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Using NAS for virtual machines

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Despite the limitations, there are some good reasons why you might prefer a NAS system over block storage devices.

• Many NFS storage devices use thin provisioning by default, which can help conserve disk space because virtual disks don't consume the full amount of space they've been allocated.
• File locking and queuing are handled by the NFS device, which can result in better performance vs. iSCSI/FC where locking and queuing are handled by the host server.
• NFS doesn't have a single disk I/O queue like a block storage device has, so you may get better performance. The performance of NFS is based on the size of the network connection and the capabilities of the disk array.
• Implementing NAS costs a lot less than traditional FC storage. NAS devices require only common NICs instead of expensive HBAs, and use traditional network components rather than expensive FC switches and cables.
• Because NAS takes away a lot of the complexity of managing shared storage, specialized storage administrators aren't necessary in most cases. Managing files on an NFS server is much easier than managing LUNs on a SAN.
• Virtual datastores can be expanded easily by simply increasing the disk on the NFS server; there's no need to increase the size of datastores as they'll automatically increase accordingly.
• Operations like snapshots and cloning are done at the file system level instead of at the LUN level, which can offer greater

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flexibility and more granular support.

The advantages to using NAS are many and you shouldn't be discouraged by the disadvantages that mainly apply to specific circumstances or with lower quality NAS products. With a properly sized and designed system that will handle the VM workloads on your hosts, NAS can be as good a choice as any block storage device.

Is NAS performance enough?

Many IT shops considering NAS as an alternative to block storage for their virtual servers are concerned about performance, and with good reason. In most cases, NAS performance won't equal that of an FC SAN, but a properly architected NFS solution can easily meet the performance needs of most workloads.

Some users end up comparing iSCSI to NAS as they're both low-cost alternatives to FC storage and they can each use existing Ethernet infrastructure. VMware Inc. has published test results comparing the performance of virtual machines on NAS, iSCSI and FC storage devices. The results show that the performance of NAS vs. both hardware and software iSCSI is nearly identical. As long as the CPU doesn't become a bottleneck, the maximum throughput of both iSCSI and NFS is limited by the available network bandwidth. Software iSCSI and NFS are both more efficient than Fibre Channel and hardware iSCSI at writing smaller block sizes (fewer than 16 KB), but with larger blocks more CPU cycles are used, which makes software iSCSI and NFS less efficient than hardware iSCSI and Fibre Channel. The CPU cost per I/O is greatest with NFS; it's only slightly higher than iSCSI, but much higher than hardware iSCSI and FC, but on a host with enough spare CPU capacity this shouldn't be an issue.

Achieving the best performance with NAS comes down to several factors; the first is having enough CPU resources available so the CPU never becomes a bottleneck to NFS protocol processing. It's easy enough to achieve by simply making sure you don't completely overload your virtual host's CPU with too many virtual machines. Unfortunately, there's no way to prioritize or reserve CPU resources for NFS protocol processing, so you need to make sure you adjust your workloads on your hosts accordingly and monitor CPU usage. Using a technology like VMware's Distributed Resource Scheduler will help balance CPU workloads evenly across hosts.

This was first published in February 2011

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