tree 0cc010718f63769debe1e45f7ff20bffc6db0b88
parent 07e83364845e1e1c7e189a01206a9d7d33831568
author Song Liu <songliubraving@fb.com> 1485283530 -0800
committer Shaohua Li <shli@fb.com> 1485285966 -0800

md/r5cache: disable write back for degraded array

write-back cache in degraded mode introduces corner cases to the array.
Although we try to cover all these corner cases, it is safer to just
disable write-back cache when the array is in degraded mode.

In this patch, we disable writeback cache for degraded mode:
1. On device failure, if the array enters degraded mode, raid5_error()
   will submit async job r5c_disable_writeback_async to disable
   writeback;
2. In r5c_journal_mode_store(), it is invalid to enable writeback in
   degraded mode;
3. In r5c_try_caching_write(), stripes with s->failed>0 will be handled
   in write-through mode.

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>