btrfs: stripe_alloc: report read-modify-write of uncovered stripes
The mount-time warning says which block groups stripe_alloc does not
cover. It cannot say whether anything is actually landing there, and on
a filesystem with raid56 metadata that is the interesting question: every
sub-stripe write to an uncovered stripe is a write hole window, where
parity and data reach the disk separately and a crash in between leaves
the stripe unreconstructible.
Report it from the one place that knows the write is going out as a
read-modify-write rather than as a full or padded stripe. Rate limited,
because a raid56-metadata filesystem does this continuously and the point
is to make the exposure visible rather than to fill the log, and paired
with a meta_rmw counter in the existing stripe_park_stats sysfs file so
the rate can be read off without grepping dmesg.
Silent when stripe_alloc is off: there the whole filesystem works this
way and the user has asked for nothing else.
projects / linux / commit