VSAN Part 26 – Does Disk Size Matter?
I was involved in an interesting thread recently with one of our VSAN partners regarding disk sizes used in VSAN, and what impact smaller drives may have. In an earlier post, I discussed reasons why VSAN would stripe a VMDK storage object even though a stripe width was not requested in the VM Storage Policy – Why is my Storage Object striped?
In that post, I highlighted the fact that if the VMDK storage object is too big to fit onto the free space of a single hard disk, then it will automatically be striped across multiple hard disks. However there is another VSAN object that disks size may also impact – the VM Home Namespace.
The VM Home Namespace is described in great detail in another blog post so I won’t go over it here again. However I do want to call out the fact that the VM Home Namespace is 255GB in size. This is important to know when it comes to designing your VSAN environment because if you choose a magnetic disk that is, for example, 146GB in size, you will end up having a striped VM Home Namespace. This means that for each VM that is deployed, it is now going to require four components rather than two for the namespace object (because the VM Home Namespace is mirrored – it has a FailuresToTolerate = 1 setting associated with it). Because the physical disk size is 146GB, and the VM Home Namepsace is 255GB, VSAN will stripe the VM Home Namespace across two physical disks to accommodate its size. With 100 VMs deployed on the host (the current maximum), that is an additional 200 components just for the VM Home Namespace. Another consideration is the witness component. As more components are used to back a particular object, additional witness components may be needed to attain quorum in the event of failures or split brain scenarios.
Considering the maximum number of components per host is 3,000, consideration should definitely be given to the size of magnetic disk used when designing a VSAN environment.
6 Replies to “VSAN Part 26 – Does Disk Size Matter?”
Quotation from VSAN part 18
“The VM Home Namespace is where we store all the virtual machine configuration files, such as the .vmx, .log, digest files, memory snapshots, etc.”
It means that it is not possible to do a snapshot with memory for a VM with 256 GB of RAM.
Are there any plans to decrease VM Home Namespace?
Good question – let me test it.
Seems like it is by design. I’ll write something up on it shortly.
Another question Cormac, Does a Datastore spans more than one disk groups ?
Yes – since there is only one VSAN datastore, it is made up from “all” of the diskgroups.
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