[EMAIL PROTECTED] said: > You are confusing unrecoverable disk errors (which are rare but orders of > magnitude more common) with otherwise *undetectable* errors (the occurrence > of which is at most once in petabytes by the studies I've seen, rather than > once in terabytes), despite my attempt to delineate the difference clearly.
I could use a little clarification on how these unrecoverable disk errors behave -- or maybe a lot, depending on one's point of view. So, when one of these "once in around ten (or 100) terabytes read" events occurs, my understanding is that a read error is returned by the drive, and the corresponding data is lost as far as the drive is concerned. Maybe just a bit is gone, maybe a byte, maybe a disk sector, it probably depends on the disk, OS, driver, and/or the rest of the I/O hardware chain. Am I doing OK so far? > Conventional approaches using scrubbing provide as complete protection > against unrecoverable disk errors as ZFS does: it's only the far rarer > otherwise *undetectable* errors that ZFS catches and they don't. I found it helpful to my own understanding to try restating the above in my own words. Maybe others will as well. If my assumptions are correct about how these unrecoverable disk errors are manifested, then a "dumb" scrubber will find such errors by simply trying to read everything on disk -- no additional checksum is required. Without some form of parity or replication, the data is lost, but at least somebody will know about it. Now it seems to me that without parity/replication, there's not much point in doing the scrubbing, because you could just wait for the error to be detected when someone tries to read the data for real. It's only if you can repair such an error (before the data is needed) that such scrubbing is useful. For those well-versed in this stuff, apologies for stating the obvious. Regards, Marion _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
