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Disk-based backup for SAN and NAS

Disk-as-disk backups involve using disk drives behaving as disk drives -- the disks aren't pretending to be tape.

This article first appeared in "Storage" magazine in their February issue. For more articles of this type, please visit

What you will learn from this tip: The differences between SAN and NAS disk-as-disk configurations.

Disk-as-disk backups involve using disk drives behaving as disk drives -- the disks aren't pretending to be tape. In a SAN disk-as-disk configuration, a disk array is connected to one or more backup servers via a SAN, and a disk volume is assigned to each server. Each server then puts a filesystem on that volume, and backups are sent to that filesystem. In a network-attached storage (NAS) disk-as-disk architecture, the disk resides behind a filer head that shares filesystems via NFS or CIFS, and backups are sent to those filesystems.

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. The backup server 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.

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 you have multiple backup servers. When using a tape library or VTL, most backup software packages know how to share these devices. 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 speaking, such systems are easier to maintain than traditional disk arrays. But that easier management comes with a price. Both the filer head and filer operating system 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, however, performance may not be an issue. If you're a NAS shop with many other filers, a NAS disk-as-disk target makes perfect sense -- especially if you're going to use replication-based backup.

Disk-as-disk targets provide a quick and inexpensive way to start backing up to disk. Yet they also 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. In the next article in this series, we'll explore how to use VTLs with your backup system. We'll describe their advantages and disadvantages vs. disk-as-disk systems, and explain the advantages and disadvantages of the two different kinds of VTLs.

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For more information:

Tip: Think before you invest in disk-to-disk backup

Backup School: Lesson 2 -- Which backup media is right for you?

Tip: How to architect tiered backup with D2D2T

About the author: W. Curtis Preston is the vice president of GlassHouse Technologies, Framingham, Mass. He is also the author of "Using SANs and NAS," "Unix Backup & Recovery" and the "Storage Security Handbook."

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