Date: Thu, 23 Jul 1998 00:47:19 -0400 (EDT) From: Mikhail Teterin <mi@aldan.algebra.com> To: grog@lemis.com (Greg Lehey) Cc: questions@FreeBSD.ORG Subject: Re: ccd questions Message-ID: <199807230447.AAA05997@rtfm.ziplink.net> In-Reply-To: <19980723133831.P8993@freebie.lemis.com> from "Greg Lehey" at "Jul 23, 98 01:38:31 pm"
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Greg Lehey once stated: => I'm trying to do this under a day old -stable. There are four disks => -- 2 2Gb and 2 4Gb -- involved. I set up two ccd mirroring disks => 2Gb and 4Gb big each with different ileave numbers (from 2 to 6000, => being powers of 2 and primes). Here are my problems: => The 4 disks are on an ahc of their own with an idle tape-drive. I => never tested both arrays at the same time. The 3 system disks are => on a separate ahc. The machine has a single PPro-200 with 256Kb of => cache, 128Mb of RAM, 192Mb of swap split evenly among three system => drives. => All four disks are different, so I do not expect "optimum" => performance, but my results were still disappointing :( => => According to iozone benchmark, the write speed went down 50% => compared to when using the disks by themselves -- without => ccd. I would expect it to stay the same, really -- it is => about 3.5Mb/sec and is far from saturating the 10Mb/s top => of this SCSI interface. The ileave number does not seem to => matter once it is above 32. =Yes, I'd consider this result disappointing as well. May I assume =that the performance improved with increasing the ileave factor? My =investigations suggest that 128 is about the optimum, though there's =not much improvement beyond 32. Yes, it grows, but very little... And is always 50-60% of the single disk speed. => The read speed is about the same -- according to `systat 1 => -iostat' the data is read only from the first disk of an => array -- I'd expect it to double as the things can be read => in parallel from each of the drives. Again the ileave number => does not seem to matter once it is above 32. =This would only happen if you're runnning multiple processes. =Otherwise you'll be single-threaded by the test. Khmmm... Why is not it reading, say 10% of a file from one disk and the next 10% from the other, in parallel? No buffers, I guess... Oh, well... => Features/stability: => => I tried to create the third ccd to concatenate the two => mirroring disks into one 6Gb big chunk. It "did not work" => most of the time, and crashed the system once when it seemed => to succeed and I started to => newfs /dev/rccd2c => Is this combination supposed to work at all? =I'm not sure what you're trying to do here. What does your =/etc/ccdconfig look like? Are you trying to join ccds together into =second-level ccds? I don't see any reason to want to do this, and I'm =pretty sure nobody expects it to work. In any case, when you have such =problems, there's little anybody can do without a kernel dump. Yes, it is like this: ccd0 2047 0x05 /dev/sd3s1e /dev/sd5s1e ccd1 6000 0x05 /dev/sd4s1e /dev/sd6s1e ccd2 0 none /dev/ccd0c /dev/ccd1c The reason is to have a system which can be quickly brought back up in case of a drive failure (any one of the 4 disks can go, even two different ones can go), while providing one big partition for a big file-system. =You might like to try vinum (http://www.lemis.com/vinum.html) and see =how that compares. Bear in mind, though, that this is still =pre-release software. You shouldn't use it on production systems. This is a production system. The array(s) will replace several disks scattered accross several aging SGIs... Thanks! -mi To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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