Storage IO Control and Storage vMotion?

My colleague Duncan posted an article on yellow-bricks regarding storage vMotion (sVMotion) of a virtual disk placed in a storage IO control (SIOC) enabled datastore. I thought of providing some more information on this topic..

Yes, sVMotion will be treated as a regular stream of I/O requests coming from a particular VM to a vmdk that is placed on a SIOC enabled datastore. If the datastore wide I/O latency exceeds the congestion threshold of the datastore, SIOC kicks in and adjusts the device queue in the host according to the aggregate disk shares of all the VMs on the host that share the datastore. Within a particular host, I/O requests of each VM is given preferential priority based on VM’s disk shares. The I/O requests can be from an application needing data or from ESX requesting  sVMotion of the vmdk on a non-VAAI compatible storage.

How does SIOC treats sVMotion’s I/O traffic? When SIOC is active, a VM is allowed to have a certain number of concurrent I/O requests queued in the host for the SIOC enabled datastore. If sVMotion is initiated on the VM when it was actively issuing I/O requests, the VM’s quota of concurrent I/O requests will be shared by both sVMotion traffic and the other I/O traffic from the VM to the datastore. If the VM is sparsely issuing I/O requests to the datastore, then its quota of concurrent requests will be dominated by sVMotion traffic.

Note in both cases, the total concurrent I/O requests (sVMotion + other I/O traffic) is limited to a value proportional to the disk shares of the VM on the datastore.

What happens when the storage is VAAI compatible? ESX issues the sVMotion command to the storage. The storage initiates sVMotion on behalf of ESX. ESX doesn’t even see the sVMotion traffic. In this case, the VM is free to use its full quota of concurrent I/O requests.

You will not be able to see the exact number of each I/O request types in the device queue. Good news is, that you don’t have to worry about them. SIOC is capable of  handling these varying traffic conditions for you. If you feel geeky, and really want to get into this, your best bet will be monitoring the difference in sVMotion’s completion time under different load conditions of your VM. But know this – irrespective of the VM’s load situation, SIOC will not let sVMotion affect the I/O traffic on the datastore from any other VM. Though, the response time of I/O operations in the VM on which sVMotion was initiated will be affected by sVMotion.

What if the datastore is not congested? SIOC lets sVMotion use its full quota of bandwidth until datastore becomes congested (datastore wide latency > congestion threshold). Then SIOC does what it is designed to do.

Here is a question for you – If you have to sVMotion a vmdk on a SIOC enabled datastore when do you do it? 😉


About @_chethankumar
I currently work at PernixData as a Technical Marketing Engineer.

2 Responses to Storage IO Control and Storage vMotion?

  1. Sean Clark says:

    “If you have to sVMotion a vmdk on a SIOC enabled datastore when do you do it?”

    **With VaaI – Any time you want since offloaded svMotion doesn’t go against it’s “quota” of I/O allowed by SIOC.
    **No VaaI – Preferably at time when VM has low I/O requirements so that svMotion I/O doesn’t hog the “quota” of I/O allowed to it by SIOC and consequently, adversely affect the performance of the VM being svMotioned.

    Great post!

  2. Pingback: My vlinks choice – 4-2011 « VirtualByte

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