Safety of vxresize and dynamic lun expansion on live volumes

[rtheriault]

Hello, I've been using Veritas vxvm/SF for a while (currently mostly on 5.0mp3) and have been able to use vxresize on live volumes/filesystems to resize them in the past. But I'm unclear on, when vxresize is running, resizing a filesystem if it is blocking reads and writes to the filesystem? The man page doesn't make this clear. It does say :

 o  In some situations, when resizing large volumes, vxresize

        may take a long time to complete.

But I'm not sure if it's blocking during this time and would cause IO failures for the applications trying to access the fs if the resize takes too long.

So does vxresize block IO when resizing? If yes, are there any best practices/guidelines (or suggestions) on how to safely use this?

I'm also interested in starting to use the "dynamic lun expansion" feature, (i.e. vxdisk resize) to expand our filesystems after growing a SAN lun, but am also unsure if this is a safe operation to perform on a live filesystem? 

Thank you for any insight.

Romeo

[Gaurav_S · Accepted solution]

Hello Romeo,

To answer the first part, the time might be taken for resize of large filesystems is because "If there are filesystem resources in use in the sectors being moved, the time needed for relocation depends on number of sectors being moved" There are cases when filesystem resize fails when it was 100% used, it is always recommended to resize a filesystem when it has some space for vxfs structural movement. Same problem may also appear if filesystem is too busy. To my understanding, IOs to filesystem are not put on hold.

To answer the second part, the most important thing about Dynamic Lun expansion (DLE) is, the array should support this feature. Because when DLE is performed, it should increase the Lun at the bottom. If it increases on top of the Lun, chances are there that it may corrupt the private region of veritas lying on the top of the Lun.

Also, do read the Dynamic Lun Expansion article in VxVM admin guide, which can be found here:

https://vos.symantec.com/documents

DLE is there is market from quite a while & people are using it, so not to worry on that part.

Gaurav

[rtheriault]

Hi Gaurav, Thanks for replying. I decided to do a little testing to see if there is any blocking that happens when one vxresizes a mounted filesystem. I created a 10g lun with vxfs and vxvm, then resized the lun to 110g on the storage side. Then went through the process of getting vxvm seeing the resized lun. Once I was all ready to start the vxresize command I started a verbose rsync of a directory of small files (/etc) onto the filesystem that was going to resize. Once the rsync was under way I set off the vxresize command. I've tried it a few times and every time the vxresize makes the rsync pause for about 5 seconds or so. While 5 seconds isn't bad what worries me is if this pause would ever take a long time as the quote I pointed out suggested.

I'm also having a bit of trouble with the "Dynamic Lun Resize" feature. I get all kinds of errors if I try to do this on a mounted filesystem. I can get it to work on an unmounted filesystem but still with errors. I've got to do a bit more testing to see if having more than one lun in the diskgroup will allow me to do the dynamic lun resizing on  a mounted diskgroup/volume.

[rtheriault]

To answer the second part, the most important thing about Dynamic Lun expansion (DLE) is, the array should support this feature. Because when DLE is performed, it should increase the Lun at the bottom. If it increases on top of the Lun, chances are there that it may corrupt the private region of veritas lying on the top of the Lun.

We use Netapp SAN's. Do you know where I could find out if netapp supports DLE?

Thank you,

[rtheriault]

To answer my own question, I found a netapp knowledgebase article that uses DLE, so I guess Netapp supports the feature.