Date: Wed, 04 Oct 2006 17:35:57 +0100 From: Pete French <petefrench@ticketswitch.com> To: fullermd@over-yonder.net Cc: jasen.gibson@ge.com, freebsd-amd64@freebsd.org Subject: Re: suggestions for SATA RAID cards Message-ID: <E1GV9iv-0007hZ-T9@dilbert.firstcallgroup.co.uk> In-Reply-To: <20061004152125.GK75501@over-yonder.net>
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> I would guess that's nothing to do with blocks or striping per se. A > single big file won't be laid out 'sequentially' on the disk; the > filesystem tries to avoid filling up cylinder blocks. It puts enough > in given places that the seek costs are pretty low. So, after reading > one bit a while, it'll seek somewhere and start another read. The > controller could issue that read to another component of the mirror; yes, the filesystenm layout may well make it leap around the drives. But what I am wanting to ensure is that if I issue a 128k block read to the drives then it will satisfy this by reading two lots of 64k, one from each mirror, in parallel. If this is true, and if I am sequentially reading blocks from a file, then even if the files moves about on the drives, the requests for blocks should always come from both drives at once. It isn't doung this, and I found a couple of interesting things - one being the 'stripe size' parameter inside the RAID controller which is set to 64k, and the oter being a comment somewhere that FreeBSD never issues more than 64k read requests at once. So I was speculating that what is happening is that I am reading 64k chunks, and is this is less than the stipre size then controller is almost always satisfying these from a single drive. Of course I am making some large assumtions here about the internal workings of the RAID controller, which may well be wrong - but then thats why I asked the question. > I'd expect a sufficiently smart RAID controller to even out across the > disks, even when there's only one process reading (not that the > controller knows anything about 'processes', but...) Yes, so would I - and it isn't doing so. -pete.
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