Re: [rfc patch 2/2] direct-io: remove address alignment check

[Badari Pulavarty <pbadari@us.ibm.com>]

On Fri, 2005-07-15 at 17:23 +0900, Tejun Heo wrote:
> Badari Pulavarty wrote:
...
> >>  I don't know why you wanna relax the alignment requirement, but 
> >> wouldn't it be easier to just write/use block-aligned allocator for 
> >> such buffers?  It will even make the program more portable.
> >>
> > 
> > I can imagine a reason for relaxing the alignment. I keep getting asked
> > whether we can do "O_DIRECT mount option".  Database folks wants to
> > make sure all the access to files in a given filesystem are O_DIRECT
> > (whether they are accessing or some random program like ftp, scp, cp
> > are acessing them). This was mainly to ensure that buffered accesses to
> > the file doesn't polute the pagecache (while database is using O_DIRECT
> > access). Seems like a logical request, but not easy to do :(
> > 
> > Thanks,
> > Badari
> 
>   I don't know much about VM, but, if that's necessary, I think that 
> limiting pagecache size per mounted fs (or by some other applicable 
> category) is easier/more complete approach.  After all, you cannot mmap 
> w/ O_DIRECT and many programs (gcc, ld come to mind) mmap large part of 
> their memory usage.

I agree. I guess for mmap()ed access we can kick it back to buffered
mode.

I don't think limiting pagecache use per filesystem is an acceptable
option. In fact, database folks exactly want this -  to limit the
pagecache use by filesystems - but I don't think its right thing to do,
so I am trying to propose mount O_DIRECT as an alternative (if its
feasible).

Thanks,
Badari

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