LTO-4 fed from staged data on RAID-0 array

IT-Monkey-Dave used Ask the Experts™
I'm scoping out a new enterprise backup solution for us and don't want to make any big mistakes.  My goal is to build a viable backup solution that will work for us for the next 4 years, minimum.

We're running ArcServe for Windows R12.5, with various agents etc.  It works well and we're a Windows house, so that is unlikely to change.

Currently we're using a Sony AIT-4 8-slot library.  The nature of the data we back up is such that it doesn't compress well at all, so compression is not even a consideration.  I get the native 200GB capacity of the AIT-4 media and no more.  A full backup right now is ~1.1TB, which takes 6 AIT-4 media.  Staging that data to disk takes ~12 hours.  Then committing it to tape takes ~16-17 hours.  That works out to *roughly* 65-70GB/minute to tape, which is probably all the AIT-4 drive is capable of doing under perfect conditions.

I can see a 2x or even 3x increase in our backup load looming in the next 12 months.  That's just not going work with the current AIT-4 system unless I split up into multiple backup jobs, buy more media, etc.  Not appealing, too much hassle and too long backup window(s).

I'm looking at a Dell TL2000 LTO-4 library.  It has 24 media slots; native capacity per tape is 800GB.  Claimed throughput is many times faster than the AIT-4 drive.

I want to make darn sure the backup server can stage all this data and feed it to the LTO-4 drive as fast as the drive can take it.  No "shoeshining".  

I'm looking at a Dell R710 server to host ArcServe.  Probably will run Windows Server 2008 Std. R2 x64.  The question is, what kind of drives to use in this server for staging the data.  I plan on 2 smallish drives running RAID-1 for the OS and apps.  Then 4 drives of some type configured in RAID-0 as the staging device.

If I go with, say 4 x 1TB 7200 RPM SAS drives configured as RAID-0, I'll have about 4TB for staging.  

My question is: Can those drives, configured that way, feed data to the LTO-4 tape drive as fast as it can take it?  Any thoughts or insight on this is greatly appreciated.  Likewise if you see a major gaping hole in my plan.
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It's unlikely that you will avoid shoeshining altogether, but your goal is a worthy one.  LTO4 drives can easily write at 100+ MB/s sustained throughput.  Not much is capable of throwing that much data at it continuously.  With a 4-drive RAID-0, I think the bottleneck would probably be your system bus and/or your RAID controller.  If your server is pretty old, you might be better served by just getting a new system with modest disks than a crazy-fast disk system on an old server.  

Median file size also plays a big role in determining how much throughput you're going to get.  Larger files allow you to ramp up to higher speeds vs a bunch of small files, as processing header information on reads and cataloging each file creates a lot of overhead which steals cycles and prevents the system from keeping the write buffers full.  

Which brings me to my last will definitely need to tweak the buffers in your backup software.  I'm not familiar with ArcServe, but most backup software allows you to tweak the size and number of write buffers.  You will need to do a fair amount of testing different sizes and combinations of buffer parameters until you find the combination that gives you the best performance.  If you can look through the software documentation, look for a section on performance tuning.  

Oh, and make sure the system attached to the tape library and disks has plenty of RAM available.  For instance, if you have your backup software running locally on that box as well, you'll need at least 1 GB RAM for Windows Server 2003, 0.5-1 GB for your backup software, and at least 1 GB for your buffers.  Add on memory requirements of anything else that's running on that box.  

Best of luck,
Top Expert 2014
Here's a speed table with how fast, and equally as important how slow, they can go. It doesn't matter if you leave compression on with uncompressable data, it just doesn't write the compressed version to the tape if the uncompressed version is smaller, useful feature of LTO.

You scould do the sizing assuming you get 3:1 compression on some parts of the data, so supply the drive with 3 * 120 = 360MB/s, I don't see you getting this with just 4 SATA disks, I'd get an external SAS/SATA enclosure and fill it with a dozen 500GB ones. For once I won't say get fast SAS disks since staging area should be sequential access so not many disk seeks.


I'll get back to this question soon, have not abandoned it.  Just been busy with other issues.  Appreciate the responses!


Appreciate the comments, it is helpful.

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