VMware vSphere Snapshots (draft-WIP)

This post aims to condense and place into a single page important information with regards to snapshots, svmotion (snapshots are used), cloning (snapshots used there too!) and some general issues  and questions which I've encountered in my working environment. (quiescing errors, during Avamar backup, during cloning of "hardened" windows GOS)

I started out looking for supporting articles but ended up going in and out of KBs and losing track of what belongs to what, where belongs to where. Hence this post. It's mostly my notes of what I think will be useful and important while troughing through the maze of KB articles.

Start here (Understanding how Snapshots work on different versions of ESX/ESXi)
http://kb.vmware.com/selfservice/microsites/search.do?cmd=displayKC&docType=kc&docTypeID=DT_KB_1_1&externalId=1015180

• Quiesce: If the  flag is 1 or true, and the virtual machine is powered on when the snapshot is taken, VMware Tools is used to quiesce the file system in the virtual machine. Quiescing a file system is a process of bringing the on-disk data of a physical or virtual computer into a state suitable for backups. This process might include such operations as flushing dirty buffers from the operating system's in-memory cache to disk, or other higher-level application-specific tasks.
  
  Note: Quiescing indicates pausing or altering the state of running processes on a computer, particularly those that might modify information stored on disk during a backup, to guarantee a consistent and usable backup. Quiescing is not necessary for memory snapshots; it is used primarily for backups.
• If the virtual disk is larger than 2TB in size, the redo log file is of  --sesparse.vmdk format.
• .vmsd
  The .vmsd file is a database of the virtual machine's snapshot information and the primary source of information for the snapshot manager. The file contains line entries which define the relationships between snapshots as well as the child disks for each snapshot.
• Snapshot.vmsnThe .vmsn file includes the current configuration and optionally the active state of the virtual machine.
• The above files will be placed in the working directory by default in ESX/ESX 3.x and 4.x.
• In ESXi 5.x and later snapshots descriptor and delta VMDK files will be stored in the same location as the virtual disks (which can be in a different directory to the working directory). 
• When removing a snapshot, the snapshot entity in the snapshot manager is removed before the changes are made to the child disks. The snapshot manager does not contain any snapshot entries while the virtual machine continues to run from the child disk. 
•  During a snapshot removal, if the child disks are large in size, the operation may take a long time. This can result in a timeout error message from either VirtualCenter or the VMware Infrastructure Client.

The child disk

The child disk, which is created with a snapshot, is a sparse disk. Sparse disks employ the copy-on-write (COW) mechanism, in which the virtual disk contains no data in places, until copied there by a write. This optimization saves storage space. The grain is the unit of measure in which the sparse disk uses the copy-on-write mechanism. Each grain is a block of sectors containing virtual disk data. The default size is 128 sectors or 64KB

The disk chain

Generally, when you create a snapshot for the first time, the first child disk is created from the parent disk. Successive snapshots generate new child disks from the last child disk on the chain. The relationship can change if you have multiple branches in the snapshot chain.

This diagram is an example of a snapshot chain. Each square represents a block of data or a grain as described above:

• Reverting virtual machines to a snapshot causes all settings configured in the guest operating system since that snapshot to be reverted. The configuration which is reverted includes, but is not limited to, previous IP addresses, DNS names, UUIDs, guest OS patch versions, etc.

When performing Storage vMotion
http://blogs.vmware.com/vsphere/2011/09/storage-vmotion-storage-drs-virtual-machine-snapshots.html
"It should also be noted that if you do a Storage vMotion of a VM with snapshots and the VM has the workingDir parameter set, theworkingDir setting will be removed from the .vmx & the .vmsn snapshot data file will be moved to the home folder of the VM on the destination datastore. You do get a warning in the migration wizard about this"

"Therefore, if you use the snapshot.redoNotWithParent = "TRUE" parameter, you should refrain from doing Storage vMotion operations."

This happens regardless even if you set the parameters above - in other words, try as best as possible to avoid putting the snapshot files on a datastore away from the parent -flat file disks if all the datastores involved are backing an SDRS cluster...

Troubleshooting http://kb.vmware.com/selfservice/search.do?cmd=displayKC&docType=kc&docTypeID=DT_KB_1_1&externalId=1031200
Disable selective VSS writers for troubleshooting

http://kb.vmware.com/selfservice/microsites/search.do?cmd=displayKC&externalId=5962168
Using custom "pre-freeze" and "post-thaw" scripts.

Covers SYNC and LGTO_SYNC drivers, not VSS.

This article details why the VM may become unresponsive and seem "hung" during a snapshot process.

http://kb.vmware.com/selfservice/microsites/search.do?cmd=displayKC&externalId=1007696
Details VSS  troubleshooting. This article also includes the services that need to be running on the GOS., Issues with quiescing.

When performing cloning on vSphere v5.x on a VM with snapshots
This is what's been observed: Base disk + snapshot will be copied over to the destination VM merging the snapshot(s) into a single VMDK at destination.

When you've run out of space on the datastore and snapshots cannot be deleted
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1004545
This post details the steps to take with a command line tool provided you already have another datastore with sufficient space or have been able to increase the space on the same datastore that had run out of space.

There is a limit on how many open vmdk files an ESXi host can address depending on the VMFS version. 
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1004424
This article is very interesting technically. Covers all versions of ESXi till date. There are changes to the HEAP size between version updates. Useful. Here's the table of limits reproduced:

Version/build	Default heap amount	Default allowed open VMDK storage per host	Minimum heap amount	Maximum heap amount	Maximum heap value	Maximum open VMDK storage per host
ESXi/ESX 3.5/4.0	16 MB	4 TB	N/A	N/A	N/A	N/A
ESXi/ESX 4.1	80 MB	8 TB	N/A	128 MB	128	32 TB
ESXi 5.0 Update 2 (914586) and earlier	80 MB	8 TB	N/A	256 MB	255	25 TB
ESXi 5.0 Patch 5 (1024429) and later	256 MB	60 TB	256 MB	640 MB	255	60 TB
ESXi 5.1 Patch 1 (914609) and earlier	80 MB	8 TB	N/A	256 MB	255	25 TB
ESXi 5.1 Update 1 (1065491) and later	256 MB	60 TB	256 MB	640 MB	255	60 TB

Disks (VMDK) larger than 2TB (for ESXi 5.5 with VMFS5 only. If using NFS, backend must be on file system that has large file support like EXT4. Extending disks beyond 2TB also requires the use of the Web Client or vCLI)

http://kb.vmware.com/selfservice/search.do?cmd=displayKC&docType=kc&docTypeID=DT_KB_1_1&externalId=2058287

Changes in virtual machine snapshots for VMDKs larger than 2 TB:

• Snapshots taken on VMDKs larger than 2 TB are now in Space Efficient Virtual Disk (SESPARSE) format. No user interaction is required. The redo logs will be automatically created as SESPARSE instead of VMFSSPARSE (delta) when the base flat VMDK is larger than 2 TB.
• Extending a base flat disk on VMFSSPARSE or SESPARSE is not supported.
• The VMFSSPARSE format does not have the ability to support 2 TB or more.
• VMFSSPARSE and SESPARSE formats cannot co-exist in the same VMDK. In a virtual machine, both types of snapshot can co-exist, but not in the same disk chain. For example, when a snapshot is taken for a virtual machine with two virtual disks attached, one smaller than 2 TB and one larger than 2 TB, the smaller disk snapshot will be VMFSSPARSE the larger disk snapshot will be SESPARSE.
• Linked clones will be SESPARSE if the parent disk is larger than 2 TB.

What else can cause snapshots consolidation to fail?
Main reference article in spanish:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2046576
1. Locks (files are locked)
http://kb.vmware.com/selfservice/microsites/search.do?cmd=displayKC&docType=kc&docTypeID=DT_KB_1_1&externalId=10051
2. Temporary loss of communication between vCenter and ESXi hosts during confirmation - this does not mean that the ESXi hosts are shown to be disconnected from vCenter. To "restore" connectivity restart management agents from the host. (My note from field experience - there is a chance that during the restart of the management agents, your host may really get disconnected from vCenter AND if your cluster is EVC enabled, you will have to shutdown all the running VMs on that host in order for that host to rejoin the EVC cluster - so beware!)
3. A snapshot configuration file with extension .vmsd in the VM home directory may interfere. Rename, move or delete that file.
http://kb.vmware.com/selfservice/microsites/search.do?cmd=displayKC&docType=kc&docTypeID=DT_KB_1_1&externalId=1003490