btrfs: stripe_alloc: isolate nodatacow and preallocated extents by stripe
Preallocated extents and nodatacow files' extents are candidates for
write-in-place, which reintroduces the raid56 write hole for every
stripe such a write touches: the RMW recomputes parity that also covers
whatever else shares the stripe. Before write-in-place can be
re-enabled for them (a later change; stripe_alloc still forces COW
today), their placement must guarantee the blast radius: such an extent
must never share a stripe with any other file's data.
Steer them into private per-inode stripe runs of a new NOCOW class,
reusing the log-active inode machinery: runs owned by one inode, never
in the shared band slots, found by owner-and-class lookup. Successive
allocations of the same file pack sequentially into the file's own
stripes; different files, and the datacow/relocation/log classes, never
share a stripe with them. A preallocation signals itself through a new
btrfs_reserve_extent() parameter; nodatacow files are recognized by the
inode flag. Placement remains best effort: when no fully-free stripes
are left for a private run the allocation falls back to the shared
runs, which is safe -- an extent that lands in a shared stripe simply
stays force-COWed when write-in-place arrives.
projects / linux / commit