Bill Davidsen <davidsen@xxxxxxx> writes:
> Goswin von Brederlow wrote:
>> Hi,
>>
>> I would welcome if someone could work on a new feature for raid5/6
>> that would allow replacing a disk in a raid5/6 with a new one without
>> having to degrade the array.
>>
>> Consider the following situation:
>>
>> raid5 md0 : sda sdb sdc
>>
>> Now sda gives a "SMART - failure iminent" warning and you want to
>> repalce it with sdd.
>>
>> % mdadm --fail /dev/md0 /dev/sda
>> % mdadm --remove /dev/md0 /dev/sda
>> % mdadm --add /dev/md0 /dev/sdd
>>
>> Further consider that drive sdb will give an I/O error during resync
>> of the array or fail completly. The array is in degraded mode so you
>> experience data loss.
>>
>>
> That's a two drive failure, so you will lose data.
>> But that is completly avoidable and some hardware raids support disk
>> migration too. Loosly speaking the kernel should do the following:
>>
>>
> No, it's not "completly avoidable" because have described sda is ready
> to fail and sdb as "will give an I/O error" so if both happen at once
> you will lose data because you have no valid copy. That said, some of
But sda has not failed _yet_. I just suspect it will. As long as it
doesn't actualy fail it can compensate for sdb failing. The problem is
that you had to remove sda to replace it despite it still working.
> what you describe below is possible to *reduce* the probability of
> failure. But if sdb is going to have i/o errors, you really need to
> replace two drive :-(
> See below for some thoughts.
>> raid5 md0 : sda sdb sdc
>> -> create internal raid1 or dm-mirror
>> raid1 mdT : sda
>> raid5 md0 : mdT sdb sdc
>> -> hot add sdd to mdT
>> raid1 mdT : sda sdd
>> raid5 md0 : mdT sdb sdc
>> -> resync and then drop sda
>> raid1 mdT : sdd
>> raid5 md0 : mdT sdb sdc
>> -> remove internal mirror
>> raid5 md0 : sdd sdb sdc
>>
>>
>> Thoughts?
>>
>
> If there were a "migrate" option, it might work something like this:
> Given a migrate from sda to sdd, as you noted and raid1 between sda
> and sdd needs to be created, and obviously all chunks of sdd need to
> be marked as needing rebuild, but in addition sda needs to be made
> read-only, to minimize the i/o and to prevent any errors which might
> come from a failed write, like failed sector relocates, etc. Also, if
> valid data for a chunk is on sdd, no read would be done to sda. I
> think there's relevant code in the "write-mostly" bits to implement
> keep i/o to sda to a minimum, no writes and only mandatory reads when
> no valid chunk is on sdd yet. This is similar to recovery to a spare,
> save that most data will be valid on the failing drive and doesn't
> need to be recreated, only unreadable data must be done the slow way.
It would be nice to reduce the load on sda as much as possible if it
is suspect of failing soon. But that is rather an optimization for the
case I described. To keep things simple lets assume sda will be just
fine. So we just setup a raid1 over sda/sdd and do a rebuild. All
reads can go to sda, all writes go to both disks. If sda gives an
error then the raid1 can fail completly, if sdd gives an error kick it
from the raid1. Just like with a normal raid1.
Consider the case of wanting to regulary migrate data from one disk to
the spare disk so all disks age the same. Say every 3 month you
migrate a disk to make a different disk the hot-spare. You wouldn't
want extra considerations for the "to be spare" disk. It is not
suspected to fail soon.
> Care is needed for sda as well, so that if sdd fails during migrate, a
> last chance attempt to bring sda back to useful content can be made,
> I'm paranoid that way.
>
> Assuming the migrate works correctly, sda is removed from the array,
> and the superblock should be marked to reflect that. Now sdd is a part
> of the array, and assemble, at least using UUID, should work.
>
> I personally think that a migrate capability would be vastly useful,
> both for handling failing drives and just moving data to a better
For actually failing drives (say it developes bad blocks but is mostly
still intact) it would be usefull if special care is taken. A read
error on sda should recompute the parity for the block and write it to
sdd. But I would be fine with kicking out sda if it actually fails. We
don't have a raid mode in the kernel to cope with a raid1 where one
mirror is flaky. It would need some new coding in the bitmap for that.
> place. As you point out, the user commands are not *quite* as robust
> as an internal implementation could be, and are complex enough to
> invite user error. I certainly always write down steps before doing
> migrate, and if possible do it with the system booted from a rescue
> media.
The problem with the userspace commands is that you can't do that
live. You have to stop the raid to set up the mirroring. Unless you
always run your raid with device mapper mapped drives just in case you
want to migrate in the future.
I want this in the kernel so that you can take just any running
raid5/6 and migrate. No downtime, no device mapper preparations
beforehand.
MfG
Goswin
PS: Some customers swap out drives in a raid when their waranty
expires even if they work perfectly. That is another use case.
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