Re: [Networker] Tape Compression defined

Thanks Terry

I still don't see any problems with my numbers (although one could argue
that the tape drive is writing data to tape at 6MB/s always (assuming you
have a fast enough source such that whatever compression is done to the data
stream there is sufficient data so that after compression there's still
6MB/s left).

John

-----Original Message-----
From: lemons_terry AT emc DOT com [mailto:lemons_terry AT emc DOT com]
Sent: Wednesday, December 04, 2002 7:37 AM
To: Ballinger, John M
Cc: 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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