Date: Mon, 17 Mar 2003 21:35:17 -0700 From: "Kenneth D. Merry" <ken@kdm.org> To: Jeff Roberson <jroberson@chesapeake.net> Cc: Terry Lambert <tlambert2@mindspring.com>, Brooks Davis <brooks@one-eyed-alien.net>, Julian Elischer <julian@elischer.org>, Alfred Perlstein <bright@mu.org>, FreeBSD current users <current@FreeBSD.ORG>, fs@FreeBSD.ORG Subject: Re: Anyone working on fsck? Message-ID: <20030317213517.A75211@panzer.kdm.org> In-Reply-To: <20030317230147.I66343-100000@mail.chesapeake.net>; from jroberson@chesapeake.net on Mon, Mar 17, 2003 at 11:02:38PM -0500 References: <3E769970.3A05ECAF@mindspring.com> <20030317230147.I66343-100000@mail.chesapeake.net>
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On Mon, Mar 17, 2003 at 23:02:38 -0500, Jeff Roberson wrote: > On Mon, 17 Mar 2003, Terry Lambert wrote: > > Jeff Roberson wrote: > > > On Mon, 17 Mar 2003, Brooks Davis wrote: > > > > I am still intrested in improvements to fsck since I'm planning to buy > > > > several systems with two 1.4TB IDE RAID5 arrays in them soon. > > > > > > For these types of systems doing a block caching layer with a prefetch > > > that understands how many spindles there are would be a huge benefit. > > > > I call that layer "Vinum" or "RAIDFrame", since that's a job I > > expect that code to do for me. 8-). > > They are not responsible for data caching. Only informing the upper > layers how many spindles they have. Software RAID should be a transform > only in my opinion. There is no reason to have duplicate block caches in > system memory. There are times when a software RAID layer should do some caching. Hopefully your software RAID layer will be integrated enough into the system that it can avoid doing any copies, though. The place where you really want some sort of caching is when you try to coalesce buffers to get full stripe writes with RAID-5. Otherwise, you have to do read-modify-write, which is more expensive. There are other cases where caching is needed for RAID-1 and RAID-5, but they generally require specialized hardware. (Hint: why do many controllers that support RAID-1 and RAID-5 have battery backed caches?) Other than stripe coalescing, and those cases that come up when you have specialized hardware, you're right, RAID should be a transform that doesn't require copying. (Caching is another story.) Ken -- Kenneth Merry ken@kdm.org To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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