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|So, what exactly is a LUN?|
The best news for Windows admins is that many enterprise storage components are now included in the operating system. Microsoft made a big splash with the introduction of Windows Storage Server in 2003 and Windows Data Protection services in 2004. Although these technologies are of little immediate use in most environments, the quality of the bundled storage technologies in Windows has been quietly and steadily improving. Still, Windows systems are often the targets of criticism. Let's take on the common myths of Windows storage.
File system and drive-letter limitations
Most Windows people understand the importance in the shift from the old File Allocation Table (FAT) file system to Windows NT's NT File System (NTFS). NTFS removed FAT's restrictions on file naming and drive sizing, and added support for advanced and flexible access controls (permissions) for files. It's also faster, more stable and far more flexible. Even by Unix standards, NTFS is a fine file system.
Windows admins are much less familiar with a major change in the way Windows drives are managed. A common complaint from Windows naysayers is the supposed drive-letter limitation. That is, thanks to its DOS roots, Windows file systems are traditionally mounted (or mapped) to an alphabet letter. Because A: and B: are reserved for floppy disks, and C: and D: are typically used for a boot disk and CD-ROM, the common belief is that only 22 drives can be mounted on a Windows system. Add in a dozen network shares and you're left with very little space for enterprise storage. That's why Windows servers typically use a few large LUNs when they're attached to a SAN.
Ironically, this supposed limitation was removed a half-dozen years ago. All of Windows 2000's descendents support mounted drives, which are drives mapped to a folder rather than a drive letter. They function almost exactly like file system mounts in Unix, creating a hierarchical tree of drives instead of a flat lettered list. Like Unix, Windows allows you to mount an "unlimited" number of drives. Practically speaking, approximately 75 drives can be mapped this way before the system bogs down, but that should be sufficient for just about any server.
Inflexible storage configuration
Another criticism often leveled at Windows systems is their lack of flexibility in accessing shared storage. Like the drive-letter limitation issue, this is more perception than reality. In a study of storage utilization, I found Windows systems often had vast amounts of free drive space and very little free raw disk space. Most Windows systems have their storage formatted and mapped to drive letters, but leave it empty until needed.
This practice springs from the manner in which Windows systems are purchased and used, rather than from an inherent limitation in the operating system. Unlike Unix systems, which typically run a few different applications, most Windows systems are purchased for a specific application and remain dedicated to that single use. Because they'll never be reconfigured and are "spec'd out" with all of the disk space they'll ever need, just one or two large drives are normally created. This storage is locked in with no easy method for reconfiguration or sharing.
But it doesn't have to be that way. Windows 2000 also added a flexible volume manager, although it remains well hidden. Just as Sun Microsystems equipped Solaris with Veritas Volume Manager to augment its arcane Solstice DiskSuite, Microsoft turned to Veritas for Windows 2000. Veritas developed a basic version of Volume Manager in the hopes that the full version would be purchased later. Although it's unnamed by Microsoft, Veritas calls this basic version Logical Disk Manager (LDM).
LDM lets you create logical disks, which are often used as software RAID. You can combine a number of LUNs to create a single larger drive by either concatenating them (spanning) or using RAID. The basic LDM is somewhat limited in the RAID configurations offered, but it allows mirroring of LUNs, striped drives and (in Server versions of Windows) RAID 5 sets.
LDM also allows flexible drive management. You can expand a spanned drive by adding a new LUN without dismounting it, and can reconfigure other drive types without a reboot.
Windows is built to assume that all of the storage it sees belongs to it, but it has no idea how to use LUNs. In fact, most Windows systems still use a single LUN per drive, with the mapping information stored in the registry. If a server is swapped out or a LUN presentation is changed on the SAN, the admin has to manually determine which LUN should be mapped to which drive letter.
Once again, this is more tradition than a technical fact, and Windows' dynamic disks come to the rescue. Even if you don't plan to use any of the advanced features of dynamic disks (resizing, LUN spanning, mirroring and the like), you can still benefit. When you switch to dynamic disks, all of the drive configuration is stored on the LUN instead of in the registry. The upside is that LUNs can be moved from system to system without any manual reconfiguration.
This is especially important when using SAN array features like snapshots and replication. If you take a snapshot of a complicated RAID 5 set, it will be correctly configured when a new system gains access to the LUNs. This is useful for backups, populating test systems and disaster recovery.
Getting more than you pay for
Everything I've mentioned in this column is free. A premium version of Veritas' Volume Manager for Windows allows you to use these features in a cluster, and also allows you to use bigger and better RAID sets. Volume Manager allows RAID 0+1 sets, more LUNs and dynamic online growth of all RAID configurations. It also includes a nice GUI with SNMP monitoring, which is important in data center environments.
In addition, Volume Manager allows you to break off a mirror of a drive for use as a development or testing copy--a nice feature that's also offered by storage arrays. Similar functionality is included free in Windows Server 2003 as Volume Shadow Copy Service (VSS).
Even if you don't upgrade to Veritas Volume Manager, you should implement dynamic disks. The added flexibility is especially important in SAN environments. To begin using LDM, just open the Disk Management snap-in in the Computer Management tool. Right-click on a disk and convert it to a dynamic disk.
This was first published in April 2005