Date: Fri, 12 Oct 2001 16:53:18 -0700 From: David Greenman <dg@root.com> To: Mike Smith <msmith@freebsd.org> Cc: Scott Long <scott_long@btc.adaptec.com>, Alexey Koptsevich <alex@astro.su.se>, scsi@freebsd.org, msmith@mass.dis.org Subject: Re: Adaptec 3210S: RAID5 performance and bus throughput Message-ID: <20011012165318.C18001@nexus.root.com> In-Reply-To: <200110122337.f9CNbUk05018@mass.dis.org>; from msmith@freebsd.org on Fri, Oct 12, 2001 at 04:37:30PM -0700 References: <scott_long@btc.adaptec.com> <200110122337.f9CNbUk05018@mass.dis.org>
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>> >> RAID5 read performance will be almost identical to RAID0 read performance. >> Write performace will be considerably less because it involves more than >> just writing to one disk; if you are writing a piece of data that is less >> than the size of a full stripe, you will also have to read the parity >> disk, compute the parity, and then write out the data plus the parity. > >This is (often but not always) correct, but misleading. > >RAID5 parity is typically byte-parallel, ie. if you have N disks and >you are writing one byte, you only actually *need* to read N-1 bytes >(one from each of the disks to which you are not writing) to compute >the parity, and write 2 bytes. You only need to do two reads, one from the disk you are updating and one from the disk that has the parity. When you XOR the old data from the parity data, you've removed it and can XOR in the new data. Two reads, two writes. -DG David Greenman Co-founder, The FreeBSD Project - http://www.freebsd.org President, TeraSolutions, Inc. - http://www.terasolutions.com Pave the road of life with opportunities. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message
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