EVA 4K Presenting singe vdisk to multiple hosts

[Kaukonen]

Hi,

I have heard about presenting single vdisk to multiple hosts. It seems to work on my test server. And the company we purhaced the EVA from said that it should be ok. However I now have heard that this could cause corruption to the filesystem.

So what kinds of experiances you have about presenting single vdisk to multiple hosts?

[Rob Leadbeater · Accepted solution]

Hi,

It's certainly possible to present a single vdisk to multiple systems. However, those systems must be running some form of clustering software to control access to the vdisk.

If they don't then you'll get file system corruption.

Cheers,

Rob

[OFC_EDM]

The corruption I think is from users on the various systems you present the disk too not knowing what the other users are doing.

So two people could be writing to the same file.

The OS may not know that a user from a different server is accessing the file. Don't know if it's handled at a different level.

Which OS are you presenting to? Windows/Unix?

It would be interesting to know if people do present disk to multiple systems and what they do to avoid corruption.

I;m assuming you don't have any cluster software involved?

[Vladimir Fabecic]

Like Rob said, it is certainly possible to present a single vdisk to multiple systems.
But those file systems must not be mounted at multiple systems without some clustering software!
You can, however, present vdisk to multiple systems but you only one system can mount it at the time. When this system fail you can mount this vdisk on another system.
Once I had such a case with two linux servers.
But the point is that multiple systems will cause file system corruption when writing to it at the same time without some distributed lock manager (clustering software).

[Kaukonen]

Okay so it is as I have heard. The files would be used by 3 Windows servers unable to be in cluster. The software using the files is smart enough not to use files used by the other server, but most likely Windows accesses the vdisk so that it would corrupt.

Funny that the partner who sold us EVA said it would work :D

[Sheldon Smith]

The EVA will make the vdisk available to as many hosts as you want. However: The EVA has no knowledge of file system or file structure on the vdisks. It is up to the hosts sharing the vdisk to *cooperate* in writing to the vdisk(s). Some OSes can mount a vdisk read-only; Windows can not.

If multiple Windows hosts want to share the contents of a vdisk, choices are:
1) make a Windows cluster
2) use HP Polyserve (sp?) to coordinate access to the vdisk between non-clustered servers (distributed file locking)
3) have one host mount the vdisk and create a Windows Share to serve it to the others

[Uwe Zessin]

> The software using the files is smart enough
> not to use files used by the other server

Is this an Oracle environment?
I am not an expert on this matter, but as far as I can tell, they have their own implementation of a special shareable filesystem for their database. Windows does not understand the format and does not attempt to mount it.

[Kaukonen]

One of the systems has oracle, however it's not the oracle that would access the vdisk. The software uses simple xml files.

The system is acctually quite smart not to read same folder as the other. But I think I'm going to have to go with the Windows share.

[Uwe Zessin]

OK, if you are using NTFS, then only one server is allowed to access the virtual disk.

When the second server attempts to mount the disk, it finds out the the volume has already been mounted (open volumes are marked as 'dirty'). As it does not know that another server has opened the volume, it assumes it has to clean up and roll back uncomitted changes - the corruption has started!

[Clint Placette_2]

All cluster managers are supposed to manage the sharing dis that is presented to 2 are more servers. But I have seen a user bypass Veritas mgr by using the Veritas cli to present the same disk to 2 different servers. A year later the DB was corrupted. The reason it took so long was because the occupancy of the disk was 80% which caused both servers to change the same sector. The above technical points are true. Never present the same disk to other servers unles managed by a cluster manager or sooner or later you will lose data.