On Tue, Aug 06, 2024 at 05:23:58PM -0700, Darrick J. Wong wrote: > On Wed, Aug 07, 2024 at 09:52:06AM +1000, Dave Chinner wrote: > > On Tue, Aug 06, 2024 at 11:56:51AM -0700, Darrick J. Wong wrote: > > > On Thu, Aug 01, 2024 at 04:30:46PM +0000, John Garry wrote: > > > > From: Dave Chinner <dchinner@xxxxxxxxxx> > > > > > > > > We currently align extent allocation to stripe unit or stripe width. > > > > That is specified by an external parameter to the allocation code, > > > > which then manipulates the xfs_alloc_args alignment configuration in > > > > interesting ways. > > > > > > > > The args->alignment field specifies extent start alignment, but > > > > because we may be attempting non-aligned allocation first there are > > > > also slop variables that allow for those allocation attempts to > > > > account for aligned allocation if they fail. > > > > > > > > This gets much more complex as we introduce forced allocation > > > > alignment, where extent size hints are used to generate the extent > > > > start alignment. extent size hints currently only affect extent > > > > lengths (via args->prod and args->mod) and so with this change we > > > > will have two different start alignment conditions. > > > > > > > > Avoid this complexity by always using args->alignment to indicate > > > > extent start alignment, and always using args->prod/mod to indicate > > > > extent length adjustment. > > > > > > > > Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx> > > > > [jpg: fixup alignslop references in xfs_trace.h and xfs_ialloc.c] > > > > Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx> > > > > > > Going back to the 6/21 posting[1], what were the answers to the > > > questions I posted? Did I correctly figure out what alignslop refers > > > to? > > > > Hard to say. > > > > alignslop is basically an temporary accounting mechanism used to > > prevent filesystem shutdowns when the AG is near ENOSPC and exact > > BNO allocation is attempted and fails because there isn't an exact > > free space available. This exact bno allocation attempt can dirty > > the AGFL, and before we dirty the transaction *we must guarantee the > > allocation will succeed*. If the allocation fails after we've > > started modifying metadata (for whatever reason) we will cancel a > > dirty transaction and shut down the filesystem. > > > > Hence the first allocation done from the xfs_bmap_btalloc() context > > needs to account for every block the specific allocation and all the > > failure fallback attempts *may require* before it starts modifying > > metadata. The contiguous exact bno allocation case isn't an aligned > > allocation, but it will be followed by an aligned allocation attempt > > if it fails and so it must take into account the space requirements > > of aligned allocation even though it is not an aligned allocation > > itself. > > > > args->alignslop allows xfs_alloc_space_available() to take this > > space requirement into account for any allocation that has lesser > > alignment requirements than any subsequent allocation attempt that > > may follow if this specific allocation attempt fails. > > > > IOWs, args->alignslop is similar to args->minleft and args->total in > > purpose, but it only affects the accounting for this specific > > allocation attempt rather than defining the amount of space > > that needs to remain available at the successful completion of this > > allocation for future allocations within this transaction context. > > Oh, okay. IOWs, "slop" is a means for alloc callers to communicate that > they need to perform an aligned allocation, but that right now they want > to try an exact allocation (with looser alignment) so that they might > get better locality. However, they don't want the exact allocation scan > to commit to a certain AG unless the aligned allocation will be able to > find an aligned space *somewhere* in that AG if the exact scan fails. > For any other kind of allocation situation, slop should be zero. > > If the above statement is correct, It is, except for a small nit: alignslop isn't exact bno allocation specific, it allows any sort of unaligned -> fail -> aligned allocation fallback pattern to select an AG where the fallback aligned allocation will have space to succeed. > could we paste that into the > definition of struct xfs_alloc_arg so that I don't have to recollect all > this the next time I see something involving alignslop? Sure. > After which > I'm ok saying: > > Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx> Thanks! -Dave. -- Dave Chinner david@xxxxxxxxxxxxx
