[linux-lvm] Wierd lvm2 performance problems
Sven Eschenberg
sven at whgl.uni-frankfurt.de
Mon Apr 20 13:15:12 UTC 2009
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.
>
> --
> Luca Berra -- bluca at comedia.it
> Communication Media & Services S.r.l.
> /"\
> \ / ASCII RIBBON CAMPAIGN
> X AGAINST HTML MAIL
> / \
>
> _______________________________________________
> linux-lvm mailing list
> linux-lvm at redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>
More information about the linux-lvm
mailing list