Feature

How disk has changed backup

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SAN disk-as-disk targets
A SAN disk-as-disk target is simply a disk array connected to the SAN and attached to one or more backup servers. (Some products, like IBM's TSM, can back up to raw disk.) The backup server typically puts a filesystem on the array and writes to that filesystem. The advantage over a NAS disk-as-disk system is the better write performance typical of a high-end SAN disk array compared to an Ethernet NAS filer.

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TIP: DISK-AS-DISK LICENSING IS CHANGING
  • Most backup software companies will begin charging to back up to a disk-as-disk target.
  • Ask your backup software vendor what its plans are.
However, when you use a disk array as your backup target, you replicate into your secondary storage all of the provisioning issues of your primary storage. All of that hassle with associating disks to RAID groups, RAID groups to servers, and volumes to filesystems now needs to be done on the back end of your backup system. This problem is compounded when there are multiple backup servers. When using a tape library or VTL, most backup software packages know how to share these devices; they don't know how to share a SAN-based filesystem. If you're using a SAN disk-as-disk target with multiple backup servers, you'll have to decide how large each backup server's volume needs to be and allocate the appropriate amount of space to each backup server.

NAS disk-as-disk targets
A NAS disk-as-disk target solves the provisioning issues of a SAN disk-as-disk target by putting the disks behind a NAS head, making a giant volume and sharing that volume via NFS. Generally, such systems are easier to maintain than traditional disk arrays. But easier management comes with a price. The filer head and filer OS increase the cost of the system, and performance will be limited to the throughput of the filer head. Depending on the size of your backups, performance may not be an issue. If you're a NAS shop with many other filers, a NAS disk-as-disk target makes sense--especially if you're using replication-based backup.

TIP: SAN VS. NAS PERFORMANCE
  • SAN arrays will offer better backup system performance.
  • NAS filers will be easier to manage and maintain, but throughput will be limited by the filer head.
Disk-as-disk targets provide a quick and inexpensive way to start backing up to disk. Yet they have many disadvantages when used with a traditional backup system. If you're going to use a disk-as-disk system, you'll need to choose either a SAN or a NAS unit. A SAN device may be more powerful than a NAS unit, but the SAN device will be more difficult to maintain and share.

Next we'll explore how to use VTLs with your backup system; describe their advantages and disadvantages vs. disk-as-disk systems, and explain the advantages and disadvantages of the two different kinds of VTLs.

VTL advantages
VTLs offer two main advantages over disk-as-disk backup targets: ease of management and better performance. As described earlier, there are various ways to use disk to protect data. A disk-as-disk target requires all of the usual provisioning steps of standard shared storage arrays. In contrast, if you tell a VTL how many virtual tape drives and virtual cartridges it should emulate, the VTL software automatically handles all of the provisioning and allocates the appropriate amount of disk to each virtual cartridge.

If the VTL needs to be expanded (not all VTLs are expandable), you just connect the additional storage, tell the VTL it's there and the VTL will automatically begin using the new storage. There's no volume manager to run and no RAID groups to administer.

Another important management advantage of VTLs is how easy it is to share VTLs among multiple servers and apps (see "Should a virtual tape library be shared?"). To share a VTL among multiple backup servers running the same software, use the built-in library sharing capability most backup products have. To share a VTL among multiple servers running different apps, partition the VTL into multiple smaller VTLs, assign a number of virtual cartridges to each VTL and associate each VTL with a different backup server. These scenarios are much easier than what's required to share a disk-as-disk target among multiple backup servers.

This was first published in November 2005

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