Some things that you need to be aware of...
LTO-4's native speed is 120MB/second. LTO-4 has adaptive write speed that goes down in HP's case to 1/3 of the native full speed. So 120/3 = 40MB/sec. Multiply that 40MB/sec by the compressibility of your data -- if your data isn't compressible, keep the 40MB/sec. If it is 1.5:1 compressible, you get 60MB/sec. 2:1, 80MB/sec. I'll assume here that your data is not compressible, but factoring in compression is important..
If you can't feed your tape drive at least 40MB/second times compression, you're going to be putting both the tape drive mechanism and the physical tape media under a lot of stress, leading to premature drive failure and unreliable tape writes and reads.
Now, you're talking about striping four LTO-4 drives together. That means that you'll need to feed data to them at at least 4*40 or 160MB/second. In the Real World, you need to pump more data than that to make up for times when the data comes across slower, or the data happens to be more compressible than your "average" -- possibly you'll need to push significantly more data to the tape drives than that.
Few "real world" disk subsystems can be read at 160MB/second, unless you're talking about extremely fast, tuned disk with almost exclusively large files (10MB and up in size). And remember, this assumes that your data is not compressible. If it's compressible at all, you have to multiply that 160MB/sec by the compression ratio.
One other thing that makes this even worse... if you're coming off a single array or disk, then additional simultaneous tape jobs mean that you'll be putting the disk heads under increasing contention. The more the disk controller has to move the head from Stream A to Stream B to Stream C... the more you'll be wasting time on head movement and settle times, and the worse your performance will be.
That said -- if you really want to do this, there was, and possibly still is, a company called Ultera in Southern California that provided a very slick tape RAID (properly, RAIT) device that would mirror, or stripe, or stripe with parity, so that you could survive the loss of up to one physical tape cartridge.
If I had a situation like yours where I had a limited backup window, one server to back up, and four available tape drives, I'd consider one of the following options:
1) Divide the data on the disk in to four approximately equal portions (based on top-level directories). Perform a full backup of portion 1 on Friday night. A full backup of portion 2 on Saturday night. A full backup of portion 3 on Sunday night. A full backup of portion 4 on Monday night. Any night when a portion is not getting a full backup, perform an incremental or differential backup on it (So Sunday is full backup of 3, and differential backup of 1, 2, 4, for instance). Heck, you could split your data into six or seven "portions" with this scheme, and it will still work well.
2) Move to a backup application like HP Data Protector or IBM Tivoli Storage Manager that perform Incremental Forever backups. In this case, you create a backup-to-disk partition. You perform a full backup on day 1 to disk. From then on, you perform only incremental backups to disk, daily, so your backup time is much less than before -- say 5% or less. Periodically you generate a "synthetic full" backup to physical tape, where the backup application uses that initial full and the intervening incremental backups to create a physical tape that is the same as if you'd done a full backup on that particular day.
3) Depending on how much data you have, your budget, and your retention policies, it might be easier to use a D2D Backup System or VLS to give you a disk target, with deduplication and replication, so that you can replicate to a second site for disaster recovery.
Remember: LTO-4 is fast... the tape drive is not likely to be your bottleneck; when I evaluate customer performance, the bottleneck is almost always the disk subsystem... or occasionally the network if performing network backups.
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by: TapeDudePosted on 2009-11-03 at 13:41:01ID: 25734096
NetBackup will do this, but Backup Exec won't.
When BackupExec was owned by Veritas, they had a program called RAIDDirector that would do this, but I don't think it's supported anymore.
I believe the reason BE won't do it is that it uses MTF (Microsoft Tape Format) as the data format, and Microsoft didn't write striping into the spec.