Virtualization without a storage area network (SAN) is like an iPod without iTunes – sure, it's still useful, but...
not nearly as much as when you have the whole package. Without an external SAN on which to store virtual machines (VMs), users cannot use any live migration feature -- for example, VMware VMotion -- nor any of the tools that build upon them, such as VMware High Availability (HA).
But, as it stands, a lot of IT shops that want to virtualize don't have a SAN in-house, said Nik Simpson, a storage analyst with Burton Group. And with entry prices for Fibre Channel SANs often starting at $60,000, "it can be quite a roadblock."
Simpson, who is giving a talk entitled "iSCSI: an Inevitable Future" at Burton Group's Catalyst conference in San Francisco this week, says that ever since VMware added native iSCSI support to its ESX hypervisor, the pairing of iSCSI and virtualization has started to take off.
VMware ESX 3, introduced last summer, supports iSCSI, as do virtualization offerings from Virtual Iron, XenSource and others.
But the desire for iSCSI storage dates back several years to when VMware introduced VMotion, said Eric Schott, director of product management at EqualLogic Inc., an iSCSI storage vendor in Nashua, N.H. Initially for Fibre Channel only, "VMotion helped [VMware] because it was an interesting feature, but it hurt some of their sales because it required a FC SAN," he said. If you wanted VMotion, "it made the purchase of virtualization a whole other class of buying decision."
At the time, people got around that limitation by relying on software iSCSI initiators loaded directly into the guest, said Schott. These days, folks occasionally still use software initiators if, for example, their servers are in a Microsoft cluster. But the vast majority of shops rely on whatever iSCSI initiator is available out of the box.
iSCSI – cheap to buy and to run
At the low end, some iSCSI SANs can be purchased for a very modest entry price – as low as $10,000 to $15,000 for an iSCSI solution from DataCore Software, said Burton Group's Simpson. Other iSCSI arrays may not cost much less than their Fibre Channel counterparts, but iSCSI's proponents argue that these arrays are much easier to install, configure, and generally get your head around.
iSCSI has proved particularly popular with virtualization users, said Phil Brotherton, senior director of enterprise solutions with the Mountain View, Calif. storage vendor Network Appliance Inc.; the firm counts over 4,000 systems connected to VMware ESX. Of those, he estimates that 40% are connected via iSCSI -- on par with Fibre Channel, and double the percentage of folks connected via NFS (20%).
When it comes to virtualization, which protocol a NetApp customer chooses "depends primarily on their expertise," Brotherton said. "If they're SAN experts and not so focused on cost, they stick with Fibre Channel. But if they're coming from the world of [direct attached storage], and they're faced with the choice to learn iSCSI or Fibre Channel, the learning curve for iSCSI is much easier."
That was the case for Matt Simmons, the IT director for Golf Savings Bank in Mountlake Terrace, Wash. Several years ago, Simmons went shopping for his first SAN and looked at an offering from EMC Corp. in Hopkinton, Mass. Compared with the EqualLogic iSCSI array he eventually settled on, "it was about three times the cost for half the storage – and that's not counting the cost of the Fibre network," Simmons said. Furthermore, "I learned to bring up the box in about thirty minutes, and it's really easy to train people on," he said.
Simmons started out playing with VMware GSX, now VMware Server, in its training center. Since then, the bank has moved on to ESX and today has four VMware ESX hosts connected to EqualLogic storage, running a total of about 50 VMs. Simmons virtualizes all sorts of applications, including Citrix, and relies heavily on virtual machine snapshots and VMotion for everyday maintenance.
iSCSI performance up to snuff
What about performance? "I can't complain," Simmons said of the VMware ESX/EqualLogic combo. "I wouldn't go throwing several hundred users in an application at it," but for his applications, it's been fine. In Golf Savings Bank's case, the amount of data traveling over the IP storage network is light enough to fit on a single VLAN. "If we needed to add more users or VMs, we might need to put [the iSCSI storage traffic] on its own network."
Indeed, the iSCSI vendors themselves are rather cocky about their wares' performance characteristics. "Sure, it's not as fast as Fibre Channel, but it's close – so close that for most users, it's a barnburner," said Ziya Aral, chief technology officer at DataCore Software Corp. "For nine out of ten people, it's terrific."
NetApp's Brotherton concurred. "I don't mean to underplay performance, but my take is that people are overly concerned about it. There are a few exceptional cases where Fibre Channel tips the needle, but most people don't need to make a decision based on it."
Part of the public's perception that iSCSI storage performs worse than Fibre Channel may be a legacy of a bygone era. Back in iSCSI's early days, most of these arrays were built with slow ATA disk arrays, inferior RAID controllers and so on. These days, though, iSCSI arrays can be equipped with high-performance Serial Attached SCSI (SAS) drives, and multiple network cards can be trunked together to provide more bandwidth.
If a storage subsystem performs badly, that's not iSCSI or virtualization's fault, said EqualLogic's Schott. "In any environment, storage can be the bottleneck due to the physics of disk drives. If you have too few spindles, you're not going to get the performance you're looking for," he said. When selling a system, EqualLogic helps customers determine their usage rates, and configures their systems with the appropriate number of drives, cache, network links and RAID type, Schott said.
10 GigE on the horizon
Whatever the case, any perceived lack of bandwidth presented by today's Gigabit Ethernet will become moot over the next couple of years as 10 Gigabit Ethernet becomes mainstream – and cheap -- said Burton Group's Simpson.
We're already seeing the warning signs of 10 GigE's price decline, he said. Take, for example, recent announcements from Mellanox and Broadcom about single chip implementations for 10 GigE. That would push 10 GigE down onto the motherboard, driving down costs.
And, by the end of this year, expect tier-one server OEMs to announce systems with more economical 10 GigE connections based on copper GBICs, said EqualLogic's Schott. Today, there are two types of cabling for 10 GigE: CX4, and the more expensive fibre optic. Like Sony Betamax, Schott expects CX4 to become obsolete, replaced by copper cabling for short runs. When that happens, "the 10 GigE stuff will really plummet in price."
Let us know what you think about the story; email: Alex Barrett, News Director.