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SAN booting alternatives for data storage managers

SAN booting and server virtualization are fueling the trend toward diskless servers. Learn about the move from internal to external storage, as well as other booting alternatives.

What you will learn in this tip: Storage-area network (SAN) booting and server virtualization are fueling the trend toward diskless servers. Learn about the move from internal to external storage and other booting alternatives available for your organization.

Enterprise storage has always implied the use of hard disk drives, but times are changing. The spread of server virtualization and the increase of SAN booting has created a new trend toward diskless servers. Server manufacturers have recognized this shift and are offering USB flash drives and SD card slots. This trend away from disk might seem threatening, but it actually increases the impact of enterprise data storage technologies.

Internal to external and networked storage

Most enterprise application data resides on external storage, whether it's SAS, a direct-attached storage (DAS) array, an iSCSI or Fibre Channel (FC) SAN, or using network-attached storage (NAS). Networked shared storage (SAN and NAS) has become increasingly valuable in the modern data center because it provides performance, efficiency and flexibility unmatched by DAS or internal hard disk drives.

One holdout in this shift to external and networked storage is the initial loading of the operating system (OS). A PC's BIOS expects to boot from an internal hard disk drive, so most servers include one or two small drives for boot even if all their data is on a SAN, LUN or NAS filer. Although booting from SAN has long been possible in both Fibre Channel and iSCSI environments, it never gained acceptance from server administrators. They simply felt more comfortable booting from an internal hard disk drive.

Server virtualization has changed this attitude. Because virtual machine (VM) hypervisors require high-bandwidth I/O, most use SAN or NAS for the majority of their storage already. Guest virtual machines running inside use this shared storage as their boot drive exclusively. This means that highly virtualized environments already boot the majority of their machines from SAN or NAS.

Server virtualization and blade servers

As server administrators have gained confidence in both networked storage and heavily utilized server hardware, they have begun to question their use of internal hard disk drives for booting. Using USB flash drives to load the VMware ESX hypervisor has become a major trend among server virtualization specialists, and they're rewarded with reduced part counts, increased machine density and greater flexibility to move workloads from machine to machine.

Nowhere is the density of computing more pronounced than in the world of blade servers. Although most vendors still specify on-board hard disk drives for their blades, they've rapidly moved to more compact 2.5” mechanisms. The blade server market is trending toward “no-personality” approaches, where the physical blade isn't tied to a specific running OS instance. This makes on-board disks even more of a liability, spurring adoption of SD cards and booting from SAN.

The same trend is happening in the high-performance computing (HPC) space, where booting from SAN is commonplace. In all cases, server architects who prize flexibility and dynamic operations favor eliminating booting from an internal hard disk drive. As they gain comfort with booting from flash media (USB or SD) and SAN, it's likely this trend will impact standalone servers as well.

Impacting SAN design

Whether booting from SAN or flash, the availability and performance of the storage network becomes critical. With all server I/O traveling over the network, IT architects must redouble their efforts to provide quick and reliable connections from the host bus adapter (HBA) to the array.

Attaining this kind of bulletproof reliability isn't new to enterprise storage. SAN administrators are committed to the use of high-availability technologies like multipathing software, and Fibre Channel SANs are particularly efficient at delivering high-performance I/O. In most cases, the same equipment and techniques are applicable to server virtualization and blade server environments as to conventional enterprise applications.

But booting from iSCSI and NAS is something new, and the practice can impact SAN design. iSCSI SANs are often engineered for performance, but they're not always designed for reliability. In my experience, the use of multiple networks and high-end switches is rare in iSCSI SANs, as is the use of advanced security features like mutual CHAP that improves availability. NFS typically runs over the “regular” LAN, with competing workloads and iffy reliability. These networks must be improved if they're to support OS booting.

Is SAN booting right for you?

SAN booting has everything to do with server deployment strategies and very little to do with the needs or desires of data storage pros. It's likely this trend will grow and spread as next-generation server architectures are deployed, so SAN designs and storage product selection must take it into account. As always, architect high-performance, low-latency, high-availability storage networks regardless of the protocol used. And don’t be surprised if you start seeing USB drives and SD cards showing up as alternative boot drives.

There's another related storm on the horizon: Virtual desktop infrastructure (VDI), which by its very definition, is an alternative boot architecture. Most VDI implementations function similarly to the server virtualization scenarios outlined above, relying on a server to supply a virtual disk with both operating system and application data. But VDI tends to be more demanding of storage resources, with “boot storms” looming as employees arrive at work. Preparing for alternative server boot solutions will help make you ready for VDI.

BIO: Stephen Foskett is an independent consultant and author specializing in enterprise storage and cloud computing. He is responsible for Gestalt IT, a community of independent IT thought leaders, and organizes their Tech Field Day events. He can be found online at, and on Twitter at @SFoskett.

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