[linux-lvm] Wierd lvm2 performance problems

[Sven Eschenberg]

Hi Luca,

On Mon, April 20, 2009 15:46, Luca Berra wrote:
> 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.

I didn't think of this 'yet', then again all the preliminary tests I did
so far were on a 4D raid10 - Didn't have the time to setup the raid5
volume yet, because the performance issues on the raid10 were so amazing
:-D.

>
> 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.
>

Since you mentioned it: What is the specific option (for xfs mainly) to
modify this behavior?

> 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.

Ouch, I see, I'm gonna be as lucky as you :-).

Another question arose, when I thought about something: I actually wanted
to place the OS on a stripe of mirrors, since this gives me the
statistically best robustness against two failing disks. From what I could
read in the md man page, non of the offered raid10 modes provides such a
layout. Would I have to first mirror two drives with md and then stripe em
together with md on top of md?

>
> Regards,
> L.

Regards

-Sven

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