From mboxrd@z Thu Jan 1 00:00:00 1970 From: Georgi Alexandrov Subject: Re: write-behind performance ... or how behind can write-behind write Date: Mon, 16 Feb 2009 12:39:31 +0200 Message-ID: <49994263.4090204@amln.net> References: <4995A170.4000000@amln.net> <4995BF95.1010908@steeleye.com> <4996C965.9020205@tmr.com> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enigF4940BC2D8DA54A006F943B2" Return-path: In-Reply-To: <4996C965.9020205@tmr.com> Sender: linux-raid-owner@vger.kernel.org To: Bill Davidsen Cc: Paul Clements , linux-raid@vger.kernel.org List-Id: linux-raid.ids This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enigF4940BC2D8DA54A006F943B2 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Bill Davidsen wrote: > 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. Well, at least write-mostly is suitable for reading from the SSD disk only in a setup like mine. If writes get really problematic maybe it's better to consider a SSD-only solution. --=20 regards, Georgi Alexandrov key server - pgp.mit.edu :: key id - 0x37B4B3EE Key fingerprint =3D E429 BF93 FA67 44E9 B7D4 F89E F990 01C1 37B4 B3EE --------------enigF4940BC2D8DA54A006F943B2 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEARECAAYFAkmZQmkACgkQ+ZABwTe0s+7l4ACbBX3KubMurPz+/qEd6TVn6s6m rBEAoKFVtVmjVBTYkiw2J/FV//BRrNfm =rQ+H -----END PGP SIGNATURE----- --------------enigF4940BC2D8DA54A006F943B2--