mismatch_cnt != 0, member content mismatch, but md says the mirror is good

[Tom Buskey]

On Mon, Feb 22, 2010 at 3:14 PM, Benjamin Scott <dragonhawk at gmail.com>wrote:

> On Mon, Feb 22, 2010 at 1:39 PM, Michael ODonnell
> <michael.odonnell at comcast.net> 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         that ZFS knows to be out  of  date  (for  example,  when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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