On Tue, Sep 29, 2015 at 08:10:19AM -0400, Austin S Hemmelgarn wrote:
> On 2015-09-29 08:00, David Sterba wrote:
> >On Mon, Sep 28, 2015 at 05:57:05PM +0000, Gabríel Arthúr Pétursson wrote:
> >>The attached patches to linux and btrfs-progs add support for filtering
> >>based on the number of strips in a block when balancing.
> >
> >What usecase do you want to address? As I understand it, this would help
> >the raid56 rebalancing to process only blockgroups that are not spread
> >accross enough devices.

   Exactly. Last week, I was trying to help Gabríel on IRC with a
close-to-full filesystem balance it to add some new devices in a
parity RAID configuration. He'd added the devices and balanced, but
the usage was unequal across the devices. The only way I could think
of dealing with it with the current tools was either to do a full
balance repeatedly until it worked itself out, or to delve into the
metadata with btrfs-debug-tree, and balance selected block groups
individually.

   I whinged that we needed a filter to pick just the block groups
that weren't "as full as possible", and Gabríel picked up the idea and
ran with it.

> This could also be helpful when reshaping a raid10 or raid0 setup.

   Yes, although ultimately less important in most cases, I think,
because you don't lose space by reducing the number of devices in a
block group for those. There are some corner cases where you could end
up losing space by having one (for RAID-0) or up to three (for
RAID-10) devices with more space left than the rest.

   Hugo.

-- 
Hugo Mills             | Dullest spy film ever: The Eastbourne Ultimatum
hugo@... carfax.org.uk |
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PGP: E2AB1DE4          |                                       The Thick of It