Michael,
A few ideas to consider when trying to tune a tape-only system.
1. SDLT compression 'inverts' already compressed data, eg you will see
slightly slower throughput and less capacity on tape when sending
already compressed data to sdlt drives. LTO compression does not have
this problem.
2. SDLT320 tapes store 160GB of data - if you send data to a SDLT volume
and it fills up at say 200GB the you can figure out what the net
compression effect was by dividing 200/160. if you see it filling up at
120GB you know you have problems with double compression..
3. NW 7.3+ systems ship with a tool for testing performance of your tape
drives - under UNIX its called tape_perf_test - you put in a blank tape
and run the command - it tries to write test data of varying
compressibility, it does this for a bunch of different blocksizes too.
This will let you know if the hardware compression is working or not. I
presume there's a windows version of it if you go looking under Program
Files..
4. compression directives on the client usually result in a backup of
about the same duration as one without compression - eg the cpu usage
will go up on the client, but the network bandwidth usage will go down.
People generally expect the backup to be quicker because they think the
network is the bottleneck, but I find that frequently the bottleneck is
the client's resources, you can do really well if there's a lot of
spare cpu and memory on a client, but people choose the size of the box
based on the main application on the client, and don't much consider the
backup software, so that's why it's usually short on cpu when the backup
comes along... YMMV as a result - if you have a lot of clients, and you
put on compression everywhere you may not see shorter backup windows,
but you do gain the ability to back more clients up.
5. see point 1. client side compression with SDLT HW compression will
result in you using more tapes.
6. performance - HW compression on the drive demands higher data rate
from the server to the drive - eg for SDLT320 the tape is spooling
internally at 16GB/sec. If you have no compression, or the data doesn't
compress, you will be sending data at 16GB/sec over the SCSI or FC
connection, you therefore have to have your clients capable of supplying
data at that rate.
If you decide to combine client-side compression with no tape
compression, you have to consider point 4 carefully because if you can't
supply the data fast enough the tape drive will overrun and have to
retrain often.. Shoe-shining as it's called will destroy your drive
mechanism in a few months.
If you decide to not use client-compression, but use HW compression
you have to supply at a higher data rate depending on the
compressibility of the data. Say you have data flowing in at 4:1
compressibility, then you will need to be able to pass data through your
server and across the SCSI or FC to the drive at 64MB/sec.... With a few
drives sharing a bus or san path, you will probably find a bottleneck.
This will result in shoe-shining again..
7. In the real world a client will send data at varying spot-rates...
Sometimes there's a complex filesystem with lots of files, and it
struggles to get the data out fast enough, other times there's
folders with big files that compress 4:1, other times there's folders
with big files that are already compressed and compress 1:1 - having
lots of clients backup in parallel helps smoothen this out though.
So overall - it's very difficult to advise on how to configure a
tape-only system, we've been struggling for years, most of us have spent
way to much time tweaking systems, only to see the nature of the clients
or data change, and for performance to drop as soon as one takes ones
eye off the system. But don't despair - the solution to all this is
disk staging.. Put some adv_file in or a little VTL and its way easier
to get your performance up to top speed, because it divorces the
client-side issues from the tape drive issues...
> -----Original Message-----
> From: EMC NetWorker discussion
> [mailto:NETWORKER AT LISTSERV.TEMPLE DOT EDU] On Behalf Of MIchael Leone
> Sent: Tuesday, 26 February 2008 4:46 AM
> To: NETWORKER AT LISTSERV.TEMPLE DOT EDU
> Subject: [Networker] Compression on Win2003 servers question
>
> I have a Win2003 server, NW 7.4 SP1, using an HP SDLT
> autoloader. Total
> size of backup is about 600G (589G, according to mminfo).
> We'd like to do
> a full backup every day. However, I seem to be averaging 4
> tapes (3 full
> tapes, and 25% used on the last) every day. Now, an SDLT tape is 160G
> native, up to 320G compressed, so 600G would need 4 tapes
> native (3 and
> 3/4 or so, according to the math).
>
> IOW, it doesn't appear as if compression is being used, or
> just barely
> used. I see nowhere in NW how to turn on hardware compression on the
> autochanger. Am I just missing it?
>
> I did change to using the "NT with compression" directive,
> which didn't
> seem to affect the output; I am still getting 3.25 tapes per
> day used.
>
> My question: I am backing up 3 servers, 2 of which were set
> to use the
> compression directive, and 1 was not (this last server is
> very small, and
> doesn't significantly add to the total). Do all group members
> need to use
> the same compression directive, else NW won't compress? It
> shouldn't be a
> problem to make all 3 servers use the compression directive,
> but I thought
> I could use it only on the server that needs it (one server
> accounts for
> about 570G of the 590G total).
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