Re: msync() behaviour broken for MS_ASYNC, revert patch?

[Nick Piggin <nickpiggin@yahoo.com.au>]

Linus Torvalds wrote:
> 
> On Fri, 10 Feb 2006, Nick Piggin wrote:
> 
>>This is a Linux implementation detail. As such it would make sense to
>>introduce a new Linux specific MS_ flag for this.
>>..
>>Definitely. And when the app gives us a hint that it really wants the
>>data on the disk, starting it as early as possible is also a good
>>optimisation.
> 
> 
> But that's what MS_SYNC is. MS_SYNC says "I need this data written now".
> 

Yes but it is synchronous.

> MS_ASYNC moves it into the page cache. That makes 100% sense. Then it will 
> be written by the regular dirty page writeout. That makes 100% sense. 
> 

MS_INVALIDATE does that (in Linux), the spec is poorly worded but the
intention seems to be that it would push dirty state back into pagecache for
implementations such as ours.

> 
>>I don't think there's anything wrong with your fadvise additions.
>>I'd rather see MS_ASYNC start IO immediately and add another MS_
>>flag for Linux to propogate bits.
> 
> 
> Why? I miss the _reason_ you want to do this.
> 

linux@horizon.com has an application (database or logging I think), which
uses MS_SYNC to provide integrity guarantees, however it is possible to do
useful work between the last write to memory and the commit point. MS_ASYNC
is used to start the IO and pipeline work.

> The current MS_ASYNC behaviour is the sane one. It's the one that doesn't 
> cause the harddisk to start ticking senselessly. It's the one that allows 
> a person on a laptop to say "don't write dirty data every 5 seconds - do 
> it just every hour".
> 

MS_INVALIDATE

> In contrast, _your_ proposal is just inflexible and inconvenient.
> 

Currently MS_ASYNC does the same as MS_INVALIDATE. But it used to start
IO (before 2.5.something), and apparently it does in Solaris as well.

> If somebody really really wants to "start flushing data now", then he can 
> do so, but that actually has absolutely zero to do with "msync()" any 
> more. A person who wants the flushing to start "now" might want to flush 
> any random dirty buffers.
> 

I didn't quite understand what you're saying here.

> Your suggestion is no different from saying "we should make every 
> 'write()' call start the IO". Which is obviously crap.
> 

I think it is quite a bit different. Obviously what you're saying is crap,
but I think there are good arguments for changing MS_ASYNC so it is not
quite so obvious.

-- 
SUSE Labs, Novell Inc.

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