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

[Gionatan Danti]

Hi Marian,

> 1. As there are more disks in the VG, can them all work with larger
> max_sectors_kb?

All four disks and relative MD device has max_sectors_kb set to the 
required (large) 2048 value.
They all are recent (circa 2010) SATA disks, albeit from different 
vendors.

> 2. Does it work for md device without stacking LVM on top of that?

I did not try to directly use the MD device. Let me test this and I will 
report here.

> 3. What's your Physical extent size?
> 
>     vgs -ovg_extent_size vg_kvm # or simple vgs -v

PE size is at default (4MiB).

> 4. If all above is fine then it may be related to LVM.

I think so, because I did some test with a _single_ disk (an SSD, 
actually) with and without LVM on top. Without LVM, I see the normal and 
expected behavior - IO transfer size increase with max_sectors_kb. With 
LVM on top, I see the exact same behavior I am reporting: max IO 
transfer size seems capped at 512 KB, regardless the max_sectors_kb 
setting.

It really seems something related to LVM layer.

> -- Marian

Thanks.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
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