chmod() and utimes()/touch of an EC file with a data stripe on an unavailable OST block until the OST returns. chmod() reaches the OSTs because the VFS sets ATTR_CTIME, which ll_setattr_raw() sends to the objects via cl_setattr_ost(). See the LU-20212 test results.

osc_io_setattr_end < cl_io_end < lov_io_end_wrapper < lov_io_call < lov_io_end < cl_io_end < cl_io_loop < cl_setattr_ost < ll_setattr_raw < ll_setattr < notify_change < chmod_common

The MDS holds the authoritative timestamps, so the OST setattr of the unavailable object could be skipped or sent no-delay for an ECRO file, and repaired by lfs mirror resync as proposed for UID/GID/PROJID in LU-20287.