That's not how it works. You're really sending the full amount of data over
the bus into the drive, then it is compressed by the controller in the
drive.
You might be sending 12 MB/sec to the drive over the SCSI, but the drive
squashes it down enough so it can be written to the tape at 6 MB/sec.
Wes
-----Original Message-----
From: Lewis, Terry {Info~Palo Alto} [mailto:TERRY.LEWIS AT ROCHE DOT COM]
Sent: Wednesday, December 04, 2002 12:27 PM
To: NETWORKER AT LISTMAIL.TEMPLE DOT EDU
Subject: Re: [Networker] Tape Compression defined
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
>
> --
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