If the VTLs I test could only supply data at 40 MB/s, I wouldn't recommend them
either. The ability of a disk device to stream to tape is job #1, AFAIC. Just
remember that not all VTLs are created equal.
Don't get me wrong. I'm not an AFTD hater. I think they're fine for smaller
shops that want an easy solution. But when I get to a larger shop, I need the
ability to make things go more concurrently, and I think it's possible to do
that with a VTL and VERY difficult to impossible to do it with an AFTD.
VTLs aren't the end-all-be-all, and some of them are crap. (The same is true
of the disk systems behind a lot of AFTDs.) They're also more management in a
SMALL NW environment than AFTDs. But in a large environment, where
staging/cloning performance is king, I'll take a VTL any day. Yes, I'll be
doing some scripting, but at least I can clone/destage multiple backups at the
same time. That's NOT the case with an AFTD unless you make a bunch of them
and that creates other management hassles, and decreases disk utilization.
I think the concurrent restore thing is a red herring. Sure, you can't do two
restores at once from a tape, but:
1. Most people don't do more than one restore a month, let alone more than one
a day, let ALONE two at the same time.
2. Add to that the odds that the two restores you do at the same time would
need the same tape at the same time. (Multiplexing would increase the chances,
but one should not multiplex to virtual tapes.)
3. If you want better odds, just use smaller virtual tapes. It costs you
nothing, adds no management issues (like more AFTDs do), and significantly
decreases the chance that two restores would be grabbing for the same tape.
So, to summarize:
1. I think it's easier to destage multiple backups simultaneously from a VTL
than it is to do it from an AFTD, and that's something you do every night.
2. Performance counts. If your device, regardless of type, can't stream your
tape drive type, you bought the wrong one.
3. Sure I can do concurrent restores from the same "tape" with AFTDs and I
can't with VTLs, but I don't think that happens much and if it does I can
address it with smaller virtual tape sizes.
Isn't healthy discussion fun?
Curtis Preston | VP Data Protection
GlassHouse Technologies, Inc.
T: +1 760 710 2004 | C: +1 760 419 5838 | F: F: +1 760 710 2009
cpreston AT glasshouse DOT com | www.glasshouse.com
Infrastructure :: Optimized
-----Original Message-----
From: EMC NetWorker discussion [mailto:NETWORKER AT LISTSERV.TEMPLE DOT EDU] On
Behalf Of Francis Swasey
Sent: Tuesday, August 19, 2008 6:13 PM
To: NETWORKER AT LISTSERV.TEMPLE DOT EDU
Subject: Re: [Networker] Query in Staging from adv_file
With VTL, Networker limits you to one operation at a time per virtual
Tape. I've heard tales that if you are lucky enough to issue two
recover commands at the exact same instant, that NetWorker can be
tricked into doing two restores from the same tape at once.
With the AFTD, you can do multiple concurrent writes and either multiple
restores or a stage or a clone at the same time per AFTD.
In my environment, I've tested a VTL and it was a lousy performer. I
wasted an entire month and had to put up with a lot of pressure from the
sales people who really wanted me to spend a small fortune on their
appliance when they couldn't make it perform at even one third of the
I/O performance of the AFTD on a NexSAN SATABeast.
If I had to do my weekly clones with a VTL that was flat out retrieving
data at 35-38MB/second to write to LTO4 tape drives that can do
120MB/second -- I'd be looking for a new job! Thankfully, on my big
fileservers with the big savesets, I'm pushing the LTO4's to their
limits and my clones are finished in less than 24 hours. Back when I
was doing clones from LTO4 to LTO4, these same clones ran for three
days. If I was doing that with that bad VTL, I don't know that I'd
successfully be able to clone the data during the week before the next
round of weekend fulls were written and needed cloning.
In my environment, AFTD wins.
Frank
On 8/19/08 6:16 PM, Curtis Preston wrote:
> Well, that significantly decreases the value of AFTDs over VTLs in
> NetWorker, now doesn't it?
>
>
>
> ________________________________________________________
> Curtis Preston | VP Data Protection
> GlassHouse Technologies, Inc.
>
> T: +1 760 710 2004 | C: +1 760 419 5838 | F: +1 760 710 2009
> cpreston AT glasshouse DOT com | www.glasshouse.com
> <http://www.glasshouse.com/>
> Infrastructure :: Optimized
>
>
>
> ________________________________
>
> From: Francis Swasey [mailto:Frank.Swasey AT uvm DOT edu]
> Sent: Tuesday, August 19, 2008 2:22 PM
> To: EMC NetWorker discussion; Curtis Preston
> Subject: Re: [Networker] Query in Staging from adv_file
>
>
>
> Sadly, no. The nsrstage will serialize their use of the single RO side
> of the AFTD. With multiple nsrstage processes trying to access the same
> AFTD, you will get alerts that they are waiting for the AFTD.RO
> device/volume.
>
> Frank
>
> On 8/19/08 5:10 PM, Curtis Preston wrote:
>
> I understand that each staging process will only use one drive at a
> time, but is there a way that he can get more than one staging process
> reading from the same AFTD?
>
> What if he doesn't do it automatically, but kicks off several nsrstaging
> processes manually? Is that possible? Will they then each read some
> backups and stage them to tape? (I haven't done much with nsrstage.)
>
>
>
>
>
>
>
--
Frank Swasey | http://www.uvm.edu/~fcs
Sr Systems Administrator | Always remember: You are UNIQUE,
University of Vermont | just like everyone else.
"I am not young enough to know everything." - Oscar Wilde (1854-1900)
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