Linux Tape Device Emulation?

Mon Jan 16 16:51:43 PST 2006

> Oddly enough, we have the same exact solution from what I've heard, the 
> Mirapoint! Small world.

:-)  Disclosure: I work for Mirapoint.  But I run my box in production 
just like anyone else: www.imap-partners.net.

> I'm always a bit hesitant to purchase used gear, especially when it 
> comes to tapes (given the moving parts). Ideally,
> I'd just hook an lto-3 onto it, and take advantage of the speed 
> (576gb/hour), which meant if I was at full capacity (100gb)
> a full backup or restore should be around 20 minutes, which has it's 
> appeal.. and around a $7k pricetag (for which I
> could build two very redundant file servers as dedicated backup servers, 
> and have enough $$ to take my wife out to
> dinner at French Laundry).

*ahem* Tape speed isn't the primary constraint.  LTO-1s are going for 
$500 on eBay; I got a couple from a local fellow for less.  Buy two 
(more if you want to clone tapes and still keep a spare). Spend your 
money on a _new_ cleaning cart and some _new_ tapes.  They're at the 
sweet spot on the price curve, too, lately.

But here's something for nothing: Your Mirapoint box will offline the 
local tape after every operation -- can't be made not to -- and LTO's 
(unlike DLT-7000, at least) need human intervention to re-load and read 
again for a restore.  (A DLT-7000 just turns off the "operate handle" 
light and carries on.  The LTO you have to stuff back into the slot. 
Maybe that's not a problem for you.  My 80GB M300 won't fit on a 
DLT-7000 tape any more.  It's 35/"70", note quotes.)  Via RMT, you can 
point it at the no-rewind device and otherwise have appropriate control 
over this.

I measured speed writing to a local DLT-7000 from my M300.  About 4h for 
30GB doing 'backup full tape ""' (the default, and fixed, blocksize). 
Use RMT with the default blocksize ("") via a private 100Base-T network 
to a FreeBSD 4.6 box with the same drive on a wide SCSI bus, and you're 
looking at 24h for the same dump.  Crank up the blocksize argument there 
to 2MB and it's back in the range you expect.  (_That_ was a long 
weekend in the lab.)

This is the file-by-file dump, and opening files is expensive, so you 
won't saturate a tape drive, or even 100Base-T necessarily.  I'm happy 
keeping it streaming.  Via NDMP it can do image dumps and get around 
this, but yes the DMA is in another league.  For now.

> On the other hand.. lto-1 drives have the capacity I need, and a good 
> pricepoint .. around $1k it seems.  What do the datadomain
> boxes look like pricewise?

No idea.  But I used to work for Rex when he was at Mirapoint (he's the 
King of the SEs at DataDomain now), and I happen to have chatted with 
him the other day.  Sounds very interesting, but I'm old-school: I like 
my dumps removable.  Bearings die; heads crash.  Yes, tape drives have a 
lot of moving parts and they're objectively less reliable than disk 
drives.  But _a given tape_ isn't so bad.  And I can afford to be 
careful: I test my backups routinely.  If it wouldn't wear out my co-lo 
guys, I'd send two copies offsite every Monday, to two different 
tectonic plates.