Question on Snapshot when VMDKs are on different LUN

gs1uk used Ask the Experts™
Hi Experts,

I have a general question on how snapshot working..

I have a VM (IO_Tester). The VM Was 4 Drives C  D E F .

C Drive VMDK located on RAID10 (and VMConfi i.e. vmx, swap etc)
D Drive VMDK located on RAID1
E Drive VMDK located on RAID5
F Drive VMDK located on RAID6

When a Snapshot is taken there is a lock on all the DEF VMDKs and Future Data is stored in RAID10.

So from now on when I write some data on the disk it starts storing like below on the RAID10 LUN :

I wanted to write some data on the SNAPSHOT taken (50GB) and then delete the snapshot. In this I will learn how long does it take from RAID10 to RAID5 to commit the snapshots. (Hope I think im doing a usefull test here)

Depending on the Speed I will select which RAIDs are better. Since I use VMWare vDR it does snapshots to backup. When it deletes a snapshot at the final commiting stage say last 3mins it takes my VM down meaning the Website is not accessible and no IO is passed. This breaks links from DB and APP Server and the server need to establish connection manually.

So back to the testing. When I generated a 50GB file which is on the VM which is writing on the SNAPSOT on VIClient Performace Tab -Datastore I can see there is High Wirite metrics which makes sense that Files are gettings generated on the DATASTORE-LUN RAID10

But why there is READ Latency on Datastore RAID5. By the way the 50GB File is logically written on the E Drive which is pointing to RAID5 Datastore.

My Question
1. when there is a snapshot why does it need to use anything on RAID5 Datastore.
2. When I commit a snapshot from RAID10 to RAID5 there is no metrics I can see to see there is data transfer from RAID10 Datastore to RAID5 Datastore.

I have named each test LUNs like

LUN       Datastore

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Andrew Hancock (VMware vExpert / EE Fellow)VMware and Virtualization Consultant
Fellow 2018
Expert of the Year 2017

I highly recommend reading these 2 articles on snaps:

Understanding Snapshots -
Snaphot Best Practices -

Also check out the following Snapshot Articles by Eric Siebert


The location of the Snapshot files, are usually in the VM Folder, unless you've changed the location, which can be done in the VMX file.

So when a VM is in snapshot mode, it will be reading and writing to the snapshot location.


Thanks hanccocka. I will do the reading and get back to you.

Many Thanks
Andrew Hancock (VMware vExpert / EE Fellow)VMware and Virtualization Consultant
Fellow 2018
Expert of the Year 2017

Eric's article is very good, worth a read.


Just Printed them out .. Will read shortly .. Cheers
VMware and Virtualization Consultant
Fellow 2018
Expert of the Year 2017
if you are interested in scripting and scripts, there is a script that can enable Change Block Tracking, which will allow you to see, the rate of change of size/data between snapshots.

so you'll get an idea, of when the best time to schedule the backup, when rate of change is lowest.

We use a similar scripts to check rate of change of VMs.

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