[linux-lvm] Wierd lvm2 performance problems

[Sven Eschenberg]

Hi Luca,

Okay, let's assume a chunk size of C. No matter what your md looks like,
the logical md volume consists of a series of size/C chunks. the very
first chunk C0 will hold the LVM header.
If I align the extends with the chunksize and the extends even have the
chunksize, then every extens PEx of my PV equals exactly a chunk on any of
the disks.
Which in turn means, if I want to read PEx I have to read some chunk Cy on
one disk, and PEx+1 would most certainly be a Chunk Cy+1 which would
reside on a different physical disk.

So the question is: Why would you want to align the first PE to the
stripesize, rather then the chunksize?

Regards

-Sven


On Mon, April 20, 2009 07:39, Luca Berra wrote:
> On Sun, Apr 19, 2009 at 05:16:21PM +0200, Sven Eschenberg wrote:
>> 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.
>
> if you have a 2M chunk size, a full stripe is 2M*(N-1), where N-1 is the
> number of drives in your array minus redundancy. (i.e. for a 5 drive
> raid5 a stripe size would be 8M).
>
> L.
>
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> Luca Berra -- bluca at comedia.it
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