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@xxxxxxxxxxxxx >
