Date: Sun, 8 Nov 1998 09:49:16 +1030 From: Greg Lehey <grog@lemis.com> To: Bill Vermillion <bill@bilver.magicnet.net>, freebsd-fs@FreeBSD.ORG Subject: Re: RAID1 Software vs Hardware Message-ID: <19981108094916.T499@freebie.lemis.com> In-Reply-To: <199811071331.IAA10243@bilver.magicnet.net>; from Bill Vermillion on Sat, Nov 07, 1998 at 08:31:26AM -0500 References: <19981107200618.O499@freebie.lemis.com> <199811071331.IAA10243@bilver.magicnet.net>
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On Saturday, 7 November 1998 at 8:31:26 -0500, Bill Vermillion wrote: > Greg Lehey recently said: >> On Friday, 6 November 1998 at 19:42:15 -0500, Bill Vermillion wrote: >>> Christopher Nielsen recently said: >>> >>>> On Fri, 6 Nov 1998, Bagnara Stefano wrote: >>> >>>>> Date: Fri, 6 Nov 1998 17:35:58 +0100 From: Bagnara Stefano >>>>> <bago@datasail.it> To: freebsd-fs@FreeBSD.ORG Subject: RAID1 >>>>> Software vs Hardware ^^^^^ > >>>>> I need to implement a simple raid1 with 2 9gigs SCSI HDD. I . >>>>> Ineed a inexpensive solution ... so i was thinking about a . >>>>> Isoftware raid1 s it possible? is it too slow? the system will . >>>>> Ibe only a mail server . > >>>> Your really not going to see very good performance with RAID if >>>> you're using only two spindles (i.e., discs). ... > >>> I've found that I get a 50% throughput increase (typical)when >>> running RAID 0 with 2 drives. > >> That's what theory would tell you. > > No - not theory. Measured in real-life - running HW raid 0 - on a > clients SCO system. We needed more speed. It may be slightly > under 50% - but it's darn close. Theory would tell you 50%. Practice shows it's darn close. If I had said "yes, that's what happens", somebody would have looked from the other perspective and said "not quite". > I have timed the same drive on SCO and FreeBSD - a 9GB 'cudda, and > the raw SCO performance through the file-system is in the 3MB/min > range, while using the FreeBSD file-system - as shipped - no mods, > etc,. it is between 2 and 3 times faster than SCO's. Which operating system? > Controller is DPT with a hunk of cache. All timing was done with > 100MB minimum file sizes usiing iozone. > >>> RAID 1 will give no increase in some areas, and writes are slower, > >> This depends a lot on the configuration. You need to write to every >> copy (plex in vinum jargon), whereas you only need to read from one. >> In practice, if you're running SCSI disks or IDEs with UDMA on >> different controllers, you'll delay the writes to the speed of the >> slowest disk. On average that's slower than a single disk, but not >> much. > > I've only used HW RAID devices so I can't comment on the other > implementation. The RAID Standards Board (I think that's the > correct name) has permitted a pair of striped drives to be > designated RAID 0, while in reality it is not a RAID. > > They current have 6 classes - 0 thru 5 - and there is a chart > in Adaptec's book on I/O subsytems listing the pro's'/con's of > each. RAID2/3/4 aren't used, and from what I've seen drives that > use to have spindle sync for byte/sector striping aren't being made > anymore. But with drives now at 20MB/sec+ speeds, the old needs > are gone. Correct. You'd be surprised how many products offer RAID-2/3/4, though. I think the product manager got a checklist to tick off. I have deliberately left these three out of Vinum. > I can't comment on IDE as I've never used them. Shouldn't make much difference with only one drive per controller and UDMA. Otherwise you'll run into significant contention problems. >>> but it will boost the read throughput if different files are being >>> accessed, just as if you load balanced multiple single disks.. > >> BTW, ccd always reads from the same copy of the data, so this doesn't >> work. But in principle you're right. > > Reading from two disks for different files is one of the touted > features of most HW implementations. Put it this way, I don't know of any other implementation, SW or HW, which is this primitive. Vinum has a choice of round robin (default) or always reading from a specific drive (which can be an advantage if you have a ramdisk, for example). Greg -- See complete headers for address, home page and phone numbers finger grog@lemis.com for PGP public key To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-fs" in the body of the message
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