* Set if we've tried all mirrors for this read IO, if it's not set,
* the read IO will check to-be-read OSCs' status, and make fast-switch
* another mirror if some of the OSTs are not healthy.
* Set if we've tried all mirrors for this read IO, if it's not set,
* the read IO will check to-be-read OSCs' status, and make fast-switch
* another mirror if some of the OSTs are not healthy.