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Using NAS storage devices in a virtualized server environment

Are NAS storage devices a fit for a virtual server environment? We list factors in choosing these devices, and explain their impact on your storage systems.

What you will learn: We'll help you determine whether or not network-attached storage (NAS) devices are right for your virtualized server environment. In most cases, NAS performance won't equal that of a Fibre Channel storage-area network (SAN), but a properly architected NFS solution can meet the performance needs of most workloads. Learn more about NAS storage devices and their advantages.

For the most part, NAS storage devices in a virtualized server environment function similarly to block storage devices, but there may be some limitations due to their architecture.

  • If you don't use local storage on your virtual host and want to boot directly from a shared storage device, you'll need a storage resource other than a NAS system. With Fibre Channel and iSCSI adapters you can boot the hypervisor directly from a shared storage device without using any local storage.
  • NFS uses a software client built into the hypervisor instead of a hardware I/O adapter. Because of that, there's CPU overhead as the hypervisor must use a software client to communicate with the NFS server. On a very busy host this can cause degradation in performance as the CPUs are also being shared by the virtual machines.
  • In vSphere environments, while you can create virtual machine (VM) datastores on NFS devices, they don't use the high-performance VMFS file system. While this doesn't affect the use of most of vSphere's features, you can't use raw device mappings (RDMs) to attach a physical disk directly to a VM.
  • Some vendors don't recommend NFS storage for certain sensitive transactional apps (e.g., Exchange and Domino) due to latency that can occur. But there are many factors that figure into this, such as host resources/configuration and the performance of the NFS device you're using. This shouldn't be a problem for a properly sized NFS system.
  • NFS doesn't support using multipathing from a host to an NFS server. Only a single TCP session will be opened to an NFS datastore, which can limit its performance. This can be alleviated by using multiple smaller datastores instead of a few larger datastores, or by using 10 Gigabit Ethernet (10 GbE) where the available throughput from a single session will be much greater. The multipathing constraint doesn't affect high availability, which can still be achieved using multiple NICs in a virtual switch.
  • 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 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 for your virtual server workload?

    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.

    The second factor is network architecture; the performance of NAS storage is highly dependent on network health and utilization. You should isolate your NAS traffic on dedicated physical NICs that aren't shared with virtual machines. You should also ensure that you use a physically isolated storage network that's dedicated to your hosts and NFS servers, and isn't shared with any other network traffic. Your NICs are your speed limit; 1 Gbps NICs are adequate for most purposes, but to take NFS to the next level and experience the best possible performance, 10 Gbps is the ticket. There are a number of network configuration tweaks you can use to boost performance, as well as technology like jumbo frames.

    The final factor in NFS performance is the type of NAS storage devices you're connected to. Just like any storage device, you must size your NAS systems to meet the storage I/O demands of your virtual machines. Don't use an old physical server running a Windows NFS server and expect to meet the workload demands of many busy virtual machines. Generally, the more money you put into a NAS product the better performance you'll get. There are many high-end NAS systems available that will meet the demands of most workloads.

    BIO: Eric Siebert is an IT industry veteran with more than 25 years of experience who now focuses on server administration and virtualization. He's the author of VMware VI3 Implementation and Administration (Prentice Hall, 2009) and Maximum vSphere (Prentice Hall, 2010).

    This article originally appeared in Storage magazine.


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