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Re: [linux-lvm] Wierd lvm2 performance problems

Hi Milan,

Milan Broz schrieb:
Sven Eschenberg wrote:
I tried to create a PE+VG+LV ontop of a mdraid. For no obvious reason the lvm volume show extremely poor performance - The transferrates are as little as 30% of the rates on the md device itself. In contrast creating a partition table ontop of the array gives nearly no performance impact.

Does anybody have the slightest idea, wht might be going wrong here?

P.S.: I tried aligning the first PE in the VG with the mdraid chunk size, no difference whatsoever.

How did you check that it is properly aligned?
The problem is mostly that data area start is misaligned with underlying
MD chunk size

Unfortunately I don't have the box at hand for 2 days, but I asked md to use a chunksize of 2048K and the /proc/mdstat reported 2048K, last time I checked. The LVM hat a phy-extsize of 2M and with the --dataalignment option set to 2M, pvs reported a pe_start value of 2M aswell.

Please can you paste output of
pvs -o +pe_start --unit b
cat /sys/block/<your_md_dev>/md/chunk_size

Please use at least lvm2 version 2.02.40 (see lvm version) for creating
VG - this version has automatic alignmnent option if running over MD
(see md_chunk_alignment in /etc/lvm.conf)
I need to check on lvm versions when I am back at the place it is located and will check the option then.

Also increasing readahead value can help (but only for simple linear
read operation). You can increase readahead persistenlty for LV
using (see man for lvchange -r ).

That I did already somewhen during the night and it improved everything dramatically. I set the readahead to 32768, the value md had chosen for the underlying md device.

mbroz redhat com



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