File Buffering Questions ...

[Roger Oberholtzer]

On Wed, 2006-06-14 at 09:51 -0500, Ben Duncan wrote:
> Thanks Roger,
> 
> Yes, this is a database application.
> 
> I looked at mmap, briefly.
> 
> However (correct me if I am wrong here):
> 
> mmap does not seem to support record locking in a multi user
> environment (fcntl , lockf or flock).

I think (not positive) that you can lock pages (areas of memory). Not
sure of the specifics. If a record is some unit in the file, then the
record info in the file could have a 'locked' field. Then you do not
worry about the OS having something.

> If I am the first to mmap a file, and when I am done decide to close
> the mmap'ing and you are using the file ( a second process) ,
> what happens then ? Does the mapping stay open? Does the mapping
> go away? Does your process inherit the mapped file ? Is there
> a major fubar? Does little Debbie still get to go to college ?

Debbie graduates first in the class. mmaps can go across forks. A single
app never needs to map a file more than once. Different apps can map
them. We have a config thing that does this across forks of the app. We
even have it so the memory address in each app is the same when a new
map is made after the processes forked. That way pointer references into
this are valid in all processes.

> I could not find the answer when googling or RTFM'ing ..
> 
> On Wed, 2006-06-14 at 08:36 -0500, Ben Duncan wrote:
> <SNIP>
> 
>  > I gather this is a database app. Why not mmap() the file? The OS does a
>  > great job of managing the cache when you do this. Not sure what MVISAM
>  > is up to. But maybe it offers this as a file access option?
> 
>  > --
>  > Roger Oberholtzer
> 
> Thanks ...
> 
-- 
Roger Oberholtzer

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