how does gmirror start up?
kraduk at googlemail.com
Tue Oct 13 13:07:25 UTC 2009
2009/10/13 Matthew Seaman <m.seaman at infracaninophile.co.uk>
> Nick Barnes wrote:
>> I am running a 6.3 system and have had various problems with disk
>> reliability on a key filesystem, probably down to SCSI hardware or
>> drivers. I'm intending to replace that SCSI disk with a pair of SATA
>> disks ad6/7, using gmirror as gm0 (while keeping ad4 as our boot
>> disk). I have set up the mirror and right now I'm part-way through
>> using rsync to transfer the data. But I have a question concerning
>> the underlying operation of GEOM, which is troubling me. I have read
>> the various man pages and handbook pages relating to GEOM and gmirror,
>> but they don't seem to answer this.
>> When I reboot the system, gmirror comes up (because of the line in
>> /boot/loader.conf) and gm0 appears, backed by ad6/7. Where is this
>> configuration information stored? That is, how does the system know
>> to make gm0, with ad6/7 as the backing store.
>> I would expect there to be a file somewhere in /etc with this config
>> information, but I don't see it in the documentation. From reading
>> gmirror(8), I understand that there is a label sector at the ends of
>> ad6 and ad7, identifying them as parts of gm0. But that config
>> information is back-to-front: at boot time the system knows from
>> /etc/fstab that it needs gm0; how does it find the underlying disks?
>> Does the system search the ends of all physically-attached disks,
>> looking for GEOM labels, and automatically make any corresponding GEOM
>> devices? Surely not (that would mean, for instance, that if I took
>> one of these disks out of this machine and put it into another FreeBSD
>> system then that machine would automatically set up gm0 with this
>> Possibly I'm just being dense. Can someone enlighten me?
> The geom configuration is stored on the disks in question -- I believe it
> uses the last cylinder of the drive, but I could be wrong about the
> On startup, if the appropriate geom modules are loaded into the kernel,
> geom will "taste" the disks discovered by enumerating the contents of all
> ATA, SCSI, USB, etc. busses (ie. read any geom metadata) to see if they are
> part of a RAID array of some type. The RAID will be automatically
> and appears to the system as a new device. If there are appropriate
> entries in
> /etc/fstab any file systems on it will be mounted.
> There's no data stored on the filesystem describing the RAID setup. Well,
> unless you make a backup of it yourself by dumping the output of 'gmirror
> This means that you can move the disks from one server to another and all
> RAIDs will survive. Actually, it's good enough that it can cope with you
> shuffling the disks or installing them with completely different underlying
> device names. You would certainly have to avoid a clash of geom device
> when doing that -- the names are entirely arbitrary and 'gm0' is only the
> de-facto default because it was used in one of the most popular tutorials on
> the subject. If you think it necessary, you could include the host name in
> the geom label or otherwise take action to make them globally unique across
> all your systems.
> You can achieve a similar effect for plain disks by using glabel -- this is
> handy way of avoiding the foot-shooting potential of a USB thumb drive that
> as da0 on reboot, in place of the real system disk.
> Dr Matthew J Seaman MA, D.Phil. 7 Priory Courtyard
> Flat 3
> PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate
> Kent, CT11 9PW
Also worth mentioning zfs config is stored on disk as well in a similar
manner albeit a bit more complicated.
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