[DRBD-user] [OT] rsync issues [Was Re: Read performance?]

Luciano Rocha strange at nsk.no-ip.org
Tue Jun 5 11:47:25 CEST 2007

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.


On Tue, Jun 05, 2007 at 11:12:12AM +0200, Lars Ellenberg wrote:
> On Mon, Jun 04, 2007 at 11:58:30AM -0500, David Masover wrote:
> > On Monday 04 June 2007 08:56:41 Lars Ellenberg wrote:
> > 
> > dd if=/dev/drbd0 of=/dev/null bs=1M count=1 skip=1
> > 
> > That one should be instantaneous,
> 
> no, since "skip" for dd litteraly means skip, it does not mean seek.
> it means read /dev/drbd0 from the start (offset 0), throw away the first
> megabyte, then ouput the second megabyte.

No it doesn't.

strace dd if=/dev/hdg of=/dev/null bs=1M count=1 skip=1
...
4391  open("/dev/hdg", O_RDONLY|O_LARGEFILE) = 0
4391  _llseek(0, 0, [0], SEEK_CUR)      = 0 -> does this support seek?
...
4391  _llseek(0, 1048576, [1048576], SEEK_CUR) = 0 -> skip 1 ibs
4391  read(0, "\0"..., 1048576) = 1048576
4391  write(1, "\0"..., 1048576) = 1048576

with skip=10:
...
4420  _llseek(0, 10485760, [10485760], SEEK_CUR) = 0


IEEE Std 1003.1, 2004 says on dd skip and seek:
skip=n
    Skip n input blocks (using the specified input block size) before
    starting to copy. On seekable files, the implementation shall read
    the blocks or seek past them; on non-seekable files, the blocks
    shall be read and the data shall be discarded.
seek=n
    Skip n blocks (using the specified output block size) from the
    beginning of the output file before copying. On non-seekable files,
    existing blocks shall be read and space from the current end-of-file
    to the specified offset, if any, filled with null bytes; on seekable
    files, the implementation shall seek to the specified offset or read
    the blocks as described for non-seekable files.

-- 
lfr
0/0
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