[linux-lvm] Wierd lvm2 performance problems

[Luca Berra]

On Mon, Apr 20, 2009 at 03:15:12PM +0200, Sven Eschenberg wrote:
>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.

correct

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

Because when you _write_ incomplete stripes, the raid code
would need to do a read-modify-write of the parity block.

Filesystem, like ext3/4 and xfs have the ability to account for stripe
size in the block allocator to prevent unnecessary read-modify-writes,
but if you do not stripe-align the start of the filesystem you cannot
take advantage of this.

The annoying issue is that rarely you have a (n^2)+P array, and pe_size
must be a power of 2.
So for example, given my 3D1P raid5 the only solution I devised was
having a chunk size which is a power of 2k, pe_start is aligned to
stripe, pe_size = chunk size, and I have to remember that every time I
extend a LV it has to be extended to the nearest multiple of 3 LE.

Regards,
L.

>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.
>>
>> --
>> Luca Berra -- bluca at comedia.it
>>           Communication Media & Services S.r.l.
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>
>
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-- 
Luca Berra -- bluca at comedia.it
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