[CentOS] C7, mdadm issues

Thu Jan 31 06:34:37 UTC 2019

> 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
>>>>>
>>>>> _______________________________________________
>>>>> CentOS mailing list
>>>>> CentOS at centos.org
>>>>> https://lists.centos.org/mailman/listinfo/centos
>>>>>
>>>>
>>>>
>>>> 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