Comment 5 for bug 1271177

a)
As for

"
This is important as it breaks backups made with Percona Xtrabackup, as it does some SHOW STATUS queries after it sets FTWRL.
The example error from innobackupex script looks like this:
"DBD::mysql::db selectall_hashref failed: Lock wait timeout exceeded; try restarting transaction at /usr/bin/innobackupex line 1652.
innobackupex: Error:
Error executing 'SHOW STATUS': DBD::mysql::db selectall_hashref failed: Lock wait timeout exceeded; try restarting transaction at /usr/bin/innobackupex line 1652."
"

It may be more suitable here if innobackupex did 'set session wsrep_causal_reads=0' before show status, will check with xtrabackup developers on this.

b)

It may be necessary to block even for show status since status
variables like wsrep_last_committed, for instance, depends on
application of the writeset.