[linux-lvm] Volume Group Size
kevcorry at us.ibm.com
Wed Jun 25 13:53:02 UTC 2003
On Wednesday 25 June 2003 13:11, Ragnar Kjørstad wrote:
> On Wed, Jun 25, 2003 at 11:46:34AM -0600, Andreas Dilger wrote:
> > On Jun 25, 2003 18:00 +0100, Stuart Fox wrote:
> > > I cant find information about the maximum volume group size in the
> > > current 2.4 series kernel.
> > >
> > > Id like to be able to join 2 1.75Tb raid arrays into 1 3.5Tb array.
> > > Is this possible?
> > No, there is a limit of 2TB for all block devices in 2.4 kernels. With
> > 2.5, there is a "large block device" configure option that lets you have
> > larger block devices. There is apparently also a patch to support this
> > on 2.4 kernels, but AFAIK it is not very widely used/tested.
> The question was regarding volum goups, not logical volums.
> So, correct me if I'm wrong, but I think the answer is yes, you will be
> able to join 2 1.75 Tb volume groups into a single 3.5 Tb volum group,
> if only an apropriate extent-size was set on VG-creation.
The limitation for the size of a volume-group is a little obscure. The
volume-group metadata (for LVM1 volume-groups - LVM2 format is probably
different) stores the total number of PEs in the group as a 32-bit number.
Thus, you are limited to 2^32 total extents. With a PE size of 4 MB, this is
2^54 bytes, or 16 exabytes (adjust accordingly for a different PE size).
> However, each Logical Volum in the Volum-Group need to be smaller than 2
> AFAIK the "large block device" feature in 2.5 does not enable > 2TB
> logical volumes - it only fixes the general block-device layer, md and
> the scsi-subsystem. At least that used to be the case - maybe someone
> knows if it has changed?
Again, the LVM1 format uses a 32-bit number to record the size (in sectors) of
each LV. So the 2 TB limitation still exists. However, the metadata also
records the number of extents allocated to the LV, so the tools *could* be
written to ignore the lv-size field and always calculate based on the number
of allocated extents. This would push the limit up to pe_size * 2^32 bytes,
but would of course break backwards-compatibility with older tools (not sure
if that really matters).
kevcorry at us.ibm.com
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