[Btrfs-devel] Re: [Btrfs-announce] [ANNOUNCE] Btrfs v0.10 available

[Bron Gondwana]

(still away at the choir festival for another week... but I got
online and downloaded a bunch of emails)

On Fri, Jan 18, 2008 at 08:46:23AM -0500, Chris mason wrote:
> On Thursday 17 January 2008, Bron Gondwana wrote:
> > On Tue, Jan 15, 2008 at 10:53:29AM -0500, Chris Mason wrote:
> > > Btrfs is still in an early alpha state, and the disk format is not
> > > finalized. v0.10 introduces a new disk format, and is not compatible with
> > > v0.9.
> > >
> > > The core of this release is explicit back references for all metadata
> > > blocks, data extents, and directory items.  These are a crucial building
> > > block for future features such as online fsck and migration between
> > > devices.  The back references are verified during deletes, and the extent
> > > back references are checked by the existing offline fsck tool.
> >
> > Just as a random datapoint, Maildir access (read only) via mutt feels
> > much more sluggish than on 0.5, which is the last release of btrfs I
> > played with.  The Maildirs have just been created by using rsync to
> > copy them from my usual reiserfs partition.  It's still massively
> > slower than reiserfs for the Maildir access pattern that I have.
> 
> With atimes on or off?  What is the maildir access pattern?  The backrefs make 
> it possible to do a bunch of interesting readdir optimizations.

That's a point, mounted btrfs as "default" but reiserfs is
noatime,nodiratime.  I'll go add a noatime to the btrfs mount
options and try that.

Access pattern is whatever mutt is doing - presumably getting
a directory listing and then reading the start of each file it
hasn't got in its cache yet.

> You'll also bang into the locking if you have a number of processes doing 
> cold-cache reads at the same time.

Generally not - it's either just the single mutt process or a mutt
process and a (multi-threaded) python offlineimap process - but I
always run them by hand, and only when I'm online (obviously).

Bron.