Re: bdflush flushing memory mapped pages.

[Andre Hedrick <andre@linux-ide.org>]

Keith,

I know what you are asking for and need.
It is a requirement to be "Enterprise".
What you are seeking will take time, and effort.

I have explained successfully to Jens (block maintainer) the issues of
data integrity.  If you can prove this becomes a data integrity issue,
which I know it is for the general case, your argument will have strength.

Nothing stops "fastfreenet.com" from funding the development time.

ASS-GAS-GRASS-CASH, Linux is free but my time is not.

If you want to discuss more of this offline, I will listen and help make
the case.

Cheers,

Andre Hedrick
LAD Storage Consulting Group


On Thu, 10 Apr 2003, Keith Ansell wrote:

> 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" <akpm@digeo.com>
> To: "Andre Hedrick" <andre@linux-ide.org>
> Cc: <keitha@edp.fastfreenet.com>; <linux-kernel@vger.kernel.org>;
> <axboe@suse.de>
> Sent: Wednesday, April 09, 2003 10:27 AM
> Subject: Re: bdflush flushing memory mapped pages.
> 
> 
> > Andre Hedrick <andre@linux-ide.org> 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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