On Mon, Feb 24, 2014 at 03:04:21PM +0900, sat wrote: > From: Satoru Takeuchi <takeuchi_satoru@xxxxxxxxxxxxxx> > > The end-of-file preallocation size is dynamic by default. > So the description about the default size of preallocation > should be removed. > > Instead, adding the description about the initial size of > dynamic preallocation is useful for users. > > Signed-off-by: Satoru Takeuchi <takeuchi_satoru@xxxxxxxxxxxxxx> > Cc: Dave Chinner <david@xxxxxxxxxxxxx> > Cc: Rob Landley <rob@xxxxxxxxxxx> > --- > Documentation/filesystems/xfs.txt | 16 ++++++++-------- > 1 files changed, 8 insertions(+), 8 deletions(-) > > diff --git a/Documentation/filesystems/xfs.txt b/Documentation/filesystems/xfs.txt > index 5be51fd..30e57b0 100644 > --- a/Documentation/filesystems/xfs.txt > +++ b/Documentation/filesystems/xfs.txt > @@ -23,16 +23,16 @@ default behaviour. > > allocsize=size > Sets the buffered I/O end-of-file preallocation size when > - doing delayed allocation writeout (default size is 64KiB). > - Valid values for this option are page size (typically 4KiB) > - through to 1GiB, inclusive, in power-of-2 increments. > + doing delayed allocation writeout. Valid values for this > + option are page size (typically 4KiB) through to 1GiB, > + inclusive, in power-of-2 increments. > > The default behaviour is for dynamic end-of-file > - preallocation size, which uses a set of heuristics to > - optimise the preallocation size based on the current > - allocation patterns within the file and the access patterns > - to the file. Specifying a fixed allocsize value turns off > - the dynamic behaviour. > + preallocation size (initial size is 64KiB), which uses a Actually, it isn't. The initial size of dynamic prealloc is determined by a set of heuristics based on file size, filesystem geometry and the existing extents on the file. It uses a default of 64k if no optimisation heuristics trigger. Therefore, it may actually be zero - we don't do any speculative preallocation for small files until they grow past a specific threshold. i.e. there is a small file optimisation heuristic that takes precendence over the default, and so the comment about the initial prealloc size being 64k is wrong. The initial prealloc size is zero.... An example at the other end of the spectrumL we preallocate a chunk of a file, then write past the end of the preallocated region. The initial speculative preallocation is triggered once we pass the preallocated region, and the heuristics can result in an initial size anywhere in the range of zero (small file) to 2^21 blocks (large, multi-GB file). Hence I think that the generalised text saying "default is 64k and is optimised based on heuristics" reflects the behaviour of the algorithm much better than saying "<algorithmic heuristic> and is optimised by algorithmic heuristics".... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
