VCB VSS Support

[nasadmin]

Without preparing my SQL or Exchange Database prior to taking a VSS based Vmware snapshot ,Can I get application consistency ? Can I just restart my database after restroing the Vm from a backup without any recoverey needed? What I mean is , without pre-freeze script can I take a VSS based Vmware snapshot for getting Database application consistency ?

[thecakeisalie]

Pre-freeze/post-thaw scripts will effectively provide you with database consistent backups, since they require custom scripts which actually stop SQL/Oracle/Exchange/any other high I/O application prior to the backup.

The VM Backup Guide explains what can be quiesced by filesystem and what can only be crash consistent - go to page 49.

[nasadmin]

What I understood from your answer is , taking a VSS based vmware snapshot without notifying the database by using custom quiescing scripts ,the application consistency is not possible ? Am I correct ???

[thecakeisalie]

Well, it might work, if your database happens to not be doing anything at the time; but usually the backup just fails because VSS can't quiesce the filesystem since the application is sending so much data through it.

[nasadmin]

Previous to Esx 3.5 update 1, if my Guest OS is windows 2003, VMware tools only provides file system consistency. But running some custom scripts for quiescing the database I/o and flushing the buffers to the disk prior to taking a SYNC driver based VMware snapshot ,I can say that my backup is application consistent .

But looks like I need to follow the same procedure (running custom scripts before taking vss based VMware snapshot) for esx 3.5 update 2 also. If the both the procedures are same, why the document is saying application consistent quiescening starting from ESX 3.5 update 2?

Is it because VSS provides better quiescen mechanism than sync driver? The vss aware applications will understand better if I use vss component

Correct me if I am wrong in my assumption?

[thecakeisalie]

You're right, VSS can only be used in 3.5U2 or higher builds, and if you didn't build the VM from scratch on a host running 3.5u2,you will have to 'manually' install the VMware tools 'VSS Component' (which is essentially a tools call to the MS VSS). To do this, just run the tools installation in 'interactive mode', and install the "VMware VSS Component".

Since the MS VSS isn't capable of quiescing all application data, you will 'sometimes' get a valid app-consistent backup, and 'sometimes' not - depending on the amount of I/O being processed at the time; which is why it is recommended to use the custom scripts (which are not the same in 3.5U2).

[nasadmin]

Thanks ,my doubt got clarified .I have one more question

Suppose I have a VM with one Vmdk of size 10GB residing on my local VMFS Datastore.I added another vmdk of size 100GB from a different datastore residing on a storage LUN . If I take a vmware based snapshot ,delta files related to both vmdk's are being created on local vmfs datastore only .Due to this, the space of only one datastore is being utilized .If my VCB Backup takes more time ,then my VM operations might fail due to the space issues.Is there any way to mention where should my delta files reside while taking a snapshot?

[thecakeisalie]

This KB will tell you how to change the snapshot directory.