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From Kannan Muthukkaruppan <Kan...@facebook.com>
Subject RE: commit semantics
Date Tue, 12 Jan 2010 19:40:00 GMT
Btw, is there much gains in having a large number of regions-- i.e. to the tune of 500 -- per
region server?

I understand that having multiple regions per region server allows finer grained rebalancing
when new nodes are added or a node goes down. But would say having a smaller number of regions
per region server (say ~50) be really bad. If a region server goes down, 50 other nodes would
pick up ~1/50 of its work. Not as good as 500 other nodes picking up 1/500 of its work each--
but seems acceptable still. Are there other advantages of having a large number of regions
per region server?

regards,
Kannan
-----Original Message-----
From: jdcryans@gmail.com [mailto:jdcryans@gmail.com] On Behalf Of Jean-Daniel Cryans
Sent: Tuesday, January 12, 2010 9:42 AM
To: hbase-dev@hadoop.apache.org
Subject: Re: commit semantics

wrt 1 HLog per region server, this is from the Bigtable paper. Their
main concern is the number of opened files since if you have 1000
region servers * 500 regions then you may have 100 000 HLogs to
manage. Also you can have more than one file per HLog, so let's say
you have on average 5 log files per HLog that's 500 000 files on HDFS.

J-D

On Tue, Jan 12, 2010 at 12:24 AM, Dhruba Borthakur <dhruba@gmail.com> wrote:
> Hi Ryan,
>
> thanks for ur response.
>
>>Right now each regionserver has 1 log, so if 2 puts on different
>>tables hit the same RS, they hit the same HLog.
>
> I understand. My point was that the application could insert the same record
> into two different tables on two different Hbase instances on two different
> piece of hardware.
>
> On a related note, can somebody explain what the tradeoff is if each region
> has its own hlog? are you worried about the number of files in HDFS? or
> maybe the number of sync-threads in the region server? Can multiple hlog
> files provide faster region splits?
>
>
>> I've thought about this issue quite a bit, and I think the sync every
>> 1 rows combined with optional no-sync and low time sync() is the way
>> to go. If you want to discuss this more in person, maybe we can meet
>> up for brews or something.
>>
>
> The group-commit thing I can understand. HDFS does a very similar thing. But
> can you explain your alternative "sync every 1 rows combined with optional
> no-sync and low time sync"? For those applications that have the natural
> characteristics of updating only one row per logical operation, how can they
> be sure that their data has reached some-sort-of-stable-storage unless they
> sync after every row update?
>
> thanks,
> dhruba
>

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