Date: Tue, 13 Oct 2009 14:02:13 +0100 From: Matthew Seaman <m.seaman@infracaninophile.co.uk> To: Nick Barnes <nb@ravenbrook.com> Cc: freebsd-questions@freebsd.org, Ravenbrook SysAdmins <sysadmins@ravenbrook.com> Subject: Re: how does gmirror start up? Message-ID: <4AD47A55.2060707@infracaninophile.co.uk> In-Reply-To: <59889.1255436565@thrush.ravenbrook.com> References: <59889.1255436565@thrush.ravenbrook.com>
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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.
>=20
> 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.
>=20
> 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?
>=20
> 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
> disk).
>=20
> 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 detai=
ls.
On startup, if the appropriate geom modules are loaded into the kernel,
geom will "taste" the disks discovered by enumerating the contents of all=
=20
ATA, SCSI, USB, etc. busses (ie. read any geom metadata) to see if they a=
re
part of a RAID array of some type. The RAID will be automatically recrea=
ted,
and appears to the system as a new device. If there are appropriate entr=
ies 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=
status'
somewhere.
This means that you can move the disks from one server to another and all=
the
RAIDs will survive. Actually, it's good enough that it can cope with you=
shuffling the disks or installing them with completely different underlyi=
ng
device names. You would certainly have to avoid a clash of geom device n=
ames
when doing that -- the names are entirely arbitrary and 'gm0' is only the=
de-facto=20
default because it was used in one of the most popular tutorials on the s=
ubject.=20
If you think it necessary, you could include the host name in the geom la=
bel 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 a
handy way of avoiding the foot-shooting potential of a USB thumb drive th=
at maps
as da0 on reboot, in place of the real system disk.
Cheers,
Matthew
--=20
Dr Matthew J Seaman MA, D.Phil. 7 Priory Courtyard
Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate
Kent, CT11 9PW
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