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SubjectRe: Volume Managers in Linux
On Tue, 3 Nov 1998, James Fidell wrote:

> > md0 : active raid1 sdc6[1] sdc7[0] [2/2] [UU] resync=89% finish=0.0min
> > md1 : active raid1 sdc9[1] sdc8[0] [2/2] [UU] resync=30% finish=0.9min
> Does this continue to work correctly if (say) sdc6 and sdc9 both fail ? I'd
> like to think that it does, but that's not the case in VxVM v2.x (for
> example).
I'm not sure I follow. md0 and md1 are entirely separate devices, so a
failure of one device each should work just fine. I know for a fact that
VxVM 2.x does indeed deal with such a situation without batting an eye. I
worked on Sequent's port of VxVM 2.2 for 2.5 years and know in detail how
it operates (but I carefully kept my nose 100% out of the code).

I should mention, however, that LVM and VxVM share nothing in conceptual
terms except the idea of a Volume Group (VxVM calls them diskgroups).
VxVM (all versions, actually) use a subdisk/plex/volume abstraction, where
you carve subdisks of arbitrary (not just 4MB) size out of physical disks,
create plexes out of them either by concatenating or striping, then put
together volumes with N plexes, which become mirrors of each other. Most
RAID levels can be constructed from this abstraction, and RAID5 can be
built from N plexes rather than just mirroring them.

This abstraction works well, too. Just before I was layed off ($$$,
wouldn't ya know), I had a machine that would do 330MB/sec disk I/O, a
setup that took me very little time to build and subsequently wasn't tuned
at all. http://www.omegacs.net/~omega/STiNG/ if you're interested in what
kind of hardware that takes.

Erik Walthinsen <omega@cse.ogi.edu> - Staff Programmer @ OGI
Quasar project - http://www.cse.ogi.edu/DISC/projects/quasar/
Video4Linux Two drivers and stuff - http://www.cse.ogi.edu/~omega/v4l2/
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