memcached 安装并正常启动，joomal 开启会话存储为memcached，无法登陆。

安装memcached启动，测试正常。用以下PHP文件检验
<?php
$a = new Memcached();
$a->addServer('localhost', 11211); 
print_r($a->getVersion());
print_r($a->getStats());
返回显示
Array ( [localhost:11211] => 1.4.36 )
Array ( [localhost:11211] =>
Array ( [pid] => 1061
[uptime] => 6923
[threads] => 4
[time] => 1506064083
[pointer_size] => 64
[rusage_user_seconds] => 0
[rusage_user_microseconds] => 506545
[rusage_system_seconds] => 0
[rusage_system_microseconds] => 611347
[curr_items] => 144
[total_items] => 399
[limit_maxbytes] => 1073741824
[curr_connections] => 35
[total_connections] => 47
[connection_structures] => 36
[bytes] => 38749
[cmd_get] => 2472
[cmd_set] => 399
[get_hits] => 2157
[get_misses] => 315
[evictions] => 0
[bytes_read] => 276149
[bytes_written] => 616975
[version] => 1.4.36 ) )
但是，joomla后台会话存储，从 database 改为 memcached，服务器重启，网站前台、后台均无法登陆，无法开始会话，无报错信息。
PHP设置文件，ini里会话存储也改为，memcached，保存路径，设为，"127.0.0.1:11211"，同时，将会话ID仅用cookies设为0,。
phpmyadmin 登陆时，重启服务器，无法登陆，并提示错误如下：
Warning in ./libraries/session.inc.php#105
 session_start(): Write of lock failed

Backtrace

./libraries/session.inc.php#105: session_start()
./libraries/common.inc.php#350: require(./libraries/session.inc.php)
./index.php#12: require_once(./libraries/common.inc.php)
Warning in ./libraries/session.inc.php#105
 session_start(): Unable to clear session lock record

Backtrace

./libraries/session.inc.php#105: session_start()
./libraries/common.inc.php#350: require(./libraries/session.inc.php)
./index.php#12: require_once(./libraries/common.inc.php)
 
请问，这是什么原因？     阿里云，centos 7.2 64位。