On 10/19/2011 08:37 PM, David Lehman wrote:
> On Wed, 2011-10-19 at 13:37 -0500, Chris Adams wrote:
>> Because ordering affects performance. For example, I set up a separate
>> LV "queue" for the mail queue on a mail server; I also have a separate
>> LV for /usr/local ("usrl", grows to fill the VG, minus space for
>> snapshots) and /var ("var", where the logs live). Anaconda orders LVs
>> alphabetically (last time I checked), which could put the mail queue and
>> mail logs at nearly opposite ends of the disk.
>
> I didn't realize lvm guarantees all lvs are allocated from adjacent
> extents.
LVM2 has allocation policies that govern the choice of where on disk to
allocate new segments. The default for a vg is normal but the contiguous
policy can be used to ask LVM2 to try to find new extents adjacent to
existing extents.
The lvm manual page has more details on how they work:
LVM(8):
--alloc AllocationPolicy
The allocation policy to use: contiguous, cling, normal, anywhere or
inherit. When a command needs to allocate physical extents from the
volume group, the allocation policy controls how they are chosen.
Each volume group and logical volume has an allocation policy. The
default for a volume group is normal which applies common-sense rules
such as not placing parallel stripes on the same physical volume. The
default for a logical volume is inherit which applies the same policy
as for the volume group. These policies can be changed using lvchange
(8) and vgchange (8) or over-ridden on the command line of any command
that performs allocation. The contiguous policy requires that new
extents be placed adjacent to existing extents. The cling policy
places new extents on the same physical volume as existing extents in
the same stripe of the Logical Volume. If there are sufficient free
extents to satisfy an allocation request but normal doesn't use them,
anywhere will - even if that reduces performance by placing two
stripes on the same physical volume.
N.B. The policies described above are not implemented fully yet. In
particular, contiguous free space cannot be broken up to satisfy
allocation attempts.
Regards,
Bryn.
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