Pick the right ATA array for backup

In the June issue, (see "Surprise! Cheap disks cure slow backup") I introduced the idea of using inexpensive ATA-based disk arrays as the primary storage medium for a backup and

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recovery system. Instead of backing up to a tape library, your initial backup goes directly to these new ATA disk arrays and is later sent to tape for off-site and long-term storage. This article further examines the options available in this new world of disk-based backup and details the pros and cons of using various products with ATA disk arrays.

RAID arrays
There are a myriad of vendors offering inexpensive RAID arrays based on ATA disk drives and FC connectivity. These boxes usually offer hardware-based RAID supporting RAID 0+1, RAID 1+0 and RAID 5. What these boxes lack is a file system. How you use these RAID arrays in your backup system depends on your configuration and what you hope to achieve.

ATA's attractiveness
The non-streaming, random accessibility of a disk is better suited to the random and inconsistent streams of data that backups produce.
If you're making copies of your backups, it's easier to copy from disk-to-disk than it is to copy from tape-to-tape.
Some of these disk arrays allow for easier sharing of hardware resources during backups.
Unlike tape, disk storage can constantly be monitored for failure. A faulty disk drive can be replaced without data loss or service interruption. In contrast, you never know a tape is good until you really need it.
Disk drives can easily be protected via RAID without significantly impacting performance. Tape RAID exists, but it actually exacerbates the streaming problem.
Creating an array of five 10MB/s tape drives creates one 40+MB/s virtual tape drive. Try streaming that tape drive without a significant level of multiplexing.
Suppose you want to back up everything across the LAN to a central backup server, and you simply want to use this inexpensive RAID array as the destination device for its backups. To do this, simply choose a RAID level, create your virtual disk drives using that RAID level, connect the array to your backup server, create a file system on the array and use this file system as the destination for your backups.

What if you had a master backup server, four media servers and a terabyte-sized disk array and you wanted each of these servers to be able to use the RAID array as the destination device for backups? There are three ways to do this. The first method is inexpensive and simple, but lacks some flexibility. Create five 200GB LUNs, attach the disk array and five servers to a storage area network (SAN), create a 200GB file system on each server using one of the five LUNs and then use this file system as the destination for backups on that server.

The disadvantage to the above method is when one server needs far less than 200GB, and another server needs more than 200GB. There are two ways to get around this problem. The least expensive way would be to attach the disk array to the back of a low-cost Windows or Linux box, and share the RAID array via Common Internet File System (CIFS) or network file system (NFS). The viability of such a solution will be based on the power and speed of your network, as well as the power and speed of the Linux or Windows server. Another option would be to connect the RAID array to a SAN and use a SAN-accessible file system, such as Sistina's Global File System or Veritas' SAN Filesystem. This option is obviously more expensive than the previous option, but it allows you to share the entire capacity of the array with each server on the SAN. Some vendors that are offering ATA-based, FC attachable disk arrays include Axus, Nexsan, and Zzyzx.

VTL appliances
The next way that ATA disks are being used in backup is with virtual tape library appliances (VTLs). VTLs consist of a RAID array, usually custom-built for the appliance, and some type of firmware or software. Physically, a VTL doesn't look much different than a FC disk array. On the outside, you would find one or two FC interfaces, and if you were to open it up, you would find an array of ATA disk drives. What makes this different from a regular RAID array is the software (or firmware) running inside the VTL. It causes the RAID array to appear to be an actual tape library. For instance, the Quantum DX 30 appears to be a Quantum/ATL P1000 library with two or six DLT tape drives and 30 slots preinstalled with media. Other VTLs - such as Hitachi's VTLA - are able to emulate many different types of tape libraries.

As of this writing, there are only two such VTLs in existence: Quantum's DX 30 and Hitachi's VTLA. The DX 30 is a complete solution built entirely by Quantum. Hitachi's product, on the other hand, was created by a cooperation of three companies. Hitachi built the hardware, Nissho Electronics helped with planning, and Alacritus wrote Securitus, the software that makes the Hitachi disk array appear as a tape library.

The biggest advantage of VTLs is they can be seamlessly integrated into your enterprise without requiring any change in how you do backups. To the storage administrator, it looks like another tape library to back up to. You stop using one tape library and start using this one. You will, of course, need to duplicate your backups from the VTL to a real tape library if you wish to ship your backups off-site. If you have connectivity to your off-site storage vendor, you could duplicate your backups from one VTL to another.

This was first published in September 2002

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