2011-06-10 20:58, Marty Scholes пишет:
If it is true that unlike ZFS itself, the replication
stream format has
no redundancy (even of ECC/CRC sort), how can it be
used for
long-term retention "on tape"?
It can't. I don't think it has been documented anywhere, but I believe that it
has been well understood that if you don't trust your storage (tape, disk,
floppies, punched cards, whatever), then you shouldn't trust your incremental
streams on that storage.
Well, the whole point of this redundancy in ZFS is about not trusting
any storage (maybe including RAM at some time - but so far it is
requested to be ECC RAM) ;)
Hell, we don't ultimately trust any storage...
Oops, I forgot what I wanted to say next ;)
It's as if the ZFS design assumed that all incremental streams would be either
perfect or retryable.
Yup. Seems like another ivory-tower assumption ;)
This is a huge problem for tape retention, not so much for disk retention.
Because why? You can make mirrors or raidz of disks?
On a personal level I have handled this with a separate pool of fewer, larger
and slower drives which serves solely as backup, taking incremental streams
from the main pool every 20 minutes or so.
Unfortunately that approach breaks the legacy backup strategy of pretty much
every company.
I'm afraid it also breaks backups of petabyte-sized arrays where
it is impractical to double or triple the number of racks with spinning
drives, but is practical to have a closet full of tapes for the automated
robot-feeded ;)
I think the message is that unless you can ensure the integrity of the stream,
either backups should go to another pool or zfs send/receive should not be a
critical part of the backup strategy.
Or that zfs streams can be improved to VALIDLY become part
of such strategy.
Regarding the checksums in ZFS, as of now I guess we
can send the ZFS streams to a file, compress this file
with ZIP, RAR or some other format with CRC and some
added "recoverability" (i.e. WinRAR claims to be able
to repair about 1% of erroneous file data with standard
settings) and send these ZIP/RAR archives to the tape.
Obviously, a standard integrated solution within ZFS
would be better and more portable.
See FEC suggestion from another poster ;)
//Jim
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