To Rebuild LUN or Not To Rebuild LUN

[Joe Redman]

If I wanted to add more space to an existing VG/LV on either a RAID0/1 or RAID5 LUN, is it better to backup data and wipe out LUN then add more disk to the LUN and rebuild or is it better to build a seperate LUN and add that device to the existing VG/LV.

Thanks in advance!!!!

[melvyn burnard]

To backup the data and recreate the LUN with a new size will take a lot more time and work than to simply create the new LUN, pvcreate it, and then vgextend the VG with the new device, unmount the file system, lvextend and extendfs
Of course a good backup is never wasted ;-}

[Roger Baptiste]

hi,

If your existing vg/pv setup is fine, then it is better not to disturb it.
Create new Raid5 groupfor the new disks and split them into luns and then use them on the
system, either as an addition to existing vg or in a new vg.

-raj

[harry d brown jr]

It's easier to add another "lun" to the VG, and not much harm there, unless the disk you are going to create is very small. Also, is the LV you plan on extending striped?

live free or die
harry

[Joe Redman]

So neither way has a positive or negative affect on I/O???

If I add another 4 disk RAID0/1 LUN (device) to an existing 4 disk RAID0/1 VG does the system then start striping over all disk evenly????

[harry d brown jr]

The disks are in a autoraid type device, like an fc60 or 12h?

live free or die
harry

[Joe Redman]

FC60.

[Jim Turner · Accepted solution]

Hi,

It may or may not matter. Since you're apparently able to choose RAID0+1 or RAID5, that lets out 12H AutoRAID. I'll assume then you're working with FC60/SC10, VA7x00, or big boys like XP or EMC.

Here's my general rule-of-thumb: Keep the I/O balanced. Make sure all your LVOLs are striped across all available LUNs with primary paths (LVM) spread equally across physical I/O paths for all LVOLs in a VG. That way any access to any LVOL lights up *all* of your I/O paths equally, yet you still have your alt links if something gets pooched-up.

A specific example: I have a VA7100 shelf with 8 disks. I create two equal-size LUNs instead of one big LUN (for reasons that should become obvious). I put both LUNs in the same VG. One LUN uses c5 as primary and c7 as alt. The other LUN uses c7 as primary and c5 as alt. SAM probably won't do it that way, so use vgreduce/vgextend to fix things up nice. All LVOLs are striped (64k) across both LUNs. What does that buy me? The full bandwidth of both FC paths when I touch anything on the shelf instead of one path burning up while the other one sits la-ti-da doing nothing waiting for the first path to fail. Now let's say I get the other 7 disks for the shelf and want to extend my existing config. No prob. I create two new equal-size LUNs, add them to the VG ensuring again that primary paths are split, and extend my LVOLs. I/O is still balanced and maximized.

Cheers,
Jim