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Date: Sat, 16 Jun 2012 21:04:26 +0100
Message-ID: <CADLo83-dZD5C2Fk3ZTH9z=pXHn8BtYzZQqUBG6HCkHZKe1=P8Q@mail.gmail.com>
From: Chris Rees <utisoft@gmail.com>
To: Xin LI <delphij@gmail.com>
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Cc: Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl>,
	freebsd-hackers@freebsd.org
Subject: Re: BIO_DELETE equivalent for file on FFS filesystem
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On Jun 16, 2012 8:37 PM, "Xin LI" <delphij@gmail.com> wrote:
>
> On Sat, Jun 16, 2012 at 12:01 PM, Chris Rees <utisoft@gmail.com> wrote:
> > On Jun 14, 2012 5:49 AM, "Wojciech Puchar" <
wojtek@wojtek.tensor.gdynia.pl>
> > wrote:
> >>>>
> >>>> file to take 900MB or... can i call some system function to "punch"
> >>>> holes?
> >>>
> >>>
> >>> I think you can only truncate the file at this time, pretty much like
> >>> brk() works for memory.
> >>
> >>
> >>
> >> BAD. suppose i keep windoze VM image on filesystem which takes 10GB but
> > uses 5GB.
> >>
> >> i could write simple program to find out what blocks are unused and
> > then...do nothing.
> >>
> >
> > What if you cp it?
>
> That would be a dd(1) unless we teach cp(1) how to do sparse.  I think
> what he wanted is to tell the OS "I don't need block XX - YY anymore"
> and the OS creates a sparse hole, which is not available at this time.

Sorry, I must have misread.  I take it cp would take a file with holes and
only copy the data part? i.e. take a 10G file of which 5G is a hole, you'd
end up with a 5G file?

Chris