Re: C7, mdadm issues

Alessandro Baggi <alessandro.baggi@xxxxxxxxx> · Thu, 31 Jan 2019 09:23:39 +0100

Il 31/01/19 07:34, Simon Matter ha scritto:
Il 30/01/19 16:49, Simon Matter ha scritto:
On 01/30/19 03:45, Alessandro Baggi wrote:
Il 29/01/19 20:42, mark ha scritto:
Alessandro Baggi wrote:
Il 29/01/19 18:47, mark ha scritto:
Alessandro Baggi wrote:
Il 29/01/19 15:03, mark ha scritto:

I've no idea what happened, but the box I was working on last
week
has a *second* bad drive. Actually, I'm starting to wonder about
that particulare hot-swap bay.

Anyway, mdadm --detail shows /dev/sdb1 remove. I've added
/dev/sdi1...
but see both /dev/sdh1 and /dev/sdi1 as spare, and have yet to
find
a reliable way to make either one active.

Actually, I would have expected the linux RAID to replace a
failed
one with a spare....

can you report your raid configuration like raid level and raid
devices
and the current status from /proc/mdstat?

Well, nope. I got to the point of rebooting the system (xfs had the
RAID
volume, and wouldn't let go; I also commented out the RAID volume.

It's RAID 5, /dev/sdb *also* appears to have died. If I do
mdadm --assemble --force -v /dev/md0  /dev/sd[cefgdh]1 mdadm:
looking
for
devices for /dev/md0 mdadm: /dev/sdc1 is identified as a member of
/dev/md0, slot 0.
mdadm: /dev/sdd1 is identified as a member of /dev/md0, slot -1.
mdadm: /dev/sde1 is identified as a member of /dev/md0, slot 2.
mdadm: /dev/sdf1 is identified as a member of /dev/md0, slot 3.
mdadm: /dev/sdg1 is identified as a member of /dev/md0, slot 4.
mdadm: /dev/sdh1 is identified as a member of /dev/md0, slot -1.
mdadm: no uptodate device for slot 1 of /dev/md0
mdadm: added /dev/sde1 to /dev/md0 as 2
mdadm: added /dev/sdf1 to /dev/md0 as 3
mdadm: added /dev/sdg1 to /dev/md0 as 4
mdadm: no uptodate device for slot 5 of /dev/md0
mdadm: added /dev/sdd1 to /dev/md0 as -1
mdadm: added /dev/sdh1 to /dev/md0 as -1
mdadm: added /dev/sdc1 to /dev/md0 as 0
mdadm: /dev/md0 assembled from 4 drives and 2 spares - not enough
to
start the array.

--examine shows me /dev/sdd1 and /dev/sdh1, but that both are
spares.
Hi Mark,
please post the result from

cat /sys/block/md0/md/sync_action

There is none. There is no /dev/md0. mdadm refusees, saying that it's
lost
too many drives.

         mark

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I suppose that your config is 5 drive and 1 spare with 1 drive failed.
It's strange that your spare was not used for resync.
Then you added a new drive but it does not start because it marks the
new disk
as spare and you have a raid5 with 4 devices and 2 spares.

First I hope that you have a backup for all your data and don't run
some
exotic command before backupping your data. If you can't backup your
data,
it's a problem.

This is at work. We have automated nightly backups, and I do offline
backups
of the backups every two weeks.

Have you tried to remove the last added device sdi1 and restart the
raid
and
force to start a resync?

The thing is, it had one? two? spares when /dev/sdb1 started dying, and
it
didn't use them.

For many years now I'm only doing RAID1 now because it's just safer then
RAID5 and easier than RAID6 if the number of disks is low.

Like you, I run always raid1 but in the last year I run a raid5 with 3tb
wd red for my personal backup server but never got an error for the time.

What about RAID10 vs RAID5, RAID6? You loss half size but is performant
as raid5 e reliable as raid1.

I did RAID10 in the past but don't do it now. If you do large linear
read/writes, RAID10 may perform better, if you have lots of independent
and random read/writes, RAID1 may perform better. It really depends a lot
on how the disk are used.

Have you tried other type of raid like RAID50 or RAID60?

Yes I did in the past it even adds more complexity than I like.

About resync process, all type of raid level are disk killer during this
procedure or only raid5 (and similar) is a disk killer?

I don't call it a disk killer, it's more that it detects disks errors but
does not produce them.

I also don't have much experience with spare handling as I also don't do
it in my scenarios.

However in general, I think the problem today is this:
We have very large disks these days. Defects on a disk are often not
found
for a long time. Even with raid-check, I think it doesn't find errors
which only happen while writing, not while reading only.

So now, if one disk fails, things are still okay. Then, when a spare is
in
place or the defective disk was replaced, the resync starts. Now, if
there
is any error on one of the old disks while the resync happens, boom, the
array fails and is in a bad state now.

I once had to recover a broken RAID5 from some linux based NAS and what
I
did was:
* Dump the complete raid partition from every disk to a file, ignoring
the
read errors on one of the disks.
* Build the RAID5 like this:

mdadm --create --assume-clean --level=5 --raid-devices=4
--spare-devices=0 \
    --metadata=1.0 --layout=left-symmetric --chunk=64 --bitmap=none \
    /dev/md10 /dev/loop0 missing /dev/loop2 /dev/loop3

* Recover 99.9% of the data from /dev/md10.

Why not recover directly from  backup? This saves time.
  From your last command why you inserted /dev/loopN?

I that case, the owner of the NAS was a photographer who had all his past
work on the NAS with no real backup :-(

What I did in that case was to dump all data from all disks of the array
to files. Then I made copies of the original dump files to work with them.
I didn't want to touch the disks more than needed.

One more hint for those interested:
Even with RAID1, I don't use the whole disk as one big RAID1. Instead, I
slice it into equally sized parts - not physically :-) - and create
multiple smaller RAID1 arrays on it. If a disk is 8TB, I create 8
paritions of 1TB and then create 8 RAID1 arrays on it. Then I add all 8
arrays to the same VG. Now, if there is a small error in, say, disk 3,
only a 1TB slice of the whole 8TB is degraded. In large arrays you can
even keep some spare slices on a spare disk to temporary move broken
slices. You get the idea, right?

About this type of configuration if you have 2 disks and create 8 raid1
on this two disks, you won't lose performances? As you said if in a

Performance is the same, with maybe 0.1% overhead.

single partition you got some bad error you save other data but if one
disk fails totally you had the same problem more you need to recreate 8

That's true, but in almost three decades of work with harddisks, complete
disk failures were rarely seen.

partition, resync 8 raid1. This could require more time to recovery and
possibly more human error.

That's true about human errors. But in this case, I usually create small
scripts to do it, and I really look at those scripts very carefully before
I run them :-)

Regards,
Simon

Hi Simon,
thank you for your reply.

Best regards,
Alessandro.
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