Pick the right ATA array for backup

VTL vendors also claimed a performance advantage over network-attached storage (NAS)-based devices, which are discussed in the next section. They say they're writing directly to the raw disk without the overhead of a file system. They might also be using a different level of RAID that's more suitable to sequentially streaming data. The DX 30, for example, uses RAID 3, which is typically used for streaming video.

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Disk-based backup information
Backup and recovery technology is changing rapidly. Some of the vendors that have disk-based backup and recovery products can be found at the Website for the enhanced backup initiative, http://www.enhancedbackup.com. Directories of such products can also be found at http://www.storagemountain.com.
Like real tape libraries, VTLs can also be shared using dynamic drive sharing software such as Veritas' SSO or Legato's DDS. An interesting point: In order to use a VTL, you must buy a robotic license from your backup software vendor for a robot that doesn't truly exist.

Emulating tape drives also gives VTLs a disadvantage. Since the backup software doesn't realize this tape drive (a sequential access device) is really a disk drive (a random access device), it will treat it like a sequential access device. The first backup will be placed at the front of the tape. Subsequent backups will be placed after previous backups on the tape. When a backup product needs to restore a backup that was placed closer to the end of the tape, it will need to read the tape sequentially until it gets to the end of the tape. Another problem is multiplexing. If a storage administrator needs to multiplex multiple backups onto a single virtual tape drive in order to achieve the maximum throughput of that device, multiplex backups will be interleaved onto the tape just as they would to a tape drive. When a backup product needs to restore or copy one image from a multiplexed backup, it will need to read and disregard the other backups within that multiplex image, just as it would with a tape drive. As you'll see in the next section, NAS-based devices don't have this disadvantage.

NAS devices
Instead of purchasing an ATA-based RAID array and connecting it to your system via FC, you could purchase a NAS device based on ATA disk drives. Some NAS filers were always based on ATA disk drives, such as Quantum's products. Other vendors' filers are based on SCSI or FC disk drives. Some of these vendors have recently created ATA-based filers, and they are actively marketing them in the archiving and backup and recovery markets. Network Appliance's NearStore is an example of such a product.

Using a NAS appliance instead of a basic RAID array solves the sharing problem described earlier. All servers that can mount the NAS appliance can use it as a destination for backups. Since each server sharing the appliance will only be writing to a separate file within a large network file system, sharing isn't a problem. NAS vendors will also tell you advances in NFS, CIFS and NAS technology will allow your backup server to write to the NAS appliance with minimal latency and maximum bandwidth - usually allowing you to fill the entire Gigabit Ethernet type with backup data.

One perceived disadvantage to NAS-based devices is that your backup software must support backing up to a file system device in order to back up to a NAS device. However, almost all backup software supports such functionality. Although integrating this device into your backup system wouldn't be as seamless as installing a VTL, it's still relatively simple. You mount the NAS device to your backup server, create a directory for each destination device you wish to create and then configure your backup software to use those directories as destination devices for backup. And, unlike VTLs, they don't require purchasing a robotic license or drive sharing software.

The advantage to NAS-based devices is your backup software knows they are random access devices. When writing the first backup to a file type device, it creates a new file within the directory you created. When making subsequent backups to the same device, it makes new files for each backup sent to the device. This is true even for multiplexing. If you send several simultaneous backups to a file type device, each backup would create a separate file on the file system. This means that when the backup product needs to read a particular backup - whether for a restore or for duplication - it knows exactly which file it wrote to, and will open only that file and read it. So restores happen instantaneously, instead of waiting while the tape reads unneeded data that it must disregard. Customers can also make the multiplexed backups from multiplexed originals with no performance impact.

As you can see, VTL and NAS vendors are able to claim both advantages for their product and disadvantages for the other type of product. Only testing in your environment will prove which one is right for you.

Backup and recovery software
Also participating in this revolution are traditional backup and recovery software vendors. Most of them are confirming they can write to virtual tape libraries and file type devices, and are making sure they can automate copying to (real) tape the backups that were sent to these devices. At least one vendor, BakBone Software, has written their own virtual tape library software allowing standard disk to behave like a tape library. At this time, this software only works with NetVault, their backup software product.

There's also a new genre of backup and recovery software. These products were written from the ground up with the understanding that the primary backup medium would be disk, which allows them to use a number of new backup methodologies that wouldn't be possible with tape. These include concepts such as Single Instance Store (SIS), a technology that backs up a single copy of the each file, regardless of how many times that file resides in your enterprise. Imagine backing up only one copy of word.exe, or df. They also include block level incremental backup, where only the bytes of a file that have changed are backed up. Traditional backup software backs up the entire file when only one byte changes. Using both of these technologies together doesn't work well when tape is your primary backup medium, because they result in a single system being backed up to many tapes.

This relatively new industry includes products such as Connected's TLM, Veritas' NetBackup Professional, EVault, Nexsan's InfiniSAN D2D, and Avamar's DPN. You should expect to see many more products like this in the near future.

This was first published in September 2002

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