Re: [PATCH] [RFC] spaceman: physically move a regular inode

[Brian Foster <bfoster@redhat.com>]

On Fri, Dec 04, 2020 at 05:10:59PM +1100, Dave Chinner wrote:
> On Wed, Dec 02, 2020 at 07:30:06AM -0500, Brian Foster wrote:
> > On Wed, Dec 02, 2020 at 08:15:57AM +1100, Dave Chinner wrote:
> > > On Tue, Dec 01, 2020 at 09:07:42AM -0500, Brian Foster wrote:
> > > > On Wed, Nov 11, 2020 at 09:59:24AM +1100, Dave Chinner wrote:
> > > > For example, might it make sense to implement a policy where move_inode
> > > > simply moves an inode to the first AG the tempdir lands in that is < the
> > > > AG of the source inode? We'd probably want to be careful to make sure
> > > > that we don't attempt to dump the entire set of moved files into the
> > > > same AG, but I assume the temp dir creation logic would effectively
> > > > rotor across the remaining set of AGs we do want to allow.. Thoughts?
> > > 
> > > Yes, we could. But I simply decided that a basic, robust shrink to
> > > the minimum possible size will have to fill the filesystem from AG 0
> > > up, and not move to AG 1 until AG 0 is full.  I also know that the
> > > kernel allocation policies will skip to the next AG if there is lock
> > > contention, space or other allocation setup issues, hence I wanted
> > > to be able to direct movement to the lowest possible AGs first...
> > > 
> > > THere's enough complexity in an optimal shrink implementation that
> > > it will keep someone busy full time for a couple of years. I want to
> > > provide the basic functionality userspace needs only spending a
> > > couple of days a week for a couple of months on it. If we want it
> > > fast and deployable on existing systems, compromises will need to be
> > > made...
> > > 
> > 
> > Yeah, I'm not suggesting we implement the eventual policy here. I do
> > think it would be nice if the userspace command implemented some
> > reasonable default when a target AG is not specified. That could be the
> > "anything less than source AG" logic I described above, a default target
> > of AG 0, or something similarly simple...
> 
> That's the plan. This patch is just a way of testing the mechanism
> in a simple way without involving a full shrink or scanning AGs, or
> anything like that.
> 
> i.e:
> 
> $ ~/packages/xfs_spaceman  -c "help move_inode" -c "help find_owner" -c "help resolve_owner" -c "help relocate" /mnt/scratch
> move_inode -a agno -- Move an inode into a new AG.
> 
> Physically move an inode into a new allocation group
> 
>  -a agno       -- destination AG agno for the current open file
> 
> find_owner -a agno -- Find inodes owning physical blocks in a given AG
> 
> Find inodes owning physical blocks in a given AG.
> 
>  -a agno  -- Scan the given AG agno.
> 
> resolve_owner  -- Resolve paths to inodes owning physical blocks in a given AG
> 
> Resolve inodes owning physical blocks in a given AG.  This requires
> the find_owner command to be run first to populate the table of
> inodes that need to have their paths resolved.
> 
> relocate -a agno [-h agno] -- Relocate data in an AG.
> 
> Relocate all the user data and metadata in an AG.
> 
> This function will discover all the relocatable objects in a single
> AG and move them to a lower AG as preparation for a shrink
> operation.
> 
> 	-a <agno>       Allocation group to empty
> 	-h <agno>       Highest target AG allowed to relocate into
> $
> 

Ah, I see. This relocate command is essentially what I was asking for,
it just wasn't apparent from the move_inode bits alone that this was
covered somewhere. I do think there's value in dropping this in
userspace early, even if it's just a crude/isolated implementation for
now, because that helps motivate keeping the kernel bits as simple as
possible for the broader feature. Thanks for the description.

Brian

> So, essentially, I can test all the bits in one command with
> "relocate", or I can test different types of objects 1 at a time
> with "move_inode", or I can look at what "relocate" failed to move
> with "find_owner" and "resolve_owner"....
> 
> An actual shrink operation will effectively run "relocate" on all
> the AGs that it wants to empty, setting the highest AG that
> relocation is allowed into to the last full AG that will remain in
> the shrunk filesystem, then check the AGs are empty, then run the
> shrink ioctl....
> 
> But to get there, I'm bootstrapping the functionality one testable
> module at a time, then refactoring them to combine them into more
> complex operations...
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com
>