Re: Replacing a failed disk "in advance"

[Ladislav Mate <ladislav.mate@gmail.com>]

On Wed, May 20, 2015 at 04:20:49PM +0200, Jan Kasprzak wrote:
> 	Hello,
Hi Jan,
> 
> I have a RAID-5 volume of 8 physical disks. One of these disks failed the SMART
> self-test with an unreadable block error. Unfortunately I have discovered
> that there is _another_ bad block on another disk. It is a different block,
> so the RAID-5 volume as a whole is still working. But as a whole, the
> RAID-5 volume has at least two unreadable sectors on two different disks.
> 
> What is the best way to replace these two failing disks one by one without
> the loss of data? I cannot mdadm --fail one of them, because the
> subsequent rebuild on a new disk would fail on reading the other bad block.
> 
> I would like to add the ninth drive to the RAID-5 volume, and put a replica
> of one of the failing drives to it. Then remove the just-replicated drive,
> and do the same with the other failing drive.
When you take a look in mdadm man page and search for --replace you'll find what you are looking for.

--replace
       Mark listed devices as requiring replacement.  As soon as a spare is available, it will be rebuilt  and  will  replace
       the marked device.  This is similar to marking a device as faulty, but the device remains in service during the recov-
       ery process to increase resilience against multiple failures.  When the replacement  process  finishes,  the  replaced
       device will be marked as faulty.

--with This  can  follow a list of --replace devices.  The devices listed after --with will be preferentially used to replace
       the devices listed after --replace.  These device must already be spare devices in the array.

HTH,

/lm
> 
> Thanks,
> 
> -Yenya
> 
> -- 
> | Jan "Yenya" Kasprzak   <kas at {fi.muni.cz - work | yenya.net - private}> |
> | New GPG 4096R/A45477D5 -- see http://www.fi.muni.cz/~kas/pgp-rollover.txt |
> | http://www.fi.muni.cz/~kas/     Journal: http://www.fi.muni.cz/~kas/blog/ |
>            Smart data structures and dumb code works a lot better
>            than the other way around.           --Eric S. Raymond
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