Date: Fri, 23 Apr 2010 08:23:32 -0600 From: Scott Long <scottl@samsco.org> To: John Baldwin <jhb@freebsd.org> Cc: Alexander Motin <mav@freebsd.org>, freebsd-current@freebsd.org, freebsd-geom@freebsd.org Subject: Re: Switchover to CAM ATA? Message-ID: <C801B33A-C677-4E6A-BCDA-AC9957302601@samsco.org> In-Reply-To: <201004230950.33999.jhb@freebsd.org> References: <4BD06BD9.6030401@FreeBSD.org> <201004230950.33999.jhb@freebsd.org>
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On Apr 23, 2010, at 7:50 AM, John Baldwin wrote: > On Thursday 22 April 2010 11:31:37 am Alexander Motin wrote: >> If ataraid(4) should be reimplemented in GEOM, then how exactly? One >> more separate RAID infrastructure in GEOM (third?) looks excessive. >> Reuse gmirror, gstripe,... code would be nice, but will make them = more >> complicated and could be not easy for RAID0+1 (due to common = metadata) >> and RAID5 (due to lack of module in a base system). >=20 > Scott's view (which sounds good to me) is that GEOM should include a = library=20 > of routines for working with common transforms such as RAID1, = striping, etc. =20 > Each ATA RAID vendor format would then consist of a small GEOM module = that=20 > used the library routines to manage all the I/O and the bulk of the = module=20 > would be managing a specific metadata format. >=20 THIS It's hard for me to talk about RAID and FreeBSD without getting into a = long sermon, so I'll try to keep this short. RAID is about data = integrity, not about mirror/stripe/parity algorithms. Those algorithms = are just a small part of RAID, and are merely tools for achieving the = goals of RAID. But RAID !=3D algorithms. It's like how we use linked = lists extensively within the kernel, but the kernel !=3D linked lists. A well-designed software raid stack is going to be an engine that = manages topology, executes and rolls back I/O transactions, and handles = error recovery. On-disk metadata is again just an algorithm that is = part of this whole picture, and should be modularized as such along with = the transforms. And to be even more brief, the existing GEOM RAID modules are designed = in completely the wrong direction from this. Caveat Emptor. Scott
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