Date: Wed, 5 Sep 2001 13:57:50 -0700 From: Doug Hardie <bc979@lafn.org> To: David Gilbert <dgilbert@velocet.ca> Cc: "Lawrence Farr" <l.farr@epcdirect.co.uk>, "'Greg Lehey'" <grog@FreeBSD.ORG>, "'Lawrence Farr'" <lawrence@epcdirect.co.uk>, "'David Gilbert'" <dgilbert@velocet.ca>, "'Chris BeHanna'" <behanna@zbzoom.net>, "'FreeBSD-Stable'" <stable@FreeBSD.ORG> Subject: RE: [stable] Re: RAID5 Message-ID: <f0433010ab7bc414d46c7@[10.0.1.100]> In-Reply-To: <15254.16593.350305.548246@trooper.velocet.net> References: <002c01c135e4$69c924d0$c80aa8c0@lfarr> <f04330116b7bbe195e610@[10.0.1.100]> <15254.16593.350305.548246@trooper.velocet.net>
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At 11:12 -0400 9/5/01, David Gilbert wrote: >Well... FreeBSD doesn't use a 'fast write' disk (although this is an >interesting idea), but writing a single block of RAID-5 data requires >a read of the previous data, a read of the parity block then a write >of the data and a write of the parity block --- 4 I/O operations. It is the distributing of the data among the disks that is required for write that makes it slower than read. As the transfer amount grows, the slowdown will also grow. I believe thats why the difference in the transfer rates quoted earlier. The fast write disk does nothing for small writes. Its benefit is when the amount transferred is close to the size of the fast write disk, but not more. By the way, recovery from a fast-write disk failure is almost impossible if there was data left on the drive that had not been written to the data disks. Since that disk gets a lot of use one would expect it to have the most failures although that was not our experience. Those disks never died. -- -- Doug To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
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