Date: Tue, 3 Aug 1999 15:59:46 +0930 From: Greg Lehey <grog@lemis.com> To: Bernd Walter <ticso@cicely.de> Cc: Stephen Hocking-Senior Programmer PGS Tensor Perth <shocking@prth.pgs.com>, hackers@FreeBSD.ORG Subject: Re: Adding disks -the pain. Also vinum Message-ID: <19990803155945.W62948@freebie.lemis.com> In-Reply-To: <19990803081216.B23148@cicely8.cicely.de>; from Bernd Walter on Tue, Aug 03, 1999 at 08:12:17AM %2B0200 References: <19990803133554.S62948@freebie.lemis.com> <199908030416.MAA16945@ariadne.tensor.pgs.com> <19990803081216.B23148@cicely8.cicely.de>
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On Tuesday, 3 August 1999 at 8:12:17 +0200, Bernd Walter wrote: > On Tue, Aug 03, 1999 at 12:16:06PM +0800, Stephen Hocking-Senior Programmer PGS Tensor Perth wrote: >> >>> No, it would cause a higher I/O load. Vinum doesn't transfer entire >>> stripes, it transfers what you ask for. With a large stripe size, the >>> chances are higher that you can perform the transfer with only a >>> single I/O. >> >> Even if I'm using really large reads? > Several month ago I beleaved the same but there are severall points here: > - UFS/FFS don't handle clustering over 64k > - modern harddisks do preread simply by having a reversed sector layout. > - without spindle syncronisation you will have additional latency > - vinum don't aggregate access to subdisks, so the transfer to the subdisks > is limited by the stripe size. Note, BTW, that this wouldn't make much sense. To aggregate access to consecutive stripes, your transfer would have to involve *all* the disks in the stripe set, which would be a ridiculous performance hit. Read http://www.lemis.com/vinum/Performance-issues.html for more details. > For UFS/FFS there is nothing worth seting the stripesize to low. > It is generally slower to acces 32k on different HDDs than to acces 64k on > one HDD. It is always slower where the positioning time is greater than the transfer time for 32 kB. On modern disks, 32 kB transfer in about 300 µs. The average rotational latency of a disk running at 10,800 rpm is 2.8 ms, and even with spindle synchronization there's no way to avoid rotational latency under these circumstances. > Spindle Sycronisation won't bring you that much on modern HDDs - I tried > it using 5 Seagate Elite 2.9G (5,25" Full-Height). It should be useful for RAID-3 and streaming video. 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-hackers" in the body of the message
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