Re: Create Big size dummy File..
From: Tony Proctor (tony_proctor_at_aimtechnology_NOSPAM_.com)
Date: 06/24/04
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Date: Thu, 24 Jun 2004 13:01:41 +0100
In principle, seeking to a very high file address (i.e. beyond the EOF) and
writing more data there should leave behind an expanse of random data from
the disk - I've certainly used some UNIX systems in the past that have
exhibited this, and very interesting it was to read through it too.
However, I'm wondering if the system file manager is having an influence
here. I know that it uses memory-mapped sections to implement its caching.
Uninitialised pages would then be created as 'demand zero', with the result
that when they're flushed, NULLs would be written to the output file. This
also explains why 'creation' is so much quicker than 'copying': Demand-zero
pages are very cheap to create, whilst paging-in all the content of a
previous file is comparatively expensive.
Tony Proctor
"NickHK" <TungCheWah@Invalid.com> wrote in message
news:e#ORakdWEHA.3284@TK2MSFTNGP12.phx.gbl...
> J,
> ~4 minutes for a copy as opposed to ~90 seconds for a create.
>
> NickHK
>
> "J French" <erewhon@nowhere.com> wrote in message
> news:40da9c34.8524506@news.btclick.com...
> > On Thu, 24 Jun 2004 16:53:42 +0800, "NickHK" <TungCheWah@Invalid.com>
> > wrote:
> >
> > >J,
> > >Possibly 100 times that for Duane's initial 20Meg file.
> >
> > It would be 100 times - as it is doing 100 times the work
> > - on the FAT alone
> >
> > Out of interest, and only if you have enough time and disk space,
> > could you /copy/ the large file and see whether that takes the same
> > time as creating it
> > - that is not a totally accurate test, but if copy takes significantly
> > longer than creation, we can be fairly sure that it is not physically
> > writing nulls to disk.
> >
>
>
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