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Recently there have been a number of queries in and around Storage I/O Control (SIOC) I/O accounting, especially when it comes to what might be called administrative functions such as virtual machine snapshots and Storage vMotion. I’ll try to answer some of those frequently asked questions in this post.

This article assumes a familiarity with the workings of Storage I/O Control. If you are unfamiliar with SIOC, then may I suggest a read of the following articles here & here, as well as this white paper.

SIOC & VM Snapshots

A common question is whether the snapshot disks of a virtual machine are included in SIOC accounting. The SIOC enforcement is VM-based, as such the total IO slots used by a VM is what matters. So, if a VM is involved in accessing its snapshot disks, all are charged to the same VM and included in the SIOC accounting.

SIOC & Storage vMotion

Another common question is whether Storage vMotion operations are included in SIOC accounting. Again, like snapshots, all IO slots used by a VM are charged, so yes, Storage vMotion operations are also charged to the VM and included in SIOC accounting.

Backups & Hot-Add disks

Many backup products hot-add VMDKs from a VM to the backup server/appliance in order to speed up the backup process (disk to disk copies rather than over the network). Once again, any limits or shares associated with the VM/VMDK continue to be honored by SIOC.

I/O Accounting behavior with multiple VMDKs

There is some difference in behavior in the I/O accounting for virtual machines with multiple disks. First, I/O shares are at set at VMDK level, but the enforcement is at VM level. Example: VMDK1 has 100 shares and VMDK2 has 100 shares. The VM as a whole has 200 shares and when both VMDKs are on the same datastore, the 200 total shares can be dynamically split between the VMDKs (in case one is not as busy as the other).

To summarize, all I/Os are billed to the same VM & SIOC will proportionally divide I/O and will not make distinction based on type of I/O.

vCenter Availability

A final question here is related to whether or not SIOC continues to function if the vCenter server becomes unavailable for some reason. SIOC does not depend on the vCenter server for any enforcement. SIOC enforcement is done at the host level. Therefore SIOC continues to function even if the vCenter server is unavailable.

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About the Author

Cormac Hogan

Cormac Hogan is a Senior Staff Engineer in the Office of the CTO in the Storage and Availability Business Unit (SABU) at VMware. He has been with VMware since April 2005 and has previously held roles in VMware’s Technical Marketing and Technical Support organizations. He has written a number of storage related white papers and have given numerous presentations on storage best practices and vSphere storage features. He is also the co-author of the “Essential Virtual SAN” book published by VMware Press.