[mythtv-users] WAY OT: Storage designs
linux at thehobsons.co.uk
Mon Aug 30 15:15:50 UTC 2010
Greg Oliver wrote:
> > If you mean what I think you mean then - no you can't. iSCSI is a block
>> protocol and you can't mount it more than once at a time unless you use a
>> clustered filesystem - if you try then you'll get instant corruption.
>You can with SCST :) That's the only reason I would consider it..
>It allows multiple accesses to the same LUNs simultaneously as well.
>I had never heard of it before, but it popped up on lkml, so I started
>reading up on it.. And of course immediately wanted to try it..
I suggest you read up some more then, because I don't think you've
understood some of the details. Yes, it is **standard iSCSI
functionality** that more than one initiator can connect to a single
target, but ...
You need to do one of three things :
1) Ensure that you never, ever (and I mean ever) mount the same
filesystem on more than one system at a time - ie you always unmount
it cleanly on one system before mounting it on another.
2) Use a clustered filesystem, or clustered LVM.
3) Mount it read-only on every machine - and no I don't mean you can
mount it r/w on one system and r/o on the others, that won't work
If you do try and mount a volume r/w in more than one place, with a
standard filesystem, then it is guaranteed to corrupt the file system
- big time. Each OS where the volume is mounted will assume that it's
the only one running the filesystem. Each OS will cache parts of the
filesystem data it's read from disk, and it will cache changes made
to the filesystem. The result is that each OS will have an
inconsistent view of what is on the disk, and multiple uncordinated
changes will be made. Eg, machine A makes a change, but it isn't all
written immediately to disk. Machine B makes changes independently of
machine A. Depending on timing, what ends up on disk is some random
mish-mash of changes from both machines. Even if machine A wrote all
changes immediately (ie no caching), then machine B would not be
aware of them because at least some of the data would be in it's read
Even if A is the only machine to mount the filesystem r/w, B would
still not see a consistent filesystem due to write caching on A and
read caching on B.
There's discussion of iSCSI etc over on the Xen mailing lists. I
don't follow the details as I'm not up to that level yet - there are
some interesting things you can do with Xen guests and iSCSI storage.
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