Re: file journal fadvise

Dave Chinner
2014-12-02T02:02:01Z

On Mon, Dec 01, 2014 at 05:24:46PM -0800, Sage Weil wrote:
> On Tue, 2 Dec 2014, Dave Chinner wrote:
> > On Mon, Dec 01, 2014 at 04:12:03PM -0800, Sage Weil wrote:
> > > On Tue, 2 Dec 2014, Dave Chinner wrote:
> > > > What behaviour are you wanting for a journal file? it sounds like
> > > > you want it to behave like a wandering log: automatically allocating
> > > > it's next block where-ever the previous write of any kind occurred?
> > >
> > > Precisely. Well, as long as it is adjacent to *some* other scheduled
> > > write, it would save us a seek. The real question, I guess, is whether
> > > there is an XFS allocation mode that makes no attempt to avoid
> > > fragmentation for the file and that chooses something adjacent to other
> > > small, newly-written data during delayed allocation.
> >
> > Ok, so what is the most common underlying storage you need to
> > optimise for? Is it raid5/6 where a small write will trigger a
> > larger RMW cycle and so proximity rather than exact adjacency
> > matters, or is it raid 0/1/jbod where exact adjacency is the only
> > way to avoid a seek?
>
> The common case is a single raw disk.

Ok, so it's an exact match that is really required. I'll have a
think about it.

> > > It's a circular file, usually a few GB in site, written sequentially with
> > > a range of small to large (block-aligned) write sizes, and (for all
> > > intents and purposes) is never read. We periodically overwrite the first
> > > block with recent start and end pointers and other metadata.
> >
> > Ok, so it's just another typical WAL file. ;)
>
> Nothing to lose sleep over if this mode doesn't already exist, but I
> expect a fair number of applications could make use of this.
>
> FWIW, while I am already distracting you from useful things, I suspect
> (batched) aio_fsync would be a bigger win for us and probably a smaller
> investment of effort. :)

If you want to test a patch that implements a basic, simple