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. > 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).
i like this answer, except for what you are implying by "extremely rare". > If the RAID card scrubs its disks 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". in such a case even "extremely rare" errors do happen, and not just to "extremely few" folks, but i would say to all enterprises. hell it happens to home users. when the difference between an unrecoverable single bit error is not just 1 bit but the entire file, or corruption of an entire database row (etc), those small and infrequent errors are an "extremely big" deal. 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
