distributed striping

[Kwhite_1]

Team,

I wanted to know why do we have to use distributed striping when the disks are already stiped on the san. Some of my servers are having distributed stripe and some are in concat. Right now we are seeing slow response on the caoncat ones. How do I answer to this question of people that its already striped on the san then why do I need OS level striping

[Duncan Edmonstone]

well that entirely depends on the type of disk array you have, and you didn't tell us that did you?

however generally its down to the fact that striping on the host allows you to use more than one controller/port on your disk array.

HTH

Duncan

[Kwhite_1]

We are having XP 12000 array with raid5.

[Tingli]

Striping on the lvm level (assume that is what you want) might limit your ability to file system modification, such as increase or decrease the file system size.

[Kwhite_1]

sorry i did not mean creating logical volumes with stripes. My question was creating volume with PVG or ncolumn in vxvm and diffrence between not using them

[Duncan Edmonstone]

So on an XP, any RAID 5 LUN is going to come off either 4 disks (3+1), 8 disks (7+1), or 16 disks (14+2).

But no doubt you have more that that number of disks in the array...

So its better to build a logical volume which spans multiple LUNs/LDEVs in the XP... instead of spanning just 4 disks, you could span many more by using host based striping.

Add to this that different LUNs come from different ACPs and are served out of different CHIP ports, and you should see thet the more LUNs you can spread your IO across, the better the performance you will get.

If this were an EVA however, where the LUNs could already be spread across all the spindles in the array, I would have said striping would not gain you much (although even then a 2 column stripe would ensure you used both EVA controllers.

HTH

Duncan
HTH

[Kwhite_1]

Okay in straight I am being asked why do we need to use PVG group in LVM or ncolumn=8 while creating logical volumes when the luns are already striped on the SAN

[Duncan Edmonstone]

well I think I answered that, but if we have to spell it out...

to be sure, ask your storage team another question... how are the RAID5 LUNs constructed? 3+1, 7+1, 14+2? Are all the LUNs tyou see carved from the same RAID group, or from different RAID groups.

If you can get those questions answered maybe we can deliver you a more quantitive answer... is the person who setup this striping originally not able to answer this question? You have to ask if they can't, why was it done?

HTH

Duncan