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I/O scheduling in SAN
Could anyone please explain to me which part of the SAN is responsible for doing I/O scheduling? Let’s say when a Virtual Machine (Linux) residing within a server initiates I/O request, is it a hypervisor that is going to do the scheduling or a Host Bus Adapter? Or maybe all requests are just sent through the SAN to the storage device and scheduling built into disks(e.g Native Command Queuing) is responsible for that?
I understand that it is better to use NOOP scheduler for the Linux kernel so that it couldn’t interfere with other schedulers in SAN. Hence, it is reasonable to configure NOOP for each VM. But what about hypervisor, HBA and built-in schedulers?
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1 Reply
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The I/O scheduler is a function of the kernel. For a VM arrangement that includes the guest and host VMs. It gets really tricky optimizing this because the caching policy for the VM makes a difference which I/O scheduler has the biggest effect. And it also matters what file system is being used. If you’re using XFS, deadline scheduler is commonly recommended on the XFS mailing list. So it’s easiest to just use the boot parameter elevator=deadline on both the host and guest.
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