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From: NeilBrown <neilb@suse.de>
To: Andras Korn <korn@raidlist.elan.rulez.org>
Cc: linux-raid@vger.kernel.org
Subject: Re: write-behind has no measurable effect?
Date: Tue, 15 Feb 2011 09:50:42 +1100	[thread overview]
Message-ID: <20110215095042.51ef7e0a@notabene.brown> (raw)
In-Reply-To: <20110214213817.GG836@hellgate.intra.guy>

On Mon, 14 Feb 2011 22:38:17 +0100 Andras Korn <korn@raidlist.elan.rulez.org>
wrote:

> Hi,
> 
> I experimented a bit with write-mostly and write-behind and found that
> write-mostly provides a very significant benefit (see below) but
> write-behind seems to have no effect whatsoever.

The use-case where write-behind can be expected to have an effect is when the
throughput is low enough to be well within the capacity of all devices, but
the latency of the write-behind device is higher than desired.
write-behind will allow that high latency to be hidden (as long as the
throughput limit is not exceeded).

I suspect your tests did not test for low latency in a low-throughput
scenario.

NeilBrown

  reply	other threads:[~2011-02-14 22:50 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-02-14 21:38 write-behind has no measurable effect? Andras Korn
2011-02-14 22:50 ` NeilBrown [this message]
2011-02-14 22:57   ` Andras Korn
2011-02-14 23:41     ` NeilBrown
2011-02-15  1:00       ` Andras Korn
2011-02-15  1:19         ` John Robinson
2011-02-15  2:19           ` Andras Korn
     [not found]             ` <AANLkTikFSOePZJXknAt=Tx6+FpdJ4tiSNwpuwuPC3RY=@mail.gmail.com>
2011-02-15  9:10               ` Roberto Spadim
2011-02-15 12:40                 ` Andras Korn
2011-02-15 13:26                   ` Roberto Spadim
2011-02-15 17:46                     ` Roberto Spadim
2011-02-16 12:00                 ` Andras Korn
2011-02-16 15:00                   ` Roberto Spadim
2011-02-14 22:56 ` Doug Dumitru
2011-02-14 23:03   ` Andras Korn

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