On 04/22/2011 10:03 AM, Eric Blake wrote:
cp can read whatever blocksize it chooses. If that block contains
zero, it would signal cp that maybe it should SEEK_DATA and skip
reading all those blocks. That's all. We are not trying to achieve
perfection. We are just trying to reduce cpu waste.
If the fs supports SEEK_*, then great. If it does not, then it is no
worse than before.
But providing just SEEK_DATA _is_ worse than before if you don't provide
the correct SEEK_HOLE everywhere. Because then your algorithm of trying
lseek(SEEK_DATA) after every run of zeros in the hopes of an
optimization is a wasted syscall, since it will just return your current
offset every time, so you end up with more syscalls than if you had used
the single lseek(SEEK_DATA) that returns the end of the file up front,
and known that the remainder of the file has no holes to even try
seeking past.
You are over-optimizing. strace any process on your box and you
will find numerous wasted syscalls.
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