[linux-lvm] LVM and I/O block size (max_sectors_kb)

[Gionatan Danti <g.danti@assyoma.it>]

Hi all,
it seems that, when using LVM, I/O block transfer size has an hard limit 
at about 512 KB/iop. Large I/O transfers can be crucial for performance, 
so I am trying to understand if I can change that.

Some info: uname -a (CentOS 6.5 x86_64)
Linux blackhole.assyoma.it 2.6.32-431.20.3.el6.x86_64 #1 SMP Thu Jun 19 
21:14:45 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

 From my understanding, maximum I/O block size for physical device can 
be tuned from /sys/block/sdX/queue/max_sectors_kb.

Some quick iostat -k -x 1 show that, for physical device, the tunable works:

# max_sectors_kb=512 (default)
dd if=/dev/zero of=/dev/sdd1 bs=2M count=16 oflag=direct
iostat -x -k 1 /dev/sdd: avgrq-sz=1024 (1024 sectors = 512KB = 
max_sectors_kb)

# max_sectors_kb=2048
dd if=/dev/zero of=/dev/sdd1 bs=2M count=16 oflag=direct
iostat -x -k 1 /dev/sdd: avgrq-sz=2048 (2048 sectors = 1024 KB)

dd if=/dev/zero of=/dev/sdd1 bs=4M count=16 oflag=direct
iostat -x -k 1 /dev/sdd: avgrq-sz=2730.67 (2730 sectors = ~1350 KB)

As you can see, I can't always reach 100% efficienty but I came 
reasonably close.


The problem is that, when using LVM2 (on software raid10), I can not 
really increase I/O transfer size. Setting both sd*, md* and dm-* to 
max_sectors_kb=2048 leads to _no_ increase in I/O blocks, while 
decreasing the max_sectors_kb works properly:

# max_sectors_kb=2048
dd if=/dev/zero of=/dev/vg_kvm/TEST bs=2M count=64 oflag=direct
iostat -x -k 1 /dev/vg_kvm/TEST: avgrq-sz=1024.00 (it remains as 512KB)

# max_sectors_kb=256
dd if=/dev/zero of=/dev/vg_kvm/TEST bs=2M count=64 oflag=direct
iostat -x -k 1 /dev/vg_kvm/TEST: avgrq-sz=512.00 (512 sectors = 256KB = 
max_sectors_kb)

Now, two questions:
1) why I see this hard limit?
2) can I change this behavior?

Thank you very much.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8