2012-01-15 19:16, Edward Ned Harvey пишет:
From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
boun...@opensolaris.org] On Behalf Of Jim Klimov

2) In particular, if I wanted to make a mirror of raidzN's,
can it be done in one ZFS pool, or would I have to play
with iSCSI and ZVOLs?

Although you can probably do something like this (which might give you some
problems in terms of mount order, etc) it's probably not a great idea.  One
part of the system will be optimizing for mirrored disks, another optimizing
for a raidz.  Block patterns may not (probably won't) align, possibly
resulting in some performance issues...  You'll be double-checksumming...

If you want to mirror raidz devices, why don't you just go with a mirror and
no raidz?  The mirror performs better.  The only reason I can think of to do
what you're saying is if you want more redundancy than a 2-way mirror, but
not as much as a 3-way mirror.

Well, the idea of RAID51 in ZFS came up while I was writing
another post, discussing whether raidzN actually protects
data against silent corruption. So far I have no answer to
that question ;)

Main reason to go with raidzN would be to have some redundancy
at expense of smaller storage space (or connection port number).
If raidzN indeed doesn't always protect data, mirroring over
that *could* provide added reliability (mirrors seem to always
know which half exactly is bad) while retaining the benefits
of large storage space given the same amount of disks and ports
(perhaps also limited to an enclosure/HBA/etc.).

Mirror could also help speed up some reads ;)

Also, if we mirror each local disk to a remote disk, in case of
losing one of the storage boxes we end up with a non-redundant
stripe of data. While mirroring local raidz to remote raidz,
we end up with a redundant raidz...

The exact balance in numbers has to be calculated, though -
since 2-way mirroring over some sheer amount of disks would
give more space than mirroring over raidzing over the same
disks ;) So, yes, probably the main rationale for THIS would
be separation of the mirror halves over hardware SPOFs in
some balance with bandwidths and latencies (sending one chunk
of data vs. many smaller chunks of mirrored+striped data).

Perhaps the idea is senseless after all - but that was part of
the question, as well ;)

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