My Scripts and Tips

When you create a snapshot in VMware Instrastructure (formerly ESX), the server stops updating the .vmdk files. Instead it creates a delta file for each .vmdk file. Any writes are written to the delta file rather than the .vmdk file.

This has a few implications:

• The amount of disk space used will increase (a delta can theoretically grow as large as the original .vmdk).
• Read performance can suffer (both the .vmdk and the delta file will now need to be read).

The delta file is stored with the virtual machine’s configuration file, not with the .vmdk file. So, if you have multiple .vmdk files on different volumes you might be in for a surprise.

In the example below I have a virtual machine called VM-Test. It has two .vmdk files, one in /vmfs/volumes/storage1/VM-Test and one in /vmfs/volumes/storage2/VM-Test. I have shown the directory listings before and after taking a snapshot. As you can see the delta for both .vmdk files is stored in the same directory as the configuration file.

Before the snapshot:

[root@vmsvr /]# cd /vmfs/volumes/storage1/VM-Test
[root@vmsvr VM-Test]# ls -l
total 8651072
-rw-------    1 root     root     268435456 Dec 14 22:08 VM-Test-38405ab5.vswp
-rw-------    1 root     root     8589934592 Dec 14 22:16 VM-Test-flat.vmdk
-rw-------    1 root     root         8664 Dec 14 22:08 VM-Test.nvram
-rw-------    1 root     root          314 Dec 14 22:11 VM-Test.vmdk
-rw-------    1 root     root            0 Dec 14 21:53 VM-Test.vmsd
-rwxr-xr-x    1 root     root         1572 Dec 14 22:08 VM-Test.vmx
-rw-------    1 root     root          252 Dec 14 22:08 VM-Test.vmxf
-rw-r--r--    1 root     root        19089 Dec 14 22:11 vmware.log
[root@vmsvr VM-Test]# ls -l ../../storage2/VM-Test/
total 4194368
-rw-------    1 root     root     4294967296 Dec 14 22:16 VM-Test_1-flat.vmdk
-rw-------    1 root     root          314 Dec 14 22:11 VM-Test_1.vmdk

After creating a snapshot:

[root@vmsvr VM-Test]# ls -l ../../storage2/BC-Test1/
total 4194368
-rw-------    1 root     root     4294967296 Dec 14 22:16 VM-Test_1-flat.vmdk
-rw-------    1 root     root          314 Dec 14 22:11 VM-Test_1.vmdk
[root@vmsvr VM-Test]# ls -l
total 9591296
-rw-------    1 root     root     671088640 Dec 14 22:26 VM-Test-000001-delta.vmdk
-rw-------    1 root     root          248 Dec 14 22:18 VM-Test-000001.vmdk
-rw-------    1 root     root     16777216 Dec 14 22:26 VM-Test_1-000001-delta.vmdk
-rw-------    1 root     root          310 Dec 14 22:18 VM-Test_1-000001.vmdk
-rw-------    1 root     root     268435456 Dec 14 22:08 VM-Test-38405ab5.vswp
-rw-------    1 root     root     8589934592 Dec 14 22:17 VM-Test-flat.vmdk
-rw-------    1 root     root         8664 Dec 14 22:08 VM-Test.nvram
-rw-------    1 root     root     273866015 Dec 14 22:18 VM-Test-Snapshot1.vmsn
-rw-------    1 root     root          314 Dec 14 22:11 VM-Test.vmdk
-rw-------    1 root     root          542 Dec 14 22:18 VM-Test.vmsd
-rwxr-xr-x    1 root     root         1510 Dec 14 22:25 VM-Test.vmx
-rw-------    1 root     root          252 Dec 14 22:25 VM-Test.vmxf
-rw-r--r--    1 root     root        30121 Dec 14 22:25 vmware.log