DragonFly kernel List (threaded) for 2007-10
DragonFly BSD
DragonFly kernel List (threaded) for 2007-10
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Re: HAMMER filesystem update - design document

From: "Thomas E. Spanjaard" <tgen@xxxxxxxxxxxxx>
Date: Wed, 10 Oct 2007 23:45:04 +0000

Matthew Dillon wrote:
But is RAID absolutely necessary? Probably not. Consider a replicated
filesystem with each copy backed by an array of disks. Now say you have a disk failure. The copy of the filesystem containing the disk
failure loses a portion of its B-Tree. It doesn't need to recover
the disk, you would just pull it and slap in a new one and the
filesystem would reload that portion of the B-Tree from one of the
other replicated copies to repair itself.

This is the functional equivalent of a RAID1, and that is all HAMMER provides; the point of RAIDZ (and RAID3,4,5,6,etc) is that you don't need 2n bytes worth of disk for n bytes worth of usable storage, yet keeping some level of resilience. There is something to be said for this kind of scheme, namely not wasting as much disk space, but in the case of RAID1,0,10,01, moving that to a different layer (e.g. Vinum) is good enough.

In a clustering environment, it's not likely that you'll want anything other than full replication, but at least on single-node storage systems, using storage more efficiently has its uses; even though it means longer recovery times.

        Thomas E. Spanjaard

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