LVM and VxVM

How long should it take pvmove?

 
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Tomas Martinez
Occasional Advisor

How long should it take pvmove?

I am looking to execute a pvmove during a maintainance window and have never done this before. I would like to be able to give a estimated time of outage.(this is a cluster in shared mode). The only thread I found in regards to the amount of time it takes pvmove, was 4 hours for a 40GB move. This can't be right, can it? I am looking to move 128GB worth of PE's from one PV to another. Can someone tell me, if all goes well, how long this might take to complete? thanks in advance for all responses...tomas
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Mel Burslan
Honored Contributor

Re: How long should it take pvmove?

the estimate is for the 40 GB of data you mentioned seemed quite a bit low but you need to understand when you do the pvmove, you are reading information from one disk device (or one set of disk devices),getting it into the system then flushing it out to another. So you are bound by the speed of the data transfer channel you have between the server and the disk devices as well as your server load.

Assuming in the above example 40GB got transferred in 4 hours, which roughly comes to 2.7 Mbytes/sec which is roughly 20 mbits/sec. If there are no other server load in question or I am making a huge mistake, it must be pretty old disk subsytem.

Assuming in your case this 128GB is ona SAN and you have 2GB fibrechannel connections all across the board, interfaces switches and everything.

2GBit/sec transfer ~=~ 200MBytes/sec
1GB will be transferred in about 5 sec.s
and 128 GB will be expected to go in about 7 minutes or less. But again no server load and full blast copy over 2GB FC connection that we are talking here. Anything less, will change this estimate, significantly.

________________________________
UNIX because I majored in cryptology...
Bill Hassell
Honored Contributor

Re: How long should it take pvmove?

pvmove, just like syncing a mirror, is a VERY slow process. The reason is that thi task takes place at an atomic level, that is, the read, the write, the verify and the update to the maps locks out all other activity. That's why you can sync a mirror or use pvmove on a live system. Extreme measures are taken at the driver level to ensure that each PE is accurately mirrored or moved.

If you use dd with a blocksize of 128k to 1024k, it will run from 8 to 20 times faster. If the disk area is inactive (ie, unmounted filesystem or a raw area with no database running, use dd to drastically reduce the time.


Bill Hassell, sysadmin
Tomas Martinez
Occasional Advisor

Re: How long should it take pvmove?

Thank you very much for your responses.

This is a scenerio, where the disks are on a SAN and tranferring over fibre channel across the board. These are raw volumes managed by Oracle, obviously Oracle not running and volumes not in cluster/shared mode while doing the move. No other load will be on the box during the maintainance.

So that being said, is it still recommended to use pvmove or is dd a more widely used practice? I am assuming LVM continues on without a hitch after using dd to move the data or PEs?...thanks, tomas
Mel Burslan
Honored Contributor

Re: How long should it take pvmove?

Thomas,

we have gone thru the same or very similar scenario about a year ago when we were migrating our data from EMC to XP1024 array and we extensively used the vxdump command to transfer data between logical volumes over the fibrechannel fabric. I am not sure about the speed comparison against dd but it definitely is faster than pvmove and I was told by more than one people that it is more efficient than dd. It may be well worth reading the manpages for a few minutes.

It sure is reliable. No data loss was encountered using vxdump in our case. Hope this helps.
________________________________
UNIX because I majored in cryptology...
Tom Danzig
Honored Contributor
Solution

Re: How long should it take pvmove?

FYI, I have used pvmove quite a few times to move data online and during production hours. You may be able to convince your client/manager to allow this during normal business hours. Performance will be affected somewhat, however, how much so is based on how hard your system is hit during this time period.
Tim Nelson
Honored Contributor

Re: How long should it take pvmove?

How about host based mirroring ? Add new disk to lvol as mirror. Once sync is complete reduce origional disk.
This could be done online over the course of a day or week. Then one by one reduce the old disk out. ( or split if you wish to keep an emergency copy for a couple days).

Best of luck !!
Tomas Martinez
Occasional Advisor

Re: How long should it take pvmove?

All,
I wnat to thank everyone very much for their insightful responses. I did ultimately stick with pvmove and everything was transferred as expected. It took about a hour to complete the move....thanks, tomas
Tomas Martinez
Occasional Advisor

Re: How long should it take pvmove?

thanks again, tomas
QUANG_4
Occasional Contributor

Re: How long should it take pvmove?

Hi,

I'm also in the process of using pvmove, and I would appreciate your advice. What I've done os to add a OPEN-E*4 device to a (SAP) VG. What I need to do is to transfer all the PEs on a smaller XP device (OPEN-E*2) that I need for another server.

I've already used pvmove to transfer the PEs of the smaller LVs to the bigger XP device. However, an LV containing 5036 PEs remains to be transferred. My interrogation is whether it is safe to run the pvmove over a (slow, low-bandwidth) remote connection? What happens if I loose my session in the middle? Can I run the command safely in the background? (Normally, I use screen for these things but I haven't been able to install it on these servers, running in HP-UX 11.00.)

Well, I may use dd instead I guess, since I do have the possibility to stop the SAP server during the weekend.

Thanks in advance for any help you can provide!

Best regards,

Frank Olsen
Steria