NTFS zeros the file whenever you extend it.  If you just use WriteFile() to

extend the file, the same zero'ing occurs.  This is for C2 security

requirements, to prevent folks from extending their files over other peoples

files and taking control of the data.



If you extend a file a little at a time, then for every extension:

Seek to cluster location

Zero out enough space for the addition

Seek back to the start of the write

Write the file data



(so, 2 seeks for the write)



Also, if something else has written to the adjoining cluster in the interim

since the last write, then the file will fragment.  This happens all the

time in logging applications.



So, if you pre-allocate the file, you only pay for the zeroing 1 time.  This

will prevent any other files from intruding on your nicely sequential

allocation and optimize write speeds.  If you only pay the price once, then

the the runtime performance is the same as bypassing the zeroing.  The

clusters do not get zero'd until you call SetEndOfFile().  So the order is:



CreateFile

SetFilePointer (size of file you intend to create)

SetEndOfFile (commit the file)

SetFilePointer(beginning)

start writing



No more zeroing will occur unless you overrun the file size allocated.

Also, fragmentation will be minimized.  The zeroing happens very, very fast

b/c there is no seeking.  A single seek to the start of the file, then

sequential writes so the writes occur at the disk speed.



You can bypass the zeroing of files by using the SetFileValidData().  This

requires an elevated privilege to run (SE_MANAGE_VOLUME_NAME), so depending

on the security context of your application you may have problems; also that

is only available on WinXP and later so if you are targeting Win2k systems

it won't work.  Whereas the SetFilePointer can be executed by any user

context that has access to the file.  Since (from your description) the

process creates the file as well, this won't be a problem.



Pat





"George M. Garner Jr." <gmgarner@newsgroup.nospam> wrote in message

news:eU9bWe9GFHA.3484@TK2MSFTNGP12.phx.gbl...

> Pat,

> 

>

> Could you be more specific as to how to allocate the file.  I see the

> function SetFileValidData.  But that assumes that you already have

> allocated the file.  Elsewhere I have read that one should not use

> SetFilePointer(Ex) because of the added overhead of writing zeros to the

> file.  Is there a way to allocate the file without incurring the overhead

> of immediately zeroing out the clusters.   I know the pagefile does this

> but I do not see an api that permits it.

>

> Regards,

>

> George.

>