On 13-Dec-07, at 6:28 PM, Frank Cusack wrote: > On December 13, 2007 11:34:54 AM -0800 "can you guess?" > <[EMAIL PROTECTED]> wrote: >> By contrast, if extremely rare undetected and (other than via ZFS >> checksums) undetectable (or considerably more common undetected but >> detectable via disk ECC codes, *if* the data is accessed) corruption >> occurs, if the RAID card is used to mirror the data there's a good >> chance >> that even ZFS's validation scans won't see the problem (because >> the card >> happens to access the good copy for the scan rather than the bad >> one) - >> in which case you'll lose that data if the disk with the good data >> fails.
Which is exactly why ZFS should do the mirroring... >> And in the case of (extremely rare) otherwise-undetectable >> corruption, if >> the card *does* return the bad copy then IIRC ZFS (not knowing that a >> good copy also exists) will just claim that the data is gone >> (though I >> don't know if it will then flag it such that you'll never have an >> opportunity to find the good copy). Ditto. > > i like this answer, except for what you are implying by "extremely > rare". > >> If the RAID card scrubs its disks A scrub without checksum puts a huge burden on disk firmware and error reporting paths :-) --Toby >> the difference (now limited to the >> extremely rare undetectable-via-disk-ECC corruption) becomes pretty >> negligible - but I'm not sure how many RAIDs below the near- >> enterprise >> category perform such scrubs. >> >> In other words, if you *don't* otherwise scrub your disks then ZFS's >> checksums-plus-internal-scrubbing mechanisms assume greater >> importance: >> it's only the contention that other solutions that *do* offer >> scrubbing >> can't compete with ZFS in effectively protecting your data that's >> somewhat over the top. > > the problem with your discounting of zfs checksums is that you aren't > taking into account that "extremely rare" is relative to the number of > transactions, which are "extremely high". ... > > considering all the pieces, i would much rather run zfs on a jbod than > on a raid, wherever i could. it gives better data protection, and it > is ostensibly cheaper. > > -frank > _______________________________________________ > zfs-discuss mailing list > zfs-discuss@opensolaris.org > http://mail.opensolaris.org/mailman/listinfo/zfs-discuss _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
