VMware的Snapshot（快照）相關知識

最近在研究VMware的快照的相關知識，在這裡做個記錄和分享。

先簡單介紹下快照。

A snapshot preserves the state and data of a virtual machine at a special point in time.

1) 快照儲存了虛擬機器在特定時間點的power state；

2) 還有就是資料。比如記憶體資訊，所以當你做了個memory snapshot的時候，那個快照檔案.vmsn的大小和你分配給虛擬機器的記憶體大小几乎一樣。

當你做了快照之後，VMware就建立一個child disk(sparse disk)，之後虛擬機器的資料變更都會寫在這個child disk中，而base disk就變成唯讀了。

base disk 和那個快照檔案.vmsn就可以還原快照時的狀態。

下面是一個老外寫的關於快照設定檔的一些內容。

VMware records snapshot information about the current VM in a .vmsd file. This file is located with the rest of the virtual machine configuration and VMDK files on the datastore. The file is normally called .vmsd. This file will contain information, even if
your VM has no snapshots in place.

It is possible that in some cases, ESX fails to properly clean up after previous snapshots were removed. The information for previous snapshots may still be recorded in the .vmsd file. The file may indicate that you still have snapshots in pace, although all
previous snapshots were removed and the dalta files have been merged. When you then try to create a snapshot, the .vmsd file will inform the ESX host that there is a delta file in place and that it has to create a second or third delta file. When the ESX host
interigates the VMFS file system, it's unable to find the snapshot delta files specified in the .vmsd file and therefore errors with "an invalid snapshot configuration was detected."

We need to clear the contents of the .vmsd file in order to create new snapshots.

 

How to fix the issue

Create a backup copy of the current .vmsd file. Then create a new file called.vmsd. This can be done with a single command from the VMware ESX CLI (Service Console) once you have browsed to the datastore where the VM resides:

    cd /vmfs/volumes///
    mv ./.vmsd ./.vmsd.old && touch ./.vmsd

Let's just have a look at the commands above

mv : This renames the .vmsd file.

&& : This basically tells the CLI that if the "mv" command was executed and completed successfully, only then execute the next part of the command which is "touch".

touch: "touch" is a Linux command that simply creates an empty file.

So, with a single line we have renamed (created a backup) our original file and created a new file that has the original file's name.

After the new .vmsd file has been created, it need to be initialised. In order to do so, use the VI client to create a snapshot. Once the new snapshot has been created, go ahead and delete the new snapshots again. This will write some information to the .vmsd
file:

    .encoding = "UTF-8"
    snapshot.lastUID = "2"
    snapshot.numSnapshots = "0"
    snapshot.current = "0"
    snapshot0.uid = "2"
    snapshot0.filename = "-Snapshot2.vmsn"
    snapshot0.displayName = "Consolidate Helper- 0"
    snapshot0.description = "Helper snapshot for online consolidate."
    snapshot0.createTimeHigh = "290676"
    snapshot0.createTimeLow = "861410424"
    snapshot0.numDisks = "2"
    snapshot0.disk0.fileName = ".vmdk"
    snapshot0.disk0.node = "scsi0:0"
    snapshot0.disk1.fileName = "_1.vmdk"
    snapshot0.disk1.node = "scsi0:1"
    snapshot.needConsolidate = "FALSE"

 The issue should now be resolved and snapshots should work as expected.

VMware中和快照相關的資訊都儲存在那個".vmsd "檔案中了，即使沒有建立快照，這個檔案依舊存在，不過裡面的內容只有簡單的2行：

.encoding = "UTF-8"
snapshot.lastUID = "2"

而VMware是根據這個檔案來定位快照的，所以即使你建立了N多快照，然後將該檔案重新修改，那麼就有可能找不到這些快照。或者你根本就沒有快照，但是這個檔案中的資訊顯示有快照，那麼VMware還是會顯示有快照，只是當你對快照做操作時，會提示“快照檔案配置出錯”之類的資訊。