Strange performance issue

[Steven D'Aprano]

On Mon, 05 Oct 2009 22:31:05 -0700, Dan Stromberg wrote:

> I'm rewriting 3 programs as one program - from Python with Tkinter to
> Python with pygtk, both on Windows XP.
> 
> My new version formats an SD card and preallocates some file space in
> about 3 minutes with "Optimize Performance" selected, and in about 30
> minutes with "Optimize for Quick Removal" selected.  Needless to say, I
> don't like the 27 minute penalty much.

I don't understand what that means. How are you formatting the SD card?

I'm guessing that Optimize for Quick Removal means that every write is 
immediately synced to disk. That will probably be slow, no matter what.


> But the old version of the program formats and preallocates in 3 minutes
> irrespective of whether the drive is in "optimize performance" or
> "optimize for quick removal".

I suspect that if there was no performance penalty in the old version, it 
was because you inadvertently were always using "Optimize Performance" no 
matter what.

BTW, if you want to pre-allocate some space, and you don't care what is 
in the file, you *may* be able to use file.truncate() to do so quickly. 
Despite the name, it doesn't just truncate files, it can also be used to 
extend them. (At least on POSIX systems.)


>>> f = open('empty', 'w')
>>> f.seek(1000)
>>> f.truncate()
>>> f.close()
>>> f = open('empty', 'r')
>>> len(f.read())
1000
>>> f.close()



-- 
Steven