Re: Ext4 and the "30 second window of death"

[Matthew Garrett <mjg59@srcf.ucam.org>]

On Thu, Apr 02, 2009 at 08:08:36PM -0700, david@lang.hm wrote:
> On Fri, 3 Apr 2009, Matthew Garrett wrote:
> >Well, yes, the administrator could hate the user. They could achieve the
> >same affect by just LD_PRELOADING something that stubbed out fsync() and
> >inserted random data into every other write(). We generally trust that
> >admins won't do that.
> 
> then trust the admins to make a reasonable decision for or with the user 
> on this as well.

What a reasonable decision is here depends on what software the user is 
running. There simply isn't a reasonable default other than to allow 
fsync() to work. Changing requires auditing every single piece of code 
the user may run.

> >There's various circumstances in which it's beneficial. The difference
> >between an optimal algorithm for typical use and an optimal algorithm
> >for typical use where there's an fsync() every 5 minutes isn't actually
> >that great.
> 
> mixing some sub-threads a bit to combine thoughts
> 
> you object to calling something like this 'laptop mode'
> 
> Ted's statements about laptop mode indicate that he believes that it 
> delays writes for a configurable time rather than accelerating writes.

As I said, the code is pretty easy to read.

(snip)

> thoughts?

I've certainly got no objection to the addition of a mode that changes 
the behaviour of fsync() - personally I think it would be an error for 
almost anyone to use it, but that's really up to the individual 
situation. But it would have a different goal to the existing 
laptop-mode and so should have a different name in order to avoid 
confusion.

-- 
Matthew Garrett | mjg59@srcf.ucam.org