Badari Pulavarty wrote:
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).
Just out of curiosity, can you tell me why you think limiting
pagecache isn't the right thing to do (tm)? O_DIRECT mount seems to me
incomplete/complex solution (DMA alignment etc...). Forgive me if this
issue has been discussed to death already.
--
tejun
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