On Mon, Feb 22, 2010 at 3:14 PM, Benjamin Scott <[email protected]>wrote:
> On Mon, Feb 22, 2010 at 1:39 PM, Michael ODonnell > <[email protected]> wrote: > > So far, then, it's looking like every Sunday at 4:22 all the RAIDs > > (all types or just RAID1?) in standard x86_64 CentOS5.4 (and RHAT?) > > boxes are broken and then resync'd. > > FWIW, I don't see this in my logs (back to 1/24/2010, not far) on Fedora 12. > All types (as I interpret the script source). > > If the documentation is to be believed, they are not being broken; > they are being checked for consistency. Not the same thing. Breaking > and rebuilding leaves the array vulnerable during the rebuild, as you > note. A consistency check just compares the supposedly identical > members to confirm they really *are* identically, and warns you if > they are not. > > > With a good RAID implementation, I/O for patrol reads is done when > the array is idle. (Kind of like "nice 19" for I/O.) I don't know if > Linux does this or not. > > The correct terminology is a scrub. I think most RAID systems can do it. It's like a fsck - something that checks the RAID structures and data to find inconsistancies so they can be dealt with. Scrubs can be done live and are a good thing do. They take IO and time. ZFS doesn't have fsck, but it does do scrubs. Hardware RAID can do scrubs as well. >From man zpool on Solaris: Scrubbing and resilvering are very similar operations. The difference is that resilvering only examines data that ZFS knows to be out of date (for example, when attaching a new device to a mirror or replacing an existing device), whereas scrubbing examines all data to discover silent errors due to hardware faults or disk operations, ZFS only allows one at a time. If a scrub is already in progress, the "zpool scrub" command ter- minates it and starts a new scrub. If a resilver is in progress, ZFS does not allow a scrub to be started until the resilver completes. How often to do them gets debated in the forums. Times vary with activity, hardware, size. My consumer grade RAIDZ of 4 500GB SATA II takes an hour. Your RAID system will be different.
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