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
_______________________________________________
freebsd-hackers@freebsd.org mailing list
http://lists.freebsd.org/mailman/listinfo/freebsd-hackers
To unsubscribe, send any mail to "[EMAIL PROTECTED]"
