On 15/11/16 18:14, Peter Sangas wrote:
Hi Wol,
-----Original Message-----
From: Wols Lists [mailto:antlists@xxxxxxxxxxxxxxx]
Sent: Monday, November 14, 2016 7:58 AM
To: Bruce Merry
Cc: linux-raid@xxxxxxxxxxxxxxx
Subject: Re: What to do about Offline_Uncorrectable and Pending_Sector in RAID1
On 14/11/16 15:52, Bruce Merry wrote:
On 13 November 2016 at 23:06, Wols Lists <antlists@xxxxxxxxxxxxxxx> wrote:
Sounds like that drive could need replacing. I'd get a new drive
and do that as soon as possible - use the --replace option of mdadm
- don't fail the old drive and add the new.
Would you mind explaining why I should use --replace instead of taking
out the suspect drive? I guess I lose redundancy for any writes that
occur while the rebuild is happening, but I'd plan to do this with the
filesystem unmounted so there wouldn't be any writes.
Because a replace will copy from the old drive to the new, recovering any failures from the rest of the array. A fail-and-add will have to rebuild the entire new array >from what's left of the old, stressing the old array much more.
Okay, in your case, it probably won't make an awful lot of difference, but it does make you vulnerable to problems on the "good" drive. To alter your wording >slightly, you lose redundancy for writes AND READS that occur while the array is rebuilding. It's just good practice (and I point it out because --replace is new and >not well known at present).
Cheers,
Wol
With respect to the --replace switch and "replacing a failed drive" documented on the wiki here:
https://raid.wiki.kernel.org/index.php/Replacing_a_failed_drive Can you clear a few things up for me ?
1. If I just want to replace a working drive in a RAID1 and the array is still redundant I can
issue the following command as in your example:
mdadm /dev/mdN [--fail /dev/sdx1] --remove /dev/sdx1 --add /dev/sdy1
which fails and removes sdx1 and replaces it with sdy1.
Question1. How is this different from first doing a fail/remove on sdx1, physically replacing sdx1 with sdy1 and doing an add on sdy1?
Not really different at all. It's just that you (obviously) can't do the
remove and add in the same command if you physically swap the drive in
the middle.
But I bang on a bit about having access to spare port to stick a drive
on, so I've assumed you can have both the old and the new drive
physically (and logically) in the system at the same time.
2. If one of the drives as an error in a RAID1 and gets kicked out of the array and the array loses redundancy the wiki has the following example:
mdmad /dev/mdN --re-add /dev/sdX1
mdadm /dev/mdN --add /dev/sdY1 --replace /dev/sdX1 --with /dev/sdY1
Question2. Is this point here to first try and re-add sdX1 with the "--re-add" (first line above) and if that fails do a replace (second line above)?
Correct. You've lost redundancy, and (you NEED a bitmap here) the idea
is to get sdX1 back in to the array to restore redundancy before you
copy its contents to sdY1.
You need the bitmap because, without it, a re-add becomes a normal add,
and it's not only a waste of time, it adds stress to the array and
increases your chances of a total failure.
Thanks,
Peter
Cheers,
Wol
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