Date: Tue, 1 Aug 2006 12:11:50 -0500 From: "Rick C. Petty" <rick-freebsd@kiwi-computer.com> To: Peter Jeremy <peterjeremy@optushome.com.au> Cc: freebsd-hackers@freebsd.org Subject: Re: [PATCH] adding two new options to 'cp' Message-ID: <20060801171150.GB3413@megan.kiwi-computer.com> In-Reply-To: <20060801072611.GA717@turion.vk2pj.dyndns.org> References: <200607271150.k6RBoM9p031745@lurza.secnetix.de> <44C8FB65.9020102@FreeBSD.org> <44CE03D2.2050803@centtech.com> <17614.4005.407223.621637@bhuda.mired.org> <44CE199C.2020500@centtech.com> <17614.8289.134373.387558@bhuda.mired.org> <96b30c400607310847s1d2f845eo212b234d03f51e9a@mail.gmail.com> <17614.10982.499561.139268@bhuda.mired.org> <ealpn1$lan$1@sea.gmane.org> <20060801072611.GA717@turion.vk2pj.dyndns.org>
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On Tue, Aug 01, 2006 at 05:26:11PM +1000, Peter Jeremy wrote:
> On Mon, 2006-Jul-31 22:42:49 +0200, Ivan Voras wrote:
> >I agree with this, and while you're in there, can you add -s to copy
> >sparse files (via the usual "if the buffer is all nulls, seek beyond eof
> >instead of writing" trick)?
>
> Note that it isn's possible to accurately distinguish between a block
> of NULs and a hole in the file through the filesystem. The only way
> to accurately copy a sparse file is with dump/restore.
Sure it is-- in a number of ways. The most useful way is to do something
of the sort:
int sd, dd; // assume these are set to source & dest descriptors
unsigned char* zeros;
unsigned char* buffer;
struct stat st;
size_t bytes, offset;
fstat(sd, &st);
zeros = malloc(st.st_blksize);
bzero(zeros, st.st_blksize);
for (offset = 0; offset < st.st_size; offset += bytes)
{
bytes = st.st_blksize;
if (offset + bytes > st.st_size)
bytes = st.st_size - offset;
read(sd, buffer, bytes);
if (0 == memcmp(buffer, zeros, bytes))
lseek(dd, bytes, SEEK_CUR);
else
write(sd, buffer, bytes);
}
Obviously, I didn't add the error checking/handling, but AFAIK this is
essentially what the -S option to gnu's tar does. In this example, you
may not mimic the allocated blocks of a sparse file, but you would
optimize the copy to use as few filesystem blocks as possible.
-- Rick C. Petty
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