[gpfsug-discuss] GPFS and replication.. not a mirror?

[Zachary Giles]

Fellow GPFS Users,

I have a silly question about file replicas... I've been playing around
with copies=2 (or 3) and hoping that this would protect against data
corruption on poor-quality RAID controllers.. but it seems that if I
purposefully corrupt blocks on a LUN used by GPFS, the "replica" doesn't
take over, rather GPFS just returns corrupt data.  This includes if just
"dd" into the disk, or if I break the RAID controller somehow by yanking
whole chassis and the controller responds poorly for a few seconds.

Originally my thinking was that replicas were for mirroring and GPFS would
somehow return whichever is the "good" copy of your data, but now I'm
thinking it's just intended for better file placement.. such as having a
near replica and a far replica so you dont have to cross buildings for
access, etc. That, and / or,  disk outages where the outage is not
corruption, just simply outage either by failure or for disk-moves, SAN
rewiring, etc. In those cases you wouldn't have to "move" all the data
since you already have a second copy. I can see how that would makes sense..

Somehow I guess I always knew this.. but it seems many people say they will
just turn on copies=2 and be "safe".. but it's not the case..

Which way is the intended?
Has anyone else had experience with this realization?

Thanks,
-Zach


-- 
Zach Giles
zgiles at gmail.com
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