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Path: mrsvr.UUCP!csd4.csd.uwm.edu!uwm.edu!mailrus!ncar!boulder!sunybcs!sbcs!mjn
From: mjn@sbcs.sunysb.edu (The Sixth Replicant)
Newsgroups: comp.os.aos
Subject: Re: The *real* file size (was: How to find *real* file sizes in AOS/VS...?)
Keywords: AOS/VS filesize fubar
Message-ID: <3540@sbcs.sunysb.edu>
Date: 23 Sep 89 00:29:22 GMT
References: <1055@mrsvr.UUCP>
Sender: news@sbcs.sunysb.edu
Reply-To: mjn@sbstaff2.UUCP (The Sixth Replicant)
Organization: Tyrell Corp.
Lines: 23

>In article <1055@mrsvr.UUCP> I (Jim Kohli) wrote
>>AOS/VS "compresses" *ANY* kind of file which has a complete
>>"element" (i.e., contiguous disk allocation) of zeroes.  This
>>...

>I just want to clarify one point here. AOS/VS doesn't do any compression.
>The file system allows users to have unallocated blocks in files. When
>read, these blocks will be returned as zeros. Certain commands (MOVE,
>DUMP, DUMP_II) exploit this property to save space. If, however I write
>an element of zeros to disk, I'll get an element of zeros on disk, the
>file systems doesn't scan for zeros to see if it can optimize the write.
>This would take far too much CPU.
>

You are right!  I had originally believed this fairy tale
because I was involved in doing some I/O benchmarking which
involved a lot of ?RDB's/?WRB's, and DG had one of their high
power support dudes (Dave Barrows) criticize our results as
follows: "well, if you're only reading and writing zeroes, they
aren't actually written to the disk..." (this is a
recollection, but it isn't vague on this point).  I guess it
was easy to rationalize at the time because it seemed possible
that the PTE's might maintain a "zero page" bit (no such bit
has been documented to my knowledge, but this was in 1982 when
the soul of the new machine was more like an unfriendly
spirit).

Sorry about that bbbbbboard readers!

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A sun 3/50 with 8 MB isn't *JUST* a conspicuous consumption of
silicon!
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Jim Kohli                    | "Oh Grammar!  Water bag icer gut!
GE Medical Systems           | A nervous sausage bag ice!"
PO Box 414                   | (Oar aesthete groin-murder???)
Milwaukee, WI	53201-414    |
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gemed!hal!kohli@crd.ge.com
sun!sunbird!gemed!hal!kohli
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