Date: Sat, 24 Jan 2004 13:03:39 -0500 From: Charles Swiger <cswiger@mac.com> To: Rishi Chopra <rchopra@cal.berkeley.edu> Cc: questions@freebsd.org Subject: Re: Adaptec 2400A Performance Message-ID: <A35EEBF2-4E97-11D8-9B19-003065A20588@mac.com> In-Reply-To: <401207EF.7030005@cal.berkeley.edu> References: <401207EF.7030005@cal.berkeley.edu>
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On Jan 24, 2004, at 12:51 AM, Rishi Chopra wrote: > I was rather disappointed with the results. Can anyone suggest what > might be causing such slow disk speeds, or whether these speeds are > out of the ordinary for a 4-disk FreeBSD RAID5 installation? I have > done nothing to configure the card aside from striping the array in > BIOS; FreeBSD seems to automatically detect the disks. For us to be able to comment beyond generalizations, it's necessary to also benchmark how a single disk performs. I can still answer your question, though: RAID-5 is slow. RAID-5 trades availability against performance and hardware costs. With RAID-0, n drives gives n drives' worth of usable space. With RAID-5, n drives gives n-1 drives' worth of usable space. The performance is between RAID-0 and RAID-1 is comparible for large accesses. For small accesses, particularly small writes, RAID-5 performance is much worse than plain RAID-0 or a plain disk. -- -Chuck
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