Date: Sat, 31 Jan 1998 15:55:27 -0600 From: Karl Denninger <karl@mcs.net> To: Brian Tao <taob@nbc.netcom.ca> Cc: current@FreeBSD.ORG Subject: Re: RAID controllers - folks, check this thing out Message-ID: <19980131155527.19192@mcs.net> In-Reply-To: <Pine.GSO.3.95.980131161942.27817Z-100000@tor-adm1>; from Brian Tao on Sat, Jan 31, 1998 at 04:28:00PM -0500 References: <19980131144604.03410@mcs.net> <Pine.GSO.3.95.980131161942.27817Z-100000@tor-adm1>
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On Sat, Jan 31, 1998 at 04:28:00PM -0500, Brian Tao wrote: > On Sat, 31 Jan 1998, Karl Denninger wrote: > > > > CCD mounted async is NOT a fair comparison - not at all. > > I note that in my message. Async mounts delay metainfo writes, > and the RAID controller delays all writes to disk via its cache. It > would be unfair to CCD, in that your chances of recovering a busy > async filesystem after a crash are nearly zero. The comparison is > useful when you consider that the CMD is almost able to keep up even > without asynchronous updates. Yes, but it doesn't quite get there, and you lose hot-rebuild. Also, CCD is only RAID 0 or RAID 1 - where there is no parity computation. The CMD controler in a RAID 0+1 configuration might be a LOT faster. > > Also, RAID 5 with other than 5 disks is sub-optimal, and some of those > > tests were run with fewer drives. > > What is magic about having 5 drives? Obviously the more drives > you add, the better the performance. I limited both RAID units to > three drives to provide a more level comparison. These were done on > eval units, and I would have liked six or more drives. > -- > Brian Tao (BT300, taob@netcom.ca) > "Though this be madness, yet there is method in't" RAID 5, due to the way it stripes parity across the volumes, has a "sweet spot" in performance at 5 spindles. It is significantly slower with either more or fewer devices. RAID 4 keeps all parity info on one disk, and doesn't have that problme (but it DOES have the problem of the single spindle being a bottleneck on writes). -- -- Karl Denninger (karl@MCS.Net)| MCSNet - Serving Chicagoland and Wisconsin http://www.mcs.net/ | T1's from $600 monthly to FULL DS-3 Service | NEW! K56Flex support on ALL modems Voice: [+1 312 803-MCS1 x219]| EXCLUSIVE NEW FEATURE ON ALL PERSONAL ACCOUNTS Fax: [+1 312 803-4929] | *SPAMBLOCK* Technology now included at no cost
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