mmap and raw disk devices...

mmap and raw disk devices...

[Christopher Quinn]

Hello list,

I tried to mmap a disk partition raw device which failed.
Can anyone tell me the reason mmap does not support such a
device? 
I would have thought a mmap/raw-device combination to be ideal as
a basis for a high performance database system.
I know there is the option of managing memory<->disk movements
oneself, but my understanding is that handling page-faults via
signal trap handling is *very* expensive. Far better to leave
such matters in the hands of the OS.

I suspect there is some fundamental reason for not mmap'ing raw
devices that is patently obvious to everyone but me! 

Thanks,
Chris Quinn
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Re: mmap and raw disk devices...

[Eric W. Biederman]

Christopher Quinn <cq@htec.demon.co.uk> writes:

> Hello list,
> 
> I tried to mmap a disk partition raw device which failed.
> Can anyone tell me the reason mmap does not support such a
> device? 
> I would have thought a mmap/raw-device combination to be ideal as
> a basis for a high performance database system.
>
> I know there is the option of managing memory<->disk movements
> oneself, but my understanding is that handling page-faults via
> signal trap handling is *very* expensive. Far better to leave
> such matters in the hands of the OS.
> 
> I suspect there is some fundamental reason for not mmap'ing raw
> devices that is patently obvious to everyone but me! 

mmap goes through the page cache.  It should work in 2.4.10+
though someone might not have enabled it.

Eric
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