extending stripes

[Roberto Gallis]

Hi all,
Is it possible to extend a Volume Group of 4 disk in stripe adding a new disk??? (The filesystem is advanced jfs)

[melvyn burnard · Accepted solution]

you could add in the new drive and then extend on to it, but whatever you extend will not be striped.
One of the drawbacks of striping is that to do what I think you are after, you would need to add in th edisk and then recerate your lvols, to stripe across all of the drives including the new one.
bummer

[Dan Hetzel]

Hi Roberto,

The number of physical volumes used for striping is defined when you create the logical volume. You won't be able to add another disk without recreating your lvol.

Regards,

Dan

[MARTINACHE]

Hi,

I am agree with melvyn.

If you want to modify your stripe, we need to recreate it.

But dont't forget this :

Do not stripe across more than 4 disks on the same SCSI controler : we will decrease performance by an SCSI bottleneck.

regards,

Patrice.

[Roberto Gallis]

Hi,
it's the same I was thinking about...
I know about more than 4 disks on scsi, but I have xp256 on fibrechannel... each lun is 7GB... I note that making stripes is faster than normal volumes... fault tolerance is given by xp (internal raid 5)... but if I need to extend filesystems online, stripes are not an optimal solution..
Thank you all

Best regards
Roberto

[Dave Wherry]

I suggest you do not use lvm striping. As others hace said it limits you later if you want to extend the logical volume.
Use extent based striping. It is much easier to work with later if you want to make any changes.
If you want to change the stripe later, and you use extent based striping, do this. Add new disks to your volume group and put them in a physical volume group. Then mirror your original to the new group. Use lvreduce -m 0 and take out the original disks. You are now restriped.
Since you are working on a XP, it is most important to stripe across your interfaces. All the writes go to cache so you will not gain much by concentrating on what drives you stripe. Spread the stripe across your interfaces and you will get your best performance.

[Tim Malnati]

Melvyns comments are correct, you can add a disk but it will not be striped. But if you have only a single logical volume in the group you will not necessarily be hurt that badly by this. The operating system will usually attempt to fill a voluma group from the first extent forward. You wont actually see much data on the added drive until you've exhausted just about all of the striped areas.

Dave's comments regarding extent striping has some merit but it means having a whole lot of extra disk available to accomplish it. But then again, you don't see anywhere near the performance with extent based striping either. The extent sizes are too large to allow the dynamics of read ahead prediction and memory caching to be as efficient as block based striping; particularly if there are multiple processes requestiong i/o from the same volume group.

The rule of thumb that I use is to build the volume group with at least two years planned growth built in. Then have the appropriate monitoring mechanisms there so you don't end up in a situation where you are attempting to restripe the world during a maintenance outage. This is very important in a large shop where the scheduled maintenace activities can get real crammed.