Expand/Extend Question

[Jefferson Humber]

I'm sure this is a really easy question for you guys to answer, I'm slightly embarrased I don't know the answer myself to be honest with you !

HP state in their docs that the approx time taken to perform an expand or extend operation can be worked out at the rate of 15mins per Gb. However what does the 1Gb refer too (I assume this is with an expand priority set to MEDIUM) ?

Is it the size of the new array/lun or the size of what you want to add to it ?

For instance if I have a 4x100Gb HDD's in a RAID 1+0 array (200Gb usable space). And I then add another 2x100Gb HDD's and expand the array, how long approx will the expand take ?

Is it 25hrs (100Gb of usable space added @ 15mins per Gb)? Or is it based on the new total size of the array, or is it based on something else ?

For instance the other day I expanded an array on a 6404 controller. The array to begin with was made up of 12x72.8Gb in a RAID 1+0 arrangement. I added another 2x72.8Gb disks and expanded the array. I did this by booting off a SmartStart CD to get ACU (the server was a VMWare box) so nothing else was accessing the disks, but the expand operation only took about 2hrs (Expand priority set to HIGH).

Using the approx 15mins per Gb rule of thumb I could work out what this referred too.

Can anyone shed any light on this ? I've never really thought about this before, and just kinda except that the operation could take a while. However, now I've expanded the array I need to extend the LUN sometime and want to be able to accurately predict how long it will take.

If anyone has the key to this formula I'd like to hear it.

Thanks in advance,

Jeff

[Steven Clementi]

Jeff:

The 1GB referes to the amount of data you have currently on the array. When you add adisk to an array, the controller had to "level" the data actoss all the drives.

The 15 minute rule is an average time with the setting of Medium Priority and with light to moderate i/o on the controller.


How much space was allocated on the 12 disks to begin with?


Expanding the lun is different. Since the data is already moved across all the drives in the array, when extending the Logical Drive... the controller only has to allocate more space in the array to the
LUN. The time should be significantly less then 15 minutes per GB, but if your doing it "offline", should also go quickly.

There really is no key since each situation is different. If you can incur the downtime, then it is best to set to HIGH and have NO i/o to the array.


Steven

[Jefferson Humber]

Steve,

On my real world example the 12 HDD were all 72.9G models, in a RAID 1+0 array. Approx 437Gb usable space in the array.

I knew this was a rule of thumb, but was still curious as to what it actually referred too.

So using the HP rule of thumb, are we saying in my situation with a setting of 'Medium' and light I/O, it should of taken in the region of 7.5hrs ?

[Steven Clementi]

No.

Not USABLE disk space... USED disk space.

How much DATA is on the array currently?


Steven

[Jefferson Humber]

Well, I made one big LUN from the disk array.... so used up all the original space and presented it to a W2K host.

I'll check on Tuesday as to how much of the LUN the server has written too.

[Jefferson Humber]

The size of the presented LUN is 407Gb, with 164Gb used under VMware.

[Tom O'Toole]

This 'rule of thumb' is going to vary greatly depending on the hardware. My experience with expanding EVA luns is that it happens at a rate of several GB/minute.

[Jefferson Humber]

Thanks for all your answers guys, I'll now officially close the thread.

It would seem that the rule of thumb is a bit of a stab in the dark, there are too many variables it seems... and with today high performance kit it can rip through these operations (as I have found).

As I mentioned the expand took 2->2.5hrs, but the extend of the LUN to about 2 seconds..... I needn't of worried so much.

[Jefferson Humber]

Closed.