HAMMER filesystem update - design document

Thomas E. Spanjaard tgen at netphreax.net
Wed Oct 10 16:49:11 PDT 2007

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 

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
        tgen at netphreax.net
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