Date: Tue, 01 Aug 2006 12:27:54 -0500 From: Eric Anderson <anderson@centtech.com> To: rick-freebsd@kiwi-computer.com Cc: freebsd-hackers@freebsd.org Subject: Re: [PATCH] adding two new options to 'cp' Message-ID: <44CF8F1A.5090506@centtech.com> In-Reply-To: <20060801171150.GB3413@megan.kiwi-computer.com> 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> <20060801171150.GB3413@megan.kiwi-computer.com>
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On 08/01/06 12:11, Rick C. Petty wrote:
> 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.
Wouldn't this be incorrect for files that are really full of zeros? It
would turn them in to sparse files when they shouldn't be, correct? Is
that what happens with other tools?
Eric
--
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Eric Anderson Sr. Systems Administrator Centaur Technology
Anything that works is better than anything that doesn't.
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