On 8/3/2011 8:54, Invisible wrote:
> They replicate at the filesystem level.

Sorta. I guess if you call the separate library with separate semantics a 
"filesystem", then yes. (And yes, I do, but many wouldn't, for some reason.)

> Then again, all their machines run the same application.

Huh? No they don't. That doesn't even make sense, since then the only 
application they'd be running would be "the file system."  Heck, even the 
file system has three or four separate applications to run it.

> Business datacenters aren't usually like that. Even so, it seems "obvious"
> to me that the network should operate above the filesystem level, not below it.

Personally, I've never figured out the draw of paying more for a terabyte on 
dedicated hardware that's not any more reliable or inexpensive than just 
plugging a terabyte drive into a PC.

>> Nothing wrong with spinning disk backup, especially if it's more
>> reliable than the tape itself.
>
> This is the thing, really. Disks spin constantly.

Not backup disks. No more than your backup tapes from last year do.

> Did I mention that tape is cheaper?

This is true, yes. That's the one benefit.

> Note that for our purposes, security information, file modification times
> and so forth must also be reliably stored and retrieved.

Sure. You can still do that with disks and hard links. If the security 
information or file modification times change, you add it to the backup. 
Good backup software will keep a checksum and either not actually copy the 
data if only the metadata has changed.

> (We /know/ that disks have an annual failure rate of about 3% to 5%. We
> don't know what the rate is for tape.)

Depends if they're spinning or not. I can guarantee that if you left a tape 
moving like you say you want to leave disks moving, you're have failures 
much more frequently.

>> Once it fills up, you disconnect it and put in a new one, and you put
>> the old on on the shelf.
>
> And I thought drives don't like power cycles...

How often are you going to power cycle it? You'll turn it off when it's 
full, and not turn it on again until you're ready to use it.

I think it's not so much that disks don't like power cycles (especially 
nowadays in the last 10 years or so), but that most failures happen when 
powering them up or down.

In any case, spinning up the drive once a day and then turning it back off 
again isn't going to significantly decrease its lifetime. Buy a disk 
designed for that sort of use, rather than a "server" disk that's optimized 
to not fail when it's always spinning.

> I already know for a fact that they haven't decided how to implement this
> yet. They're currently looking at options. I will of course make sure they
> know what the hard requirements are.

Well, that's all you can do, really. :-)

-- 
Darren New, San Diego CA, USA (PST)
   How come I never get only one kudo?

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