Greetings,
A lot of the chatter about VTL's being good or bad seems to stem
from which vendors you listen to, and what they are trying to sell you.
There are a lot of dogmatic statements made by people on both sides of
this issue, usually by people with no personal experience about what
they are talking about. Somebody has fed them a sales line and they
dutifully parrot it back.
EMC sold their CDL product for about two years before IBM
entered the market. During that time you would not believe how many
times I heard IBM pooh-pooh the CDL saying it wasn't a good fit for TSM,
didn't perform well, whatever they had to say to compete against it. I
even heard someone recently say it was against the law to use a CDL if
you used the IBM drivers to talk to it. Against what law exactly?
Then after two years IBM came out with their VTL the TS7510, and
almost immediately came out with a Redbook about it with a TSM chapter
explaining why the TS7510 was such a good fit for TSM! Huh? And not
because it was a better product than the EMC one, it was actually
slightly slower and only scaled to about a fourth the size of the
largest EMC VTL. The only difference is that now IBM had something in
the marketplace, and that changed everything.
As Wanda has said, a lot of the distinctions fall down to how
you use the VTL, and if your expectations are set correctly. It is easy
for a vendor presentation to promise the moon without qualifying it's
claims. A single-engine DL4100 from EMC can sustain a 1100MB/sec (3.7
TB per hour) write speed like they claim IF:
1) You are writing multiple simultaneous virtual tape streams (like 16
or more),
2) You balance the I/O across at least 4 FC streams coming in the VTL
engine,
3) You have at least 5 or more disk drawers to spread out the I/O load.
4) You are not compressing at the VTL engine. If you compress at the
VTL engine, your performance will drop off, perhaps as low as a third as
fast. This is because the compression is done in software. If you want
hardware compression, go with one of the DL6000 series that has an
optional hardware compression engine.
But the presentations only say 1100MB/sec performance, and so customers
install one, set up a single backup to a single virtual tape drive, and
when it pegs at ~100MB/sec they think they have been lied to.
The other complaint I hear a lot is the claim of 3:1 compression.
Almost every vendor puts that in their literature as if it is a solid
fact, and not a typical value. I had a customer once get so mad they
almost yanked the whole box out and made the vendor take it back because
they bought a 10TB VTL, which they sized on the assumption of 3:1
compression. Never mind that the compression they were getting on their
existing LTO tape library was on 1.2:1, they were told the VTL would do
3:1, so it should.
I had another customer almost throw out IBM because they bought 12 new
3592 tape drives, and they wouldn't perform anywhere near their rated
performance. Never mind the fact that data was coming in through a
single GigE connection, and the 12 tape drives had an aggregate
throughput rating at about times that.
Customers looking to purchase any tape or disk technology would be wise
to ask questions about how performance numbers were achieved, and look
at their own situation to see what results they should expect.
Best Regards,
John D. Schneider
Sr. System Administrator - Storage
Sisters of Mercy Health System
3637 South Geyer Road
St. Louis, MO. 63127
Email: schnjd AT stlo.mercy DOT net
Office: 314-364-3150, Cell: 314-486-2359
-----Original Message-----
From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf Of
Prather, Wanda
Sent: Friday, June 08, 2007 10:56 AM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: Re: [ADSM-L] How to Incorporate a CDL into TSM environment?
...I understood restore performance suffered with a VTL - the way it has
been described to me is that, should a restore need to come from a
volume that has been destaged from disk to tape in the VTL, then a
restore of a single file from the volume would first have to wait for
the vtl to rebuild the tape on disk? Or have I got the wrong end of the
stick?
Um. Both.
Most VTL's are disk-only devices that emulate tape, and do not have the
staging issue you describe.
Many VTL's will make restores FASTER because the tape mount time goes
from potentially minutes to a second or less. (You also don't have to
worry about collocating data in a VTL, so your migration times are
generally faster as well.)
Now that goes with a caveat - you have to PIN YOUR VENDOR TO THE WALL
and get documentation about throughput rates. ALL VTL's work about the
same way, but they all have different hardware inside the box, so you
can get drastically different results. You can easily create a case
where restoring 1 VERY LARGE file will take longer on a slow VTL than
with fast tape (Say a TS1120, which run get more than 100MB/sec.)
It depends on
WHICH VTL you are talking about,
the speed of the disk in it,
the size of the cache in it
the speed of your SAN connection and/or HBAs
compared to which tape drive, and
whether you are talking about restoring lots of little files or a
few huge ones.
A VTS (don't they make this confusing?) is an IBM-only mixture of
disk/tape that emulates tape. It has to pull data off tape and stage it
back to disk before you can restore. Normally the VTS is used in a
mainframe environment.
IBM also makes VTLs, the TS7510 and TS7520, for use in open
environments. They are all disk.
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