运行mysql出现错误：
The server quit without updating PID file ...
原因：
内存太小
解决办法1：
使用下面my.cnf
# Example MySQL config file for small systems.
#
# This is for a system with little memory (<= 64M) where MySQL is only used
# from time to time and it's important that the mysqld daemon
# doesn't use much resources.
#
# MySQL programs look for option files in a set of
# locations which depend on the deployment platform.
# You can copy this option file to one of those
# locations. For information about these locations, see:
# http://dev.mysql.com/doc/mysql/en/option-files.html
#
# In this file, you can use all long options that a program supports.
# If you want to know which options a program supports, run the program
# with the "--help" option.
# The following options will be passed to all MySQL clients
[client]
#password = your_password
#port = @MYSQL_TCP_PORT@
#socket = @MYSQL_UNIX_ADDR@
# Here follows entries for some specific programs
# The MySQL server
[mysqld]
port = 3306
socket = /tmp/mysql.sock
skip-external-locking
key_buffer_size = 16K
max_allowed_packet = 1M
table_open_cache = 4
sort_buffer_size = 64K
read_buffer_size = 256K
read_rnd_buffer_size = 256K
net_buffer_length = 2K
thread_stack = 128K
basedir=/usr/local/mysql
datadir=/usr/local/mysql/data
# Don't listen on a TCP/IP port at all. This can be a security enhancement,
# if all processes that need to connect to mysqld run on the same host.
# All interaction with mysqld must be made via Unix sockets or named pipes.
# Note that using this option without enabling named pipes on Windows
# (using the "enable-named-pipe" option) will render mysqld useless!
#
#skip-networking
server-id = 1
# Uncomment the following if you want to log updates
#log-bin=mysql-bin
# binary logging format - mixed recommended
#binlog_format=mixed
# Causes updates to non-transactional engines using statement format to be
# written directly to binary log. Before using this option make sure that
# there are no dependencies between transactional and non-transactional
# tables such as in the statement INSERT INTO t_myisam SELECT * FROM
# t_innodb; otherwise, slaves may diverge from the master.
#binlog_direct_non_transactional_updates=TRUE
# Uncomment the following if you are using InnoDB tables
#innodb_data_home_dir = @localstatedir@
#innodb_data_file_path = ibdata1:10M:autoextend
#innodb_log_group_home_dir = @localstatedir@
# You can set .._buffer_pool_size up to 50 - 80 %
# of RAM but beware of setting memory usage too high
#innodb_buffer_pool_size = 16M
#innodb_additional_mem_pool_size = 2M
# Set .._log_file_size to 25 % of buffer pool size
#innodb_log_file_size = 5M
#innodb_log_buffer_size = 8M
#innodb_flush_log_at_trx_commit = 1
#innodb_lock_wait_timeout = 50
[mysqldump]
quick
max_allowed_packet = 16M
[mysql]
no-auto-rehash
# Remove the next comment character if you are not familiar with SQL
#safe-updates
[myisamchk]
key_buffer_size = 8M
sort_buffer_size = 8M
[mysqlhotcopy]
interactive-timeout
即使使用方法1解决了不能启动的问题，mysql还是会偶尔因为内存不足而down掉。
解决办法2：
阿里云的主机 不是默认自带swap的 ，增加swap，可以缓解实际内存小的问题，当然性能不如实际物理内存好，但起码不会出现mysql不能启动，或者down掉的情况。增加swap的方法：
1.进入目录 欢迎访问，鄙视采集！
cd /var/ http://www.zrblog.net
2.获取要增加的SWAP文件块(这里以1GB为例)
dd if=/dev/zero of=swapfile bs=1024 count=1038336 欢迎访问，鄙视采集！
3.创建SWAP文件
/sbin/mkswap swapfile
4.激活SWAP文件
/sbin/swapon swapfile
5.查看SWAP信息是否正确
/sbin/swapon –s
6.添加到fstab文件中让系统引导时自动启动
echo "/var/swapfile swap swap defaults 0 0" >>/etc/fstab
上述2个方法仍然不能完全避免mysql偶尔down掉的情况。