Re: Working on Btrfs as topic for master thesis

[Hugo Mills <hugo@carfax.org.uk>]

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On Tue, Jan 21, 2014 at 07:25:43AM -0500, Austin S Hemmelgarn wrote:
> On 2014-01-21 01:42, Sandy McArthur wrote:
> > On Mon, Jan 20, 2014 at 7:20 AM, Austin S Hemmelgarn
> > <ahferroin7@gmail.com> wrote:
> >>
> >> On 2014-01-16 14:23, Toggenburger Lukas wrote:
> >>> 3. Improving subvolume handling regarding taking recursive snapshots ( https://btrfs.wiki.kernel.org/index.php/Project_ideas#Take_recursive_snapshots ) and taking snapshots of arbitrary directories ( https://btrfs.wiki.kernel.org/index.php/Project_ideas#Snapshot_arbitrary_directories )
> >>
> >> Another option that I would personally love to see would be support for
> >> write-mostly devices (that is, devices in a RAID1/RAID10 setup that only
> >> get written too unless the data can't be found elsewhere).  This would
> >> in particular provide an alternative to using bcache/dm-cache (namely an
> >> SSD and HDD in RAID1 with the HDD set to write-mostly).
> >> Based on the current development focus, I don't think anybody is working
> >> on this already (I would be, but I don't have either the time or the
> >> skills with kernel programming that would be needed).
> > 
> > Maybe this happens already: Might a similar effect be automatically
> > achieved by tracking per-device I/O load averages and distributing
> > reads based on the I/O loads of possible read devices?
> > 
> That might be the case, it depends on how the I/O load averages are
> calculated.  I actually hadn't realized BTRFS did this, I thought it
> behaved more like MD RAID (that is, distributing the reads among devices
> in a un-weighted round-robin fashion).

   I think David tried that a while ago, and the benchmarks were
actually worse. I'm not sure how much investigation he did into why,
though.

   Hugo.

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