Re: Suggestion: A No-write Option, e.g. --scrub.

[Antonio Diaz Diaz]

Ralph Corderoy wrote:
>      $ strace -c tee <$f 2>&1 >/dev/null | sed 4q
>      % time     seconds  usecs/call     calls    errors syscall
>      ------ ----------- ----------- --------- --------- ----------------
>       56.73    0.126982          33      3750           read
>       43.23    0.096779          25      3743           write

25 µs/call? It seems that your hardware is very slow. I have tried the 
command above on my AMD Athlon64 and it is 38000 times faster (0.65 ns/call).

$ strace -c tee < enwik8 2>&1 > /dev/null
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 82.98    0.000039           0     12210           read
 17.02    0.000008           0     12208           write

Even my AMD K6-II 450 MHz from 1999 is almost one hundred times faster (0.28 
µs/call).


> The extra writes to /dev/null are adding 0.1 s and that's about 75% of
> the time taken by all the read()s.

I guess this is because you are reading a file cached in RAM. Reading from a 
real USB pendrive in the K6 gives:

$ strace -c tee < binutils-2.32.tar.lz 2>&1 > /dev/null
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 98.77    0.056898          23      2494           read
  1.23    0.000708           0      2490           write


> So it might be a measurable saving to avoid the write()s when scrubbing
> 1 TB, say.

On my Athlon64, ddrescue copies 1 TB from /dev/zero to /dev/null in 2m40s 
(6.25 GB/s), which makes the savings insignificant. When reading from a real 
device the savings are even more insignificant. Ddrescue spends much more 
time managing the mapfile than writing to /dev/null. (Note that the total 
read/write/seek time is itself an insignificant fraction of total run time):

$ strace -c ddrescue -f -s1TB /dev/zero /dev/null 2>&1 > /dev/null
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 91.86    0.215880           0  15258794           read
  5.16    0.012119           0  30517582           lseek
  2.98    0.007014           0  15259740           write


Best regards,
Antonio.