Create lvm volume in the last cylinders of the disk

Question

In all magnetic disks, the speed difference between the first and last sectors is noticeable, up to a factor two or three. This is still true with the new multi-terabyte disks. So it still makes sense to reserve the last cylinders of the disk for infrequently used files, particularly backups.

With a partitioned disk, it is trivial to do this, with the only caveat of allowing for future changes of size.

But my configuration is unpartitioned LVM. So I need either to put two volume groups in the same physical hardware (one using the starts of a set of disks, other using the ends) or to make sure that a logical volume prefers to use the extensions in the last cylinders of the hardware. Is it possible? Do we have some control about where a LV is going to be?

Answer 1

I dont really agree this is a huge concern in most workloads due to caching, readahead policies and disk elevators you can use but this is possible with caveats.

The best way to tackle this would be to physically partition the media you want to allocate off into 'chunks' you consider each end of the disk you want to separate.

Something like this

Number  Start   End     Size    Type     File system  Flags
 1      1049kB  1075MB  1074MB  primary  ext4         boot
 2      1075MB  4TB     4TB     primary               lvm # My fast partition
 3      4TB     8TB     4TB     primary               lvm # My slow partition

Then, create a volume group(s). In this example I'm using one volume group but it might be easier to have a 'slow' VG and a 'fast' VG instead..

# pvcreate /dev/sda2 
# pvcreate /dev/sda3
# vgcreate vg /dev/sda2 /dev/sda3

Then allocate your LVs out of the said physical volumes..

# lvcreate -n myFastLV -L1TB vg /dev/sda2
# lvcreate -n mySlowLV -L1TB vg /dev/sda3

Caveats here being, bad sectors can be silently replaced by the disk controller with a 'reserve' often located elsewhere (which is entirely manufacturer independent). Also some fancier disks may internally remap the sectors in that is logically consistent to the claims offered but are physically not in the place you expected them to be.

Finally, the problem workload you're suggesting (pipelining huge files) really is a very sequential workload issue that would have bigger gains from using preallocation techniques on the files written (to keep them contiguous and not fragment).

Then setting aggressive readahead policies to read whole swathes of adjacent/upcoming sectors which are most likely going to be contiguous to the file you're reading.

A finer grained approach could also be achieved using dmsetup to map the physical sectors into whatever order and fashion you wanted, but this wouldn't be too portable and probably more effort than its worth in the long run (you'd need a script to rebuild the mapping on boot for example).