Re: vxprint, STRIPE vs CONCAT layout & choosing proper layout

MikeHT <mike2.li@xxxxxxxxx> wrote:
Output from vxprint. Disks are in an EMC Symmetrix 8830. We are seeing
high % busy for c6t17d21 to c6t17d25. Based on different disk size
below, could I have used STRIPE layout instead of CONCAT? Thank you.

Well, you ARE using striping on the relevant volumes (vol07, vol08, vol09,
vol28). But they all stripe over the same disks. If you access them
concurrently, you will get bad performance. Having a Symmetrix as back
end doesn't help much.

If the disks c6t17d21-d25 go to the same physical disk on the Symmetrix
(not very likely though), then performance does drop even more.

General recommendation:

.. stripe for maximum performance
.. *do not* create volumes using the same physical disk if they will be
accessed the same time.

If you cannot comply both (as in your case) it is better not to stripe.


Suppose you have three disks and want to create three volumes. Then you
have two opposite possibilities:

(1) stripe to a maximum:

+-----+ +-----+ +-----+
|vol01| |vol01| |vol01|
+-----+ +-----+ +-----+
|vol02| |vol02| |vol02|
+-----+ +-----+ +-----+
|vol03| |vol03| |vol03|
+-----+ +-----+ +-----+

(3) create three "concatenated" volumes:

+-----+ +-----+ +-----+
| | | | | |
| | | | | |
|vol01| |vol02| |vol03|
| | | | | |
| | | | | |
+-----+ +-----+ +-----+

If vol01, vol02 and vol03 are unlikely to be accessed at the same time then
you can stripe them as in (1) otherwise you should create simple volumes as
in (2)

Instead of creating three volumes and filesystems you could just create a
single large volume (which then can be striped) and let the OS and/or the
filesystem decide how to optimize I/O. This will be true esp. with the
introduction of ZFS.


--
Daniel
.

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