Hey tl,

  Thanks for the explanation.  Does this mean that I
should only use the uncompressed throughput rates
and ignore the compressed rates when estimating 
the total SCSI bandwidth required to support a given
number of tape drives?

tel 

> -----Original Message-----
> From: Terry Lemons [SMTP:lemons_terry AT EMC DOT COM]
> Sent: Wednesday, December 04, 2002 7:37 AM
> To:   NETWORKER AT LISTMAIL.TEMPLE DOT EDU
> Subject:      Re: [Networker] Tape Compression defined
> 
> Hi John
> 
> Thoughts below.
> tl
> 
> -----Original Message-----
> From: Ballinger, John M [mailto:john.ballinger AT PNL DOT GOV]
> Sent: Tuesday, November 26, 2002 2:34 PM
> To: NETWORKER AT LISTMAIL.TEMPLE DOT EDU
> Subject: [Networker] Tape Compression defined
> 
> 
> Tell me if I'm wrong but my understanding of tape compression is:
> 
> Let's say I have a 50GB chunk of data available on a very fast
> disk/SCSI/PCI/etc.
> and let's say I can backup that 50GB chunk of data in 1 hour.
> 
> Assume I'm using a DLT8000 tape drive which has a native streaming speed of
> 6MB/s assuming no compression and a DLTIV tape in a DLT8000 drive with no
> compression on the data will fit 40GB of data on that tape.
> 
> So that means my avg throughput is 50000MB/(1*60*60)sec or 13.88MB/s
> tl> Up to this point, I agree.
> And my compression ratio is 13.88/6  or 2.31:1
> tl> If you are assuming that no compression is being done, then your
> compression ratio is 1:1 here.
> 
> And also during the backup on the average I will see the throughput
> indicated by the NetWorker Admin GUI as 13.88
> And furthermore I'm fitting 50GB of data(compressed) onto a tape that only
> holds 40GB in native(no compression)mode.
> 
> tl> If you send an incompressible data stream to the DLT8000, and there are
> no other bottlenecks, the tape drive will write at 6 MB/s.  But, if you send
> a compressible data stream to the DLT8000, the tape drive will _appear_ to
> write at greater than 6 MB/s.  What actually happens is that the tape drive
> subsystem can only write at up to 6 MB/s.  But the tape preprocessing
> subsystem (where the compression engine lives) can make the data smaller
> before it is sent to the tape drive subsystem.  So, you can actually have,
> say, 9 MB/s of uncompressed data going into the tape preprocessing
> subsystem, and 6 MB/s of compressed data coming out of the tape
> preprocessing subsystem and going into the tape drive subsystem.
> 
> So, the effective tape write speed and the compressibility of the data are
> linked.
> 
> Does this help?
> 
> tl
> 
> --
> Note: To sign off this list, send a "signoff" command via email
> to listserv AT listmail.temple DOT edu or visit the list's Web site at
> http://listmail.temple.edu/archives/networker.html where you can
> also view and post messages to the list.
> =*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=

--
Note: To sign off this list, send a "signoff" command via email
to listserv AT listmail.temple DOT edu or visit the list's Web site at
http://listmail.temple.edu/archives/networker.html where you can
also view and post messages to the list.
=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=