From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: write-behind performance ... or how behind can write-behind write Date: Sat, 14 Feb 2009 08:38:45 -0500 Message-ID: <4996C965.9020205@tmr.com> References: <4995A170.4000000@amln.net> <4995BF95.1010908@steeleye.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4995BF95.1010908@steeleye.com> Sender: linux-raid-owner@vger.kernel.org To: Paul Clements Cc: Georgi Alexandrov , linux-raid@vger.kernel.org List-Id: linux-raid.ids Paul Clements wrote: > 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 > At least with write-mostly all of the capacity is going into saving data, not serving data. But as you note below if the writes are happening at a rate faster than the device can support it will be a bottleneck. >> 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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- Bill Davidsen "Woe unto the statesman who makes war without a reason that will still be valid when the war is over..." Otto von Bismark