan.setBatch(...) makes no difference?! (note that batching is
> > > different
> > > >> and separate from caching).
> > > >>
> > > >> Also note that the scanner contract is to return sorted KVs, so a
> > single
> > > >> scan cannot be parallelized across RegionServers (well not entirely
> > > true,
> > > >> it could be farmed off in parallel and then be presented to the
> client
> > > in
> > > >> the right order - but HBase is not doing that). That is why one vs
> 12
> > > RSs
> > > >> makes no difference in this scenario.
> > > >>
> > > >> In the 12 node case you'll see low CPU on all but one RS, and each
> RS
> > > will
> > > >> get its turn.
> > > >>
> > > >> In your case this is scanning 20.000.000 KVs serially in 400s,
> that's
> > > >> 50000 KVs/s, which - depending on hardware - is not too bad for
> HBase
> > > (but
> > > >> not great either).
> > > >>
> > > >> If you only ever expect to run a single query like this on top your
> > > >> cluster (i.e. your concern is latency not throughput) you can do
> > > multiple
> > > >> RPCs in parallel for a sub portion of your key range. Together with
> > > >> batching can start using value before all is streamed back from the
> > > server.
> > > >>
> > > >>
> > > >> -- Lars
> > > >>
> > > >>
> > > >>
> > > >> ----- Original Message -----
> > > >> From: Gurjeet Singh <gurjeet@gmail.com>
> > > >> To: user@hbase.apache.org
> > > >> Cc:
> > > >> Sent: Saturday, August 11, 2012 11:04 PM
> > > >> Subject: Slow full-table scans
> > > >>
> > > >> Hi,
> > > >>
> > > >> I am trying to read all the data out of an HBase table using a scan
> > > >> and it is extremely slow.
> > > >>
> > > >> Here are some characteristics of the data:
> > > >>
> > > >> 1. The total table size is tiny (~200MB)
> > > >> 2. The table has ~100 rows and ~200,000 columns in a SINGLE family.
> > > >> Thus the size of each cell is ~10bytes and the size of each row is
> > > >> ~2MB
> > > >> 3. Currently scanning the whole table takes ~400s (both in a
> > > >> distributed setting with 12 nodes or so and on a single node), thus
> > > >> 5sec/row
> > > >> 4. The row keys are unique 8 byte crypto hashes of sequential
> numbers
> > > >> 5. The scanner is set to fetch a FULL row at a time (scan.setBatch)
> > > >> and is set to fetch 100MB of data at a time (scan.setCaching)
> > > >> 6. Changing the caching size seems to have no effect on the total
> scan
> > > >> time at all
> > > >> 7. The column family is setup to keep a single version of the cells,
> > > >> no compression, and no block cache.
> > > >>
> > > >> Am I missing something ? Is there a way to optimize this ?
> > > >>
> > > >> I guess a general question I have is whether HBase is good datastore
> > > >> for storing many medium sized (~50GB), dense datasets with lots of
> > > >> columns when a lot of the queries require full table scans ?
> > > >>
> > > >> Thanks!
> > > >> Gurjeet
> > > >>
> > > >>
> > >
> >
>

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