Snapshot - how much space will be used?

[eversys]

I have a Windows SBS 2003 VM (on an ESXi 5.0.0.0, 504890) which will be migrated to SBS 2011. Before I start the migration process, I would like to snapshot the SBS 2003 VM. Just in case the migration fails, I can then go back to my previous snapshoted version. Before the migraiton, all data changes (mail, file, print) will be stopped - basically the network will be put on hold. The current VM is around 500GB, how much space do I need to calculate if I snapshot the VM?

Thanks for your feedback.

Regards

Oliver

[FritzBrause]

The snapshot size starts with 16 MB.

The size then depends on the I/O changes inside the VM.

But the snapshot size never goes beyond the size of the base disk.

Anything else is guessing.

Reference: http://kb.vmware.com/kb/1025279

[eversys]

I only have about 50GB left on my datastore - with this snapshot method I didn't want to run out of space during the migration. What do you mean that "the snapshot size never goes beyond the size of the base disk" - you mean those assigned to the particular VM (in my case 500GB)?

[Madhusudhanan Gangadharan]

You have a single VM with single 500 GB virtual disk ? or you have 500 GB disk as an accumulation of say 5 *100 GB  disks  like that ?

if you take a snapshot , all those disks will create its own snapshot (redo logs) and hold its own over head as well .

What type of disk you are using ? thin/thick etc , actually it depends on few many parameters

Also what kind of migration you are planning to ?  will you consolidate your snapshots while migration .

[Madhusudhanan Gangadharan]

The required overhead disk space can cause snapshot creation to fail if a VM's virtual disk is close the maximum VMDK size for a VMFS volume. If a VM's virtual disk is 512 GB on a VMFS volume with a 2 MB block size, for example, the maximum snapshot size would be 516 GB (512 GB + 4 GB), which would exceed the 512 GB maximum VMDK size for the VMFS volume and cause the snapshot creation to fail.

So if you plan on using snapshots, you should create VMs with a virtual disk size that's smaller than the maximum VMDK size by the amount of the maximum overhead (e.g., 512 GB - 4GB = 508 GB). Snapshot files will initially be small (16 MB), but will grow as writes are made to the VM's disk files.

Snapshots grow in 16 MB increments to help reduce SCSI reservation conflicts. When requests are made to change a block on the original disk, it is instead changed in the delta file. If the previously changed disk block in a delta file is changed again, it will not increase the size of the delta file because it simply updates the existing block in the delta file.

[Madhusudhanan Gangadharan]

The size of a snapshot file can never exceed the size of the original disk file. Any time a disk block is changed, the snapshot is created in the delta file and updated as changes are made. If you changed every single disk block on your server after taking a snapshot, your snapshot would still be the same size as your original disk file. But there's some additional overhead disk space that contains information used to manage the snapshots. The maximum overhead disk space varies; it's based on the Virtual Machine Files System (VMFS) block size:

[eversys]

Many thanks for your detailed explanation. During the migration, the SBS 2003 will migrate it's roles & data to the new SBS 2011. In your opinion, do you think that creating a snapshot of my SBS 2003 with only 50GB of free space on my datastore could lead to problems?

[Madhusudhanan Gangadharan]

yes,unless if you are not planning to extend or expand your data store for additional storage.

[Madhusudhanan Gangadharan]

you can always increase your data store size on these situations if possible .

[eversys]

My SBS 2003 VM has 1 x 240GB & 1 x 260GB Thick-Provision Lazy Zeroed hard disks assigned. At the end of the migation, the old SBS 2003 will be decommissioned (within 21 days) i.e. switched off and removed from the datastore, hence freeing up 500GB space!

[Madhusudhanan Gangadharan]

Both these ( 240/260) GB disks will create its own snapshot delta disks.

If you take snapshot when its I/O intensive then all its further writes go to delta disks , so you might need to take care and ensure your delta disk will not be filled around 50 GB as you have only that much remaining  in your datastore by the time mentioned..

[vfk]

Also make sure you have enough capacity on your LUN to support additional space requirement.  Snapshot can grow to the same size as the VM.

[FritzBrause]

Or if you have more space in a different datastore, redirect the snapshot folder.

See VMware KB: Creating snapshots in a different location than default virtual machine directory for VMware ESX and VMwa…

And whatever you do finally, make sure you have a good backup of the VM before you start the procedure. Just for the case.

[vfk]

Yeah that would work too, but I just like keeping things as simple as possible, easier to troubleshoot when things break, you don't really want too many moving parts.

[eversys]

Thanks for all the great feedback.

I have an additional physical Windows 2008 R2 server. I installed NFS services and created an NFS share. In ESXi I created a new datastore pointing to this NFS share. I now have over 4TB to play with - so far so good ;O)

The question is, what now? I quite like the idea of having the snapshots created on a different datastore. Would the suggestion FritzBrause mentioned work - having the snapshots point to my NFS datastore?

[FritzBrause]

Puuh. That's a tricky one. I thought more about another LUN besides the existing one.

Theoretically that works, but it all depends on the I/O performance.

I don't know how fast the NFS share and network is.

You need to consider the VM is than completely running on the snapshot and on the NFS share.

No one can really tell you if the I/O is sufficient. You know your environment best.

[vfk]

Depends the connection speed between your NFS server (on windows :smileyshocked:) and your esxi environment, remember when you create snapshot, you are running from the snapshot and original vmdk becomes read only.  If you don't have enough throughput you will have performance issues, specially if they are in different vlans .i.e routed nfs.

[eversys]

Both the VMware machine & Windows Server are nicely configured: Dell PowerEdge T710 with 2 x Intel Xeon E5645 2.4GHz CPUs, 96GB RAM with a SAS RAID10 array. Network performance is just fine - however, no experience of running a VM off a Windows NFS share.

What might be better... we have a test VM which I could temporarily shutdown & move from the datastore to the nfs share. I would then have about 200GB free on my local datastore for the snapshots - should be OK, right?

What would be the best way of moving this test VM, so that it will run off the nfs if required?

[eversys]

... is there a better way of moving my test VM than... using fastSCP & copy all files to the NFS share. Then remove the VM from the inventary and them re-add the VM to the inventory from the NFS share?

[FritzBrause]

If both datastores, FC and NFS are seen by the ESX host, power off the VM, right-click, migrate.

If this is not possible, maybe you can browse both datastores. Then power off the VM, unregister, go into the VM folder and copy all files over to the NFS datastore.

Here you can add it back into inventory. My recommendation is to first copy and NOT move. If everything is copied over, remove this test VM from the FC store.

Of course still no one knows if 200 GB is sufficient for your SBS upgrade. But I would give it a try.

Take the snapshot, run the upgrade and monitor disk space usage. In case it would really consume all disk space, you could stop and revert back.

Then another way must be chosen.

[eversys]

@FritzBrause - I followed the instructions you wrote regarding copying the test VM to the NFS datastore - there was no migration option. Worked just fine. Before I delete all files from the FC store, I wanted to start the VM on the NFS, it asks whether I copied or moved the VM - why, and what should I answer?

[eversys]

Found a good describtion here: http://techhead.co/vmware-esx-i-moved-it-or-i-copied-it-whats-the-difference/

I "moved it"

[Madhusudhanan Gangadharan]

That is great. Also if you found any of our answers helpful , please acknowledge

[FritzBrause]

Yes. "Moved it" is also what I usually take.

[King_Robert]

Hi check this video about working of snapshot in Vmware  hope you will learn more ..about snapshot

VMware KB: Understanding virtual machine snapshots in VMware ESXi and ESX