On 6/28/06, Nathan Kroenert <[EMAIL PROTECTED]> wrote:
On Thu, 2006-06-29 at 03:40, Nicolas Williams wrote:
> But Joe makes a good point about RAID-Z and iSCSI.
>
> It'd be nice if RAID HW could assist RAID-Z, and it wouldn't take much
> to do that: parity computation on write, checksum verification on read
> and, if the checksum verification fails, combinarotial reconstruction on
> read. The ZFS system (iSCSI client) would still have to verify the
> checksum on read...
>
> ...but leaving parity computation/reconstruction to the iSCSI server
> would greatly cut down the amount of I/O needed for RAID-Z to something
> similar to that needed for HW RAID-5.
>
> Sure, I don't expect HW-assisted RAID-Z anytime soon, nor iSCSI
> extensions for server-assisted RAID-Z. But at least iSCSI protocol
> extensions could be pursued now.
But - This still fails to address the design concept of ZFS's end to end
checksumming, and fails to address things gong bad over the system's
hardware bus, the IO card and the Fibre...
So - It's not nearly the same level of protection, IMO.
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Its not the same level of protection, but you get location
independence, multi-pathing throughout your device tree (all
components redundant, including the head node potentially), and if
Solaris ever got around to it, ERL2 support would do wonders to ensure
data integrity. Sure, you can still have specific components fail
partially and no know if you corrupt the data, but again, the answer
of mirroring your iscsi based storage allows for the error
correction/checksumming route to work its wonders. For exceptionally
large data pools, you'll need many systems (perhaps beyond the scope
of even FC). Just exposing each drive as a naked lun and doing layers
of raidz/mirrors will show poor performance and an overly centralized
management nightmare. Segmenting off the workload solves some
performance ills (again, referring to Roch's work on raidz's failings
for large number of luns) and does provide its own level of
compartmentabilty, redundancy, and manageability (to gain some, and
you lose some, I agree).
Let ZFS integrate as best it can based on the environment at hand. My
target use involves both tier1 (ala NetApp) and tier2 (very large
multi-location storage pools).
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