RE: [PATCH v11 00/21] Add support for NV-DIMMs to ext4

[Zuckerman, Boris]

I am trying to refocus this thread from a particular issue to more generic needs...

Regards, Boris

> -----Original Message-----
> From: Matthew Wilcox [mailto:willy@xxxxxxxxxxxxxxx]
> Sent: Tuesday, September 30, 2014 3:24 PM
> To: Zuckerman, Boris
> Cc: Matthew Wilcox; Valdis.Kletnieks@xxxxxx; Matthew Wilcox; linux-
> fsdevel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v11 00/21] Add support for NV-DIMMs to ext4
> 
> On Tue, Sep 30, 2014 at 05:10:26PM +0000, Zuckerman, Boris wrote:
> > >
> > > The more I think about this, the more I think this is a bad idea.
> > > When you have a file open with O_DIRECT, your I/O has to be done in
> > > 512-byte multiples, and it has to be aligned to 512-byte boundaries
> > > in memory.  If an unsuspecting application has O_DIRECT forced on
> > > it, it isn't going to know to do that, and so all its I/Os will fail.
> > > It'll also be horribly inefficient if a program has the file mmaped.
> > >
> > > What problem are you really trying to solve?  Some big files hogging the page
> cache?
> > > --
> >
> > Page cache? As another copy in RAM?
> > NV_DIMMs may be viewed as a caching device. This caching can be implemented on
> the level of NV block/offset or may have some hints from FS and applications.
> Temporary files is one example. They may not need to hit NV domain ever. Some
> transactional journals or DB files is another example. They may stay in RAM until power
> off.
> 
> Boris, you're confused.  Valdis is trying to solve an unrelated problem (and hopes my
> DAX patches will do it for him).  I'm explaining to him why what he wants to do is a bad
> idea.  This tangent is unrelated to NV-DIMMs.

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