Messages in this thread | | | Date | Sat, 16 Jul 2005 12:50:50 +0900 | From | Tejun Heo <> | Subject | Re: [rfc patch 2/2] direct-io: remove address alignment check |
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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.
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