Eric Schrock wrote:
1. Some sort of background process to proactively find errors on disks in use by ZFS. This will be accomplished by a background scrubbing option, dependent on the block-rewriting work Matt and Mark are working on. This will allow something like "zpool set scrub=2weeks", which will tell ZFS to "scrub my data at an interval such that all data is touched over a 2 week period".
Obviously, scrubbing and correcting "hard" errors that result in ZFS checksum errors is very beneficial. However, it won't address the case of "soft" errors when the disk returns correct data but observes some problems reading it. There are at least two good reasons to pay attention to these "soft" errors: a) Preemptive detection and rewriting of partially defective but still correctable sectors may prevent future data loss. Thus, it improves perceived reliability of disk drives, which is especially important in the JBOD case (including a single-drive JBOD). b) It is not uncommon for such successful reads of partially defective media to happen only after several retries. It is somewhat unfortunate that there is no simple way to tell the drive how many times to retry. Firmware in ATA/SATA drives, used predominantly in single-disk PCs, will typically do a heroic effort to retrieve the data. It will make numerous attempts to reposition the actuator, recalibrate the head current, etc. It can take up to 20-40 seconds! Such strategy is reasonable for a desktop PC but in it happens in an busy enterprise file server it results in a temporary availability loss (the drive freezes for up 20-40 seconds every time you try to read this sector). Also, this strategy does not make any sense if a RAID group in which the drive participates has redundant data elsewhere, which is why SCSI/FC drives give up after a few retries. One can detect (and repair) such problematic areas on disk by monitoring the SMART counters during scrubbing, and/or by monitoring physical read timings (looking for abnormally slow ones). -- Olaf _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
