We have about 50 CentOS servers with software RAID level 1 (mirroring).
Each week, we swap out one of the drives (the one in the second of four
hot-swap bays, only the first two of which contain drives) on each server
and take them offsite for safekeeping.

The problem is, the kernel seemingly randomly switches between /dev/sdb
and /dev/sdc for these devices. This makes the process slower by
requiring more manual input where a script(s) could otherwise suffice.

It also confuses smartd, which AFAIK, needs the correct device names to
report accurately.

Ideally, we'd like to force the OS at some level to always see these
devices as /dev/sda and /dev/sdb. If not, is there at least some way to
configure smartd to be "smart" and recognize which devices are in use?

TIA,
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At Wed, 16 Feb 2011 12:00:27 -0500 (EST) CentOS mailing list <centos@centos.org> wrote:

>
> We have about 50 CentOS servers with software RAID level 1 (mirroring).
> Each week, we swap out one of the drives (the one in the second of four
> hot-swap bays, only the first two of which contain drives) on each server
> and take them offsite for safekeeping.
>
> The problem is, the kernel seemingly randomly switches between /dev/sdb
> and /dev/sdc for these devices. This makes the process slower by
> requiring more manual input where a script(s) could otherwise suffice.

I'm assuming these are actually SATA disks with a controller that
supports hot-swap.

What I think is happening is that the kernel retains some 'memory' of
the pulled drive (say /dev/sdb) and when the fresh drive is installed, a
new dev file is created (/dev/sdc). Eventually, /dev/sdb is forgotten
by the time the next 'swap' and /dev/sdb is assigned to the next fresh
disk.

Question: are you always swapping in a *new* disk each week or
re-inserting the disk from the previous week?

>
> It also confuses smartd, which AFAIK, needs the correct device names to
> report accurately.
>
> Ideally, we'd like to force the OS at some level to always see these
> devices as /dev/sda and /dev/sdb. If not, is there at least some way to
> configure smartd to be "smart" and recognize which devices are in use?

The cure might be that you need to do a reboot to properly rescan the
disks.

>
> TIA,
> _______________________________________________
> CentOS mailing list
> CentOS@centos.org
> http://lists.centos.org/mailman/listinfo/centos
>
>

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> At Wed, 16 Feb 2011 12:00:27 -0500 (EST) CentOS mailing list
> <centos@centos.org> wrote:
>
>>
>> We have about 50 CentOS servers with software RAID level 1 (mirroring).
>> Each week, we swap out one of the drives (the one in the second of four
>> hot-swap bays, only the first two of which contain drives) on each
>> server
>> and take them offsite for safekeeping.
>>
>> The problem is, the kernel seemingly randomly switches between /dev/sdb
>> and /dev/sdc for these devices. This makes the process slower by
>> requiring more manual input where a script(s) could otherwise suffice.
>
> I'm assuming these are actually SATA disks with a controller that
> supports hot-swap.

Correct.

> What I think is happening is that the kernel retains some 'memory' of
> the pulled drive (say /dev/sdb) and when the fresh drive is installed, a
> new dev file is created (/dev/sdc). Eventually, /dev/sdb is forgotten
> by the time the next 'swap' and /dev/sdb is assigned to the next fresh
> disk.

Interesting...one would think that this behavior would be consistent
across all servers then, but it isn't. Most accept the same dev,
/dev/sdb, but some assign /dev/sdc. Is there a way to just disable
/dev/sdc and force the kernel to use /dev/sdb every time?

> Question: are you always swapping in a *new* disk each week or
> re-inserting the disk from the previous week?

It's a rotation, so re-inserting from the previous week.

>>
>> It also confuses smartd, which AFAIK, needs the correct device names to
>> report accurately.
>>
>> Ideally, we'd like to force the OS at some level to always see these
>> devices as /dev/sda and /dev/sdb. If not, is there at least some way to
>> configure smartd to be "smart" and recognize which devices are in use?
>
> The cure might be that you need to do a reboot to properly rescan the
> disks.

Ugh. Thanks for your reponse.

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