From mboxrd@z Thu Jan 1 00:00:00 1970 From: NeilBrown Subject: Re: write-behind has no measurable effect? Date: Tue, 15 Feb 2011 09:50:42 +1100 Message-ID: <20110215095042.51ef7e0a@notabene.brown> References: <20110214213817.GG836@hellgate.intra.guy> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20110214213817.GG836@hellgate.intra.guy> Sender: linux-raid-owner@vger.kernel.org To: Andras Korn Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On Mon, 14 Feb 2011 22:38:17 +0100 Andras Korn 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