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Re: file journal fadvise

To: Sage Weil <sweil@xxxxxxxxxx>
Subject: Re: file journal fadvise
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 2 Dec 2014 13:01:35 +1100
Cc: mnelson@xxxxxxxxxx, ceph-devel <ceph-devel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, éåæ <majianpeng@xxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <alpine.DEB.2.00.1412011719070.3471@xxxxxxxxxxxxxxxxxx>
References: <alpine.DEB.2.00.1411301013490.352@xxxxxxxxxxxxxxxxxx> <CALurOm2tEV=RqN21eFJvfU1zTtJkbz2gHDCk_Ntsy4oz9iwHoA@xxxxxxxxxxxxxx> <alpine.DEB.2.00.1411301922220.352@xxxxxxxxxxxxxxxxxx> <547CBEFA.3000204@xxxxxxxxxx> <alpine.DEB.2.00.1412011122020.3471@xxxxxxxxxxxxxxxxxx> <547CEC36.6070309@xxxxxxxxxx> <20141201225100.GO16151@dastard> <alpine.DEB.2.00.1412011559200.3471@xxxxxxxxxxxxxxxxxx> <20141202003239.GP16151@dastard> <alpine.DEB.2.00.1412011719070.3471@xxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
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
implementation of aio_fsync:



Dave Chinner

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