D2D Backup: Disk's dual role



Your backup choices are much greater and more complex compared to the days when you simply had to choose a backup application and tape library. Disk has solved the reliability and performance issues that most storage managers have experienced with traditional backup systems, but disk-to-disk-to-tape (D2D2T) has complicated the backup and restore processes. Users must now choose from among three backup architectures and four types of disk-based backup targets. This three-part series on disk-to-disk (D2D) backup describes the pros and cons of each approach to help you clarify your choices.

The traditional backup architecture
Before explaining how the different D2D backup options work, it's important to understand the backup systems they'll work with and why you might want to augment those systems with disk. In a traditional backup architecture (see Traditional backup architecture, this page), software resides on the backup client (the server to be backed up) that allows the backup server to transfer that client's data to tape, disk or virtual tape. The data may be transferred across the network (LAN-based), from the client directly to tape/disk across the storage area network (SAN) (LAN-free) or directly from primary storage to secondary storage across the SAN (server-free). In each case, the data is converted into a different format that's understood by the backup software running the backup. This format could be tar, cpio, dump, NTBackup or a custom format understood only by that particular backup package.

The biggest advantage of the traditional backup architecture is that it's well understood and mature. The biggest disadvantage is how it uses tape. It's difficult for a traditional backup system to use the streaming nature of modern tape drives efficiently. To properly stream tape drives, some backup software products (like EMC Corp.'s Legato NetWorker and Veritas Software Corp.'s NetBackup) send multiple backup jobs simultaneously to the same tape drive, a technique called multiplexing or interleaving. This technique helps backups but has a negative impact on the restore of a single backup; the backup software has to read the entire tape and disregard the data it doesn't need. Other backup software products, such as IBM Corp.'s Tivoli Storage Manager, solve the streaming issue with disk staging where backups are first sent to disk before they're sent to tape.

With the advent of lower-priced ATA-based disk arrays, however, everyone can take advantage of disk staging or disk-based backups without switching from a traditional backup architecture. You simply augment your tape library with a combination of disk and tape.

Disk backup options
There are four ways to augment your traditional backup system with disk. The first two options are called disk-as-disk because they involve using disk drives behaving as disk drives--the disks aren't pretending to be tape. In a SAN disk-as-disk configuration (see SAN disk-as-disk, this page), 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 (see NAS disk-as-disk, this page), the disk resides behind a filer head that shares filesystems via NFS or CIFS, and backups are sent to those filesystems.

The last two options employ virtual tape libraries (VTLs), where disk systems are placed behind a server running software that allows the disk array to pretend to be one or more tape libraries. Standalone virtual tape library, ( on this page) shows standalone VTLs that sit next to a physical tape library and pretend to be another tape library. Once you back up to a standalone VTL, you must use the backup server to copy its backups to physical tape if you want to send them offsite.

An integrated VTL (see Integrated virtual tape library, this page) sits between a physical tape library and a backup server, where it pretends to be a physical library. The backup server backs up to the integrated VTL, which then copies the data to the physical tape portion of its library.


D2D2T Backup
Advantages of disk-as-disk targets



Disadvantages of disk-as-disk targets





Tip: Don't forget about restores
Tip: Disk-as-disk licensing is changing
  • 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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