[linux-lvm] Re: LVM writes on raw disk of ATA RAID Mirror

Andrew Rechenberg arechenberg at shermfin.com
Wed Apr 30 12:35:14 UTC 2003

vgscan has similar problems with MD devices in a software RAID10 (but
not RAID1) configuration.

In software RAID10 one creates a number of software RAID1 devices and
then stripes across those with RAID0.  For example if you have a 4 disk
RAID10 setup you would have /dev/md0 and /dev/md1 as RAID1 devices and
then /dev/md2 would be a stripe across /dev/md[0,1]

The problem occurs when vgcreate adds the LVM metadata to the device. 
When you run 'vgcreate somename0 /dev/md2' it adds the metadata to
/dev/md2, but since /dev/md0 is the first device in the RAID10 array,
the metadata is in the exact same location on /dev/md0 and /dev/md2. 
Upon reboot (or a subsequent vgscan), LVM complains that it can't find
all of the PE's on the volume group created because it's reading the
information from md0 instead of md2

I was trying to hack vgscan to get my setup working but I needed the box
working ASAP so time dictated that I use the KludgeMethod(tm).  Someone
on the list recommended that I move /dev/md0 before I run my vgscan. 
That worked in my case and I hacked my sysint script to move md0, run
vgscan, and then move it back.  Not elegant AT ALL, but it works.

I'm not sure if a similar quick fix could work for you or not.  You
would have to make sure that the ataraid driver "built" the
/dev/ataraid* entries, then move /dev/hd* somewhere, run vgscan, and
then move them back.

Make sure you test first and have a backup (do people still do those? :)
) before trying anything.


On Tue, 2003-04-29 at 20:30, Ewen McNeill wrote:
> In message <20030429232352.DC21EAE4F5 at basilica.la.naos.co.nz>, Ewen McNeill writes:
> >In message <20030430011344.N16789 at zealot.blacknet.de>, Goetz Bock writes:
> >>during my last/first/only time i had to deal with ataraid, and IIRC,
> >>the promise IDE driver had a compile time flag where it would not touch
> >>initialised raid arrays.
> >>aktivating this flag would simply remove the offending disks from the
> >>list of ide disks and make it available via ataraid only.
> >
> >It would be nice if it were possible to do this.  Is this perhaps the
> >CONFIG_PDC202XX_FORCE flag (turned off, or on)?  
> According to this message:
> http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&selm=20030405112008%2462da%40gated-at.bofh.it
> from Andre Hedrick (Linux IDE maintainer), the CONFIG_PDC202XX_FORCE
> option is intended to force recognition of the Promise card's IDE
> channels as IDE devices, in order to be able to use the Linux pdcraid
> (ATA-RAID) driver.  (They are apparently semi-disabled in the PCI
> configuration data, by default, so that only the Promise SCSI device
> driver can find them.)  It should be on for using the pdcraid/ata-raid
> support, and off if you are using the Promise "SCSI" device driver.
> (And as I noted earlier, only certain chipsets seem to actually use this
> option; the rest just default to forced on.)
> This tends to confirm my view that if you use the ataraid driver, then
> you'll see both the raw /dev/hd* devices, and also the /dev/ataraid/d0*
> devices, since the ataraid/pdcraid driver appears to search the /dev/hd*
> IDE controllers for likely devices based on partition signatures.
> Which means that LVM needs to deal with the aliasing effects in some
> manner, either by recognising the ataraid devices first and ignoring the
> underlying devices, or by very loudly refusing to work with ataraid at
> all.
> Ewen
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