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August 2008
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2008.08.22 16:26 "Re: creating sparse files......", by Toby Thain
On 22-Aug-08, at 12:45 PM, Rogier Wolff wrote:
> On Fri, Aug 22, 2008 at 10:11:42AM -0300, Toby Thain wrote:
>>> static int isallzero (tdata_t buf, tsize_t size)
>>> {
>>> int i;
>>> for (i=0;i<size;i++)
>>> if (buf[i]) return 0;
>>> return 1;
>>> }
>>
>>
>> This appears unnecessarily inefficient. It's going to be much cheaper
>> to test whole longwords (or whatever data alignment will allow) than
>> byte by byte. (I wonder if something clever can be done with MMX/SSE
>> on newer chips?)
>
> Yes, this is the "quick and dirty" implementation of this function.
> The advantage of doing it this simple is that it will ALWAYS work.
> ...
> Anway, my philosophy is always FIRST get it to work, optimize later.
Yes my nitpicking was probably premature. :)
>
>>> static tsize_t
>>> _tiffWriteProc(thandle_t fd, tdata_t buf, tsize_t size)
>>> {
>>> if (isallzero (buf, size))
>>> return (lseek ((int) fd, (off_t)size, SEEK_CUR));
>>> else
>>> return ((tsize_t) write((int) fd, buf, (size_t)
>>> size));
>>> }
>>
>>
>> This penalises all writes to optimise a very special case. Not saying
>> it can't be rationalised, but pros and cons can be debated.
>
> And you're calling this an optimization. And I agree. However, in my
> case saving a factor of 21 on disk space is i'd say "worth it".
> ...
> But, yes, officially it could cost you some CPU cycles, which are not
> returned by saving on writes. (checking for zeroes and seeking is much
> more efficient than just doing the write! It not only saves you disk
> space, but also CPU cycles!)
Well this is the kind of debate I was hinting at. :)
I think Bob's suggestion of compression is a very good one and
perfectly portable? (Taking into account the filesystem differences
you mentioned.)
--Toby
>
> Roger.
>