Re: bdflush flushing memory mapped pages.

[Keith Ansell]

Thank you for your prompt replies.

I realise that Linux conforms to the letter of the specification, but maybe
not the spirit of the it.

I am porting a Database solution to Linux from Unix SVR4, Sco OpenServer and
AIX, where all write required memory mapped files are flushed to disk with
the system flusher, my users have large systems (some in excess of 600
concurrent connections) flushing memory mapped files is a big part of are
systems performance.  This ensures that in the event of a catastrophic
system failure the customers vitual business data has been written to disk .

Keith Ansell

----- Original Message -----
From: "Andrew Morton" <[e-mail]>
To: "Andre Hedrick" <[e-mail]>
Cc: <[e-mail]>; <[e-mail]>;
<[e-mail]>
Sent: Wednesday, April 09, 2003 10:27 AM
Subject: Re: bdflush flushing memory mapped pages.

> Andre Hedrick <[e-mail]> wrote:
> >
> >
> > Funny you mention this point!
> >
> > I just spent 30-45 minutes on the phone talking to Jens about this very
> > issue.  Jens states he can map the model in to 2.5. and will give it a
> > fling in a bit.  This issue is a must; however, I had given up on the
idea
> > until 2.7.  However, the issues he and I addressed, in combination to
your
> > request jive in sync.
>
> noooo.....   This isn't going to happen.  There are many reasons.
>
> Firstly, how can bdflush even know what pages to write?  The dirtiness of
> these pages is recorded *only* in some processor's hardware pte cache
and/or
> the software pagetables.  Someone needs to go tell all the CPUs to
writeback
> their pte caches into the pagetables and then someone needs to walk the
> pagetables propagating the pte dirty bit into the pageframes before we can
> even start the I/O.
>
> That's what msync does, in filemap_sync().
>
>
> And even if bdflush did this automagically, it's the wrong thing to do
> because the application could very well be repeatedly dirtying the pages.
> Very probably.  So we've just gone and done a ton of pointless I/O, over
and
> over.
>
> You can view MAP_SHARED as an IPC mechanism which uses the filesystem
> namespace for naming.  No way do these people want bdflush pointlessly
> hammering the disk.
>
> You can also view MAP_SHARED as a (strange) way of writing files out.  If
you
> want to do that then fine, but you need to tell the kernel when you've
> finished, just like write() does.   You do that with msync.
>
>
>
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