D2D Backup: Disk's dual role


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Tip: Don't forget about restores
  • If you only buy enough disks to hold a few nights' backups (i.e., disk caching), you'll speed up backups but won't speed up restores.
  • If you want to speed up backups and restores, you should buy enough disk to hold all onsite backups.
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.
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 to the filer head.
About the series: This article is the first in a three-part series that describes the various ways to use disk to protect data. The series explains the advantages and disadvantages of the four types of disk-based backup targets, including storage area network (SAN) disk-as-disk, network-attached storage (NAS) disk-as-disk, standalone virtual tape libraries (VTLs) and integrated VTLs. The first two articles cover how--and why--users are augmenting their traditional backup and recovery systems with disk, with the second article focusing on VTLs. The concluding article describes the new backup and recovery architectures that have been enabled by inexpensive disk-based targets, including replication-based backup and data-reduction backup.
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.

This was first published in February 2005

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