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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