[linux-lvm] Wierd lvm2 performance problems

[Sven Eschenberg]

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
> and
> 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.
> 
> Milan
> --
> mbroz at redhat.com
> 

Regards

-Sven

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