CentOS VM only using a third of available memory

Question

I have a VM slice running CentOS, on Parallels.

Here is the output of free -m

[root@server ~]# free -m
             total       used       free     shared    buffers     cached
Mem:           960        272        687          0          0          0
-/+ buffers/cache:        272        687
Swap:            0          0          0

This is a LAMP server, and the database itself is over 1000 MB.

Shouldn't the "buffers" and "cached" be showing higher values, caching the database files? It looks like only 1/3 of memory is being used.

MySQL, shouldn't Linux cache files in memory, if there is free memory? — soupagain, Sep 11 '12 at 12:03

score 1 · Answer 1 · edited Apr 13 '17 at 12:43

The buffers and caches displayed are from the point-of-view of the OS.

To see what MySQL is caching you must run the following queries

MYSQL_CONN="-uroot -prootpassword"
BPSIZE=`mysql ${MYSQL_CONN} -ANe"SHOW GLOBAL VARIABLES LIKE 'innodb_buffer_pool_size'" | awk '{print $2}'`
BPDATA=`mysql ${MYSQL_CONN} -ANe"SHOW GLOBAL STATUS LIKE 'Innodb_buffer_pool_pages_data'" | awk '{print $2}'`
BPPAGE=`mysql ${MYSQL_CONN} -ANe"SHOW GLOBAL STATUS LIKE 'Innodb_page_size'" | awk '{print $2}'`
BPDATABYTES=`echo ${BPDATA}*${BPPAGE}|bc`
BPPCTUSED=`echo ${BPDATABYTES}00/${BPSIZE}|bc`
KBSIZE=`mysql ${MYSQL_CONN} -ANe"SHOW GLOBAL VARIABLES LIKE 'key_buffer_size'" | awk '{print $2}'`
KBUSED=`mysql ${MYSQL_CONN} -ANe"SHOW GLOBAL STATUS LIKE 'Key_blocks_used'" | awk '{print $2}'`
KCSIZE=`mysql ${MYSQL_CONN} -ANe"SHOW GLOBAL STATUS LIKE 'key_cache_block_size'" | awk '{print $2}'`
KBDATABYTES=`echo ${KBUSED}*${KCSIZE}|bc`
KBPCTUSED=`echo ${KBDATABYTES}00/${KBSIZE}|bc`
echo "InnoDB Buffer Pool Size : ${BPSIZE}, (${BPPCTUSED}) Percent Full"
echo "MyISAM Key Buffer Size : ${KBSIZE}, (${KBPCTUSED}) Percent Full"

If you want to see how much data you actually have run this:

SELECT IFNULL(B.engine,'Total') "Storage Engine",
CONCAT(LPAD(REPLACE(FORMAT(B.DSize/POWER(1024,pw),3),',',''),17,' '),' ',
SUBSTR(' KMGTP',pw+1,1),'B') "Data Size",
CONCAT(LPAD(REPLACE(FORMAT(B.ISize/POWER(1024,pw),3),',',''),17,' '),' ',
SUBSTR(' KMGTP',pw+1,1),'B') "Index Size",
CONCAT(LPAD(REPLACE(FORMAT(B.TSize/POWER(1024,pw),3),',',''),17,' '),' ',
SUBSTR(' KMGTP',pw+1,1),'B') "Table Size"
FROM (SELECT engine,SUM(data_length) DSize,SUM(index_length) ISize,
SUM(data_length+index_length) TSize FROM information_schema.tables
WHERE table_schema NOT IN ('mysql','information_schema','performance_schema')
AND engine IS NOT NULL GROUP BY engine WITH ROLLUP) B,(SELECT 3 pw) A
ORDER BY TSize;

This will show how much InnoDB and MyISAM data you have in the MySQL Instance.

Please keep in mind

the sum of MyISAM Indexes is the Maximum key_buffer_size you can allocate for your instance
the sum of InnoDB Data and Index Pages is the Maximum innodb_buffer_pool_size for your instance

For more info on how to size the key_buffer_size and innodb_buffer_pool_size, please see my post in the DBA StackExchange.

CentOS VM only using a third of available memory

1 Answers1