Zabbix value cache working in low memory mode

Zabbix监控自身时告警“Zabbix value cache working in low memory mode”，出现这个问题是因为Zabbix Server的参数ValueCacheSize大小设置不合理。一般默认值为8M。

### Option: ValueCacheSize

# Size of history value cache, in bytes.

# Shared memory size for caching item history data requests.

# Setting to 0 disables value cache.

# Mandatory: no

# Range: 0,128K-64G

# Default:

# ValueCacheSize=8M

ValueCacheSize	否	0,128K-64G	8M	历史数据缓存大小, 单位为字节。 0即禁止缓存 (不建议). 当缓存大小超过共享内存时，每5分钟会向服务器日志写入一条警告信息。从 Zabbix 2.2.0开始支持该参数。
ValueCacheSize	no	0,128K-64G	8M	Size of history value cache, in bytes. Shared memory size for caching item history data requests. Setting to 0 disables value cache (not recommended). When value cache runs out of the shared memory a warning message is written to the server log every 5 minutes. This parameter is supported since Zabbix 2.2.0.