Hi Neil/folks,
I'm seeking some (hopefully) simple clarifications about the newer raid
checking and scrubbing behavior present in more recent kernels. I must
say that I was more than pleased when I learned about the new
functionality. Kudos, Neil for this addition. Unfortunately, because
this is new it's not to be found in the FAQs or HOW-TOs... with the
exception of the Gentoo "HOWTO Install on Software RAID".
I've looked at the following sources of info:
linux-2.6.16.19/Documentation/md.txt
linux-2.6.16.19/drivers/md:raid5.c and raid6main.c
(the raid5_end_read_request and raid6_end_read_request routines)
emails on the linux-raid mailing list, in particular:
http://lkml.org/lkml/2005/12/4/118
http://www.mail-archive.com/linux-raid@xxxxxxxxxxxxxxx/msg04615.html
===============================================================
In any regard:
I'm talking about triggering the following functionality:
echo check > /sys/block/mdX/md/sync_action
echo repair > /sys/block/mdX/md/sync_action
On a RAID5, and soon a RAID6, I'm looking to set up a cron job, and am
trying to figure out what exactly to schedule. The answers to the
following questions might shed some light on this:
1. GENERALLY SPEAKING, WHAT IS THE DIFFERENCE BETWEEN THE "CHECK" AND
"REPAIR" COMMANDS?
The "md.txt" doc mentions for "check" that "a repair may also happen for
some raid levels."
Which RAID levels, and in what cases? If I perform a "check" is there a
cache of bad blocks that need to be fixed that can quickly be repaired
by executing the "repair" command? Or would it go through the entire
array again? I'm working with new drives, and haven't come across any
bad blocks to test this with.
2. CAN "CHECK" BE RUN ON A DEGRADED ARRAY (say with N out of N+1 disks
on a RAID level 5)? I can test this out, but was it designed to do
this, versus "REPAIR" only working on a full set of active drives?
Perhaps "repair" is assuming that I have N+1 disks so that parity can be
WRITTEN?
3. RE: FEEDBACK/LOGGING: it seems that I might see some messages in
dmesg logging output such as "raid5:read error corrected!", is that
right? I realize that "mismatch_count" can also be used to see if there
was any "action" during a "check" or "repair." I'm assuming this stuff
doesn't make its way into an email.
4. DOES "REPAIR" PERFORM READS TO CHECK THE ARRAY, AND THEN WRITE TO THE
ARRAY *ONLY WHEN NECESSARY* TO PERFORM FIXES FOR CERTAIN BLOCKS? (I
know, it's sorta a repeat of question number 1+2).
5. IS THERE ILL-EFFECT TO STOP EITHER "CHECK" OR "REPAIR" BY ISSUING "IDLE"?
6. IS IT AT ALL POSSIBLE TO CHECK A CERTAIN RANGE OF BLOCKS? And to
keep track of which blocks were checked? The motivation is to start
checking some blocks overnight, and to pick-up where I left off the next
night...
7. ANY OTHER CONSIDERATIONS WHEN "SCRUBBING" THE RAID?
Sorry for some of these questions being so similar in nature. I just
want to make sure I understand it correctly.
Neil, again, a BIG thanks for this new functionality. I'm looking
forward to putting a system in place to exercise my drives!
Cheers,
-- roy
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