Date: Mon, 31 Mar 2003 08:47:41 +0930 From: Greg 'groggy' Lehey <grog@FreeBSD.org> To: Mattias Pantzare <pantzer@ludd.luth.se> Cc: Lukas Ertl <l.ertl@univie.ac.at> Subject: Re: vinum performance Message-ID: <20030330231741.GG1861@wantadilla.lemis.com> In-Reply-To: <3E87204C.5060304@ludd.luth.se> References: <20030330125138.K23911@leelou.in.tern> <3E870CC7.5000204@mac.com> <20030330175605.E23911@leelou.in.tern> <3E87204C.5060304@ludd.luth.se>
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--L+ofChggJdETEG3Y Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable [Format recovered--see http://www.lemis.com/email/email-format.html] Alternate long/short. On Sunday, 30 March 2003 at 18:50:20 +0200, Mattias Pantzare wrote: > Lukas Ertl wrote: > >> Ok. But I still don't understand why RAID 5 write performance is _so_ ba= d. >> The CPU is not the bottle neck, it's rather bored. And I don't understand >> why RAID 0 doesn't give a big boost at all. Is the ahc driver known to be >> slow? > > To do a RAID 5 write you do this: > 1. Read the old data on the blocks that you will write to. > 2. Read the coresponding parity data. These two go in parallel. > 3. Write the new data. > 4. Write the new parity. So do these. > For RAID0 to be faster than a singel disk you have to do reads that > match or are larger than a stripe. Or have several processes > generate the IO. Several processes is the obvious one. The random seeks test does 3 processes (and you'll note that in this one, RAID-5 exceeds the performance of a single disk), but that's still not enough. rawio uses 8 processes by default, but you can change it. Greg -- When replying to this message, please take care not to mutilate the original text. =20 For more information, see http://www.lemis.com/email.html See complete headers for address and phone numbers --L+ofChggJdETEG3Y Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.0 (FreeBSD) iD8DBQE+h3sVIubykFB6QiMRAquvAJ4uH3qKnMGwYZj17kVI6zkPMbGXqACgg4BQ 75db7faOifzEWykOk3ehIWI= =az3m -----END PGP SIGNATURE----- --L+ofChggJdETEG3Y--
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