md/raid1: don't split discard io for write behind

Currently, discad io is treated the same as normal write io, and for
write behind case, io size is limited to:

BIO_MAX_VECS * (PAGE_SIZE >> 9)

For 0.5KB sector size and 4KB PAGE_SIZE, this is just 1MB. For
consequence, if 'WriteMostly' is set to one of the underlying disks,
then diskcard io will be splited into 1MB and it will take a long time
for the diskcard to finish.

Fix this problem by disable write behind for discard io.

Reported-by: Roman Mamedov <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Reported-and-tested-by: Kirill Kirilenko <[email protected]>
Signed-off-by: Yu Kuai <[email protected]>
Signed-off-by: Song Liu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

0 files changed, 0 insertions, 0 deletions