From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paul Clements Subject: Re: write-behind performance ... or how behind can write-behind write Date: Fri, 13 Feb 2009 13:44:37 -0500 Message-ID: <4995BF95.1010908@steeleye.com> References: <4995A170.4000000@amln.net> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4995A170.4000000@amln.net> Sender: linux-raid-owner@vger.kernel.org To: Georgi Alexandrov Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids Georgi Alexandrov wrote: > Generally with the healthy array I'm getting the write performance of > the SATA disk alone (in terms of requests/sec issued to the disk and > bytes/sec written). The SATA disk is obviously a bottleneck even with > the write-behind option set(2). write-behind can help with two things: 1) overcoming latency (say one disk is on the network -- it may be the same speed as the source disk, but it takes longer round-trip for each I/O to complete) 2) temporary slowness of a device (say at a peak in I/O) -- the queue can temporarily hide the slowness of the secondary disk, but this won't last very long -- if writes continue at a pace faster than the disk can handle (i.e., the queue gets filled) then the array drops back to non-write-behind behavior > So the questions is How behind can write-behind write? And can we get a > better performance in a similar setup. By default, it queues up 256 writes. This can be increased, but I've actually seen worse performance in some cases -- not sure why. I haven't had the time to dig into it and figure it out. -- Paul