Logical Volume Striping

[John Booth_1]

I was wondering how much overhead you get from using the striping option when you create a logical volume. I have never used this option and I am not a big fan of software solutions. But need something. Any advice would be great.

Thanks,
John

[Helen French]

Hi John,

Generally disk striping can increase the performance of applications that read and write large, sequentially accessed files. Data access is performed over the multiple disks simultaneously, resulting in a decreased amount of required time as compared to the same operation on a single disk. If all of the striped disks have their own controllers, each can process data simultaneously.

Shiju

[Helen French]

Hi again,

This guide will give you a complete information about the benefits/drawbacks of LVM striping:

http://www.docs.hp.com/cgi-bin/fsearch/framedisplay?top=/hpux/onlinedocs/B2355-90672/B2355-90672_top.html&con=/hpux/onlinedocs/B2355-90672/00/00/76-con.html&toc=/hpux/onlinedocs/B2355-90672/00/00/76-toc.html&searchterms=LVM%7cstriping&queryid=20020111-124055

HTH,
Shiju

[James R. Ferguson]

Hi John:

There should be no overhead. Ultimately, every I/O must be mapped to a physical address so this matters little from that perspective.

There are two ways to stripe, or spread data across physical volumes in a volume group -- striping and distributed extents. Have a look at the man pages for 'lvcreate'. The choices are mutually exclusive. True striping gives finer granularity and for small I/O's may offer better distribution accross a large number of disks than distributed extents.

The use of distributed extents, however, allows the creation of both mirrored and "stripped" sets otherwise *not easily* achieved with the standard stripe methods (although this can be finessed).

The above comments apply to standard LVM. With VxVM, instead, *both* stiping and mirroring are supported "naturally".

Regards!

...JRF...

[Sridhar Bhaskarla]

Hi John,

The basic answer to any question related to performance is "it depends". And

"if you are not getting any complaints from the end users, then your system is outperforming and you don't need to touch it".

Striping does offer performance gains. But it depends on the environment you have and the disk subsystem.

If your application does a lot of sequential IOs, then striping will definitely help you and you should consider looking at it. Random IOs gain a very little from the striping. So, you may not want to do striping if your system does a lot of random IO.

Reads benifit largely from mirroring than striping but on the contrary writes suffer. So, you wouldn't be doing mirroing.

And if you have disk subsystems with their own caches, then again the stripe size determines the performance. Sometimes striping may decrease the performance. You need to check with your vendor.


-Sri