Re: [Firebird-devel] FB 2.1 64bit and memory usage ??

[Vlad H. <hv...@us...>]

> > > I know various db "vendors" support direct I/O as a db option
> > > on Linux, using the current O_DIRECT implementation, but they
> > > had to be careful in their code to avoid certain O_DIRECT
> > > kernel bugs and race conditions.  For example, it seems mixing
> > > O_DIRECT and non-O_DIRECT reads and writes on the same file
> > > simultaneously may result in stale data reads or writes. The
> > > workaround: don't do that!
> >
> >     posix_fadvise free from this drawback and not restrict us to use
> > aligned memory buffers, right ?
> 
> But lucks one more important support - direct I/O from/to user space. 

    If i understand correctly what Linus said about O_DIRECT implementation
details - there are no direct I/O from/to user space. It anyway must be coherent
with cache state. I may be wrong.

    Anyway - cost of context switch and page copy is far less than cost of direct 
access to disk, isn't is ?

> I suggest we begin with O_DIRECT, and only in case it has serious problems try 
> with posix_fadvise(). 

    Ok. In unix.cpp\PIO_force_write there are code i don't understant completely. 

Please explain : 

    why there are no call of fcntl( F_GETFL) ? 

    why for "hpux" platform used "union fcntlun" while i can't find it on 
        http://docs.hp.com  
        (fcntl described at http://docs.hp.com/en/B2355-60130/fcntl.2.html)

    I've attached diff with proposed fix, may you take a look at it ?

> This is even more important taking into an account that 
> 2.4 kernels do not support posix_fadvise().

    "Do not support" is :
a) not implemented but entrypoint exists
or
b) entrypoint is missed 
?

    If (a) - i think we can live with it

Regards,
Vlad