These disk-based backup and recovery systems offer two main benefits: It conserves disk space and it significantly decreases the amount of data transfer during a backup. This second benefit is why many of these products are designed to backup remote users. The amount of data transfer during a backup is small enough to be sent across a dial-up Internet connection.

What if we turned the tables on this setup? Instead of using a disk-based backup in your data center to back up remote computers, what if you made the backup system remote, and used it to back up the local computers? Place one of these disk-based backup and recovery products in a remote data center or colocation facility, and back up your computers across the Internet. Once the backup has been completed, the amount of data that's transferred during a single instance store, block-level incremental backup is actually quite small.

Putting the backup system off site lets you recover from a disaster without constantly moving tapes between your on-site and off-site locations. In this case, tapes would only need to be created for archival purposes. If you want an additional copy of your backups located off site, there are products that support automated replication to a second or third facility. Similar to the replication products mentioned earlier, these products range from Windows-only products costing less than $1,000 (NetBackup Professional and NetWorker Laptop) to costly heterogeneous products designed

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to back up your whole data center to disk (EVault's InfoStage and Avamar's Axion).

Server-free and client-free backups

Server-free backups are represented by the green arrow and the client-free backup travels the red arrow path.

Server-free and client-free backup
ATA disk arrays can be used in server-and client-free backup implementations. No backup is completely LAN-free, client-free or server-free, of course. The backup server is always communicating with the backup client, even if it's just to get the metadata about the backup. These terms illustrate that the bulk of the data is transferred via a path that doesn't involve a server or the client that's being backed up.

If the SAN connected to the disk storage supports a SCSI feature called extended copy, then the data can be sent directly from disk to tape, without going through any server. There are also other--more proprietary--methods for doing this that don't involve the extended copy command. This is the newest area of backup and recovery functionality being added to SANs. Server-free backups are represented (see "Server-free and client-free backups") by arrow no. 1 , which shows a data path starting at the disk array, traveling through the SAN switch and router and arriving at the shared tape library. The data path doesn't include a server, which is why it's called server-free backup.

If a backup client has its disk storage on the SAN, and that storage creates a mirror that's split off and made visible to the backup server, that client's data can be backed up via the backup server--the data never travels via the backup client. This is called client-free backup. Client-free backups are represented in "Client-free and server-free backups" by arrow No. 2, showing a data path starting at the disk array, traveling through the backup server, followed by the SAN switch and router, and finally arriving at the shared tape library.

Both client-and server-free backups require a consistent, frozen image of the backed up file system in order to safely transfer it to the backup system via a non-traditional data path. One of the biggest barriers to implementing a client-or server-free backup is the cost of the disks required to create this frozen image. However, there are some technologies that allow a frozen image to be created by splitting off a mirror, but they create an additional I/O on the primary disks.

Most client-and server-free backup systems use the mirroring capabilities built into enterprise class disk arrays, such as EMC's TimeFinder or Hitachi's ShadowImage. While these products are good, they can only mirror between like disks. That is, if you want to create a mirror for use in a client- or server-free backup system, you'll need to purchase enough SCSI or FC disks to create that mirror.

But if you only need this disk array for backup, does it need to be high-speed, expensive SCSI disks? The answer is no. What if you used a software volume manager (e.g., Veritas' Volume Manager) and created the third mirror on an ATA disk array? There are recently available products that can make one manufacturer's array look like another manufacturer's array. (Check out Rhapsody--a company currently being acquired by Brocade.)

You can use these disk arrays to increase the level of data protection in your environment. First, you can use them as a target for replication or replicate your entire data center to an off-site facility. You can replicate remote offices to your main data center, allowing you to back up from there. You can create a disk-based backup and recovery system and place it off site, which lets you have a completely automated backup and recovery system for off-site storage--without having to constantly move tapes back and forth. You can also use these arrays to create an additional mirror of important data that can be split off for use in a client-free or server-free backup system.

This was first published in February 2003

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