MySQL said "InnoDB: Initializing buffer pool, size = 128.0M", does it mean it is using 128MB RAM? I am just using a very few InnoDB tables (e.g. less than 1MB), but during MySQL startup, it said 

InnoDB: Initializing buffer pool, size = 128.0M


Does it mean even I am using in such a small size, the server still use 128M RAM?
According to the MySQL Documentation, the InnoDB Buffer Pool is set to 128MB by default in MySQL 5.5.

You can show how much of the InnoDB Buffer Pool is in use and reserved as follows:

SELECT
    BufferPoolUsed BytesUsed,
    (BufferPoolUsed / power(1024,1)) UsedKB,
    (BufferPoolUsed / power(1024,2)) UsedMB,
    (BufferPoolUsed / power(1024,3)) UsedGB,
    BufferPoolReserved BytesReserved,
    (BufferPoolReserved / power(1024,1)) ReservedKB,
    (BufferPoolReserved / power(1024,2)) ReservedMB,
    (BufferPoolReserved / power(1024,3)) ReservedGB
FROM
    (
        SELECT
            (A.num * B.num) BufferPoolUsed,
            (A.num * C.num) BufferPoolReserved
        FROM
            (SELECT variable_value num FROM information_schema.global_status
            WHERE variable_name='Innodb_page_size') A,
            (SELECT variable_value num FROM information_schema.global_status
            WHERE variable_name='Innodb_buffer_pool_pages_data') B,
            (SELECT variable_value num FROM information_schema.global_status
            WHERE variable_name='Innodb_buffer_pool_pages_total') C
    ) AA
;


I have it set to 256M in MySQL 5.5.12 for Windows. Here is my output:

mysql&gt; SELECT
    -&gt;     BufferPoolUsed BytesUsed,
    -&gt;     (BufferPoolUsed / power(1024,1)) UsedKB,
    -&gt;     (BufferPoolUsed / power(1024,2)) UsedMB,
    -&gt;     (BufferPoolUsed / power(1024,3)) UsedGB,
    -&gt;     BufferPoolReserved BytesReserved,
    -&gt;     (BufferPoolReserved / power(1024,1)) ReservedKB,
    -&gt;     (BufferPoolReserved / power(1024,2)) ReservedMB,
    -&gt;     (BufferPoolReserved / power(1024,3)) ReservedGB
    -&gt; FROM
    -&gt;     (
    -&gt;         SELECT
    -&gt;             (A.num * B.num) BufferPoolUsed,
    -&gt;             (A.num * C.num) BufferPoolReserved
    -&gt;         FROM
    -&gt;             (SELECT variable_value num FROM information_schema.global_status
    -&gt;             WHERE variable_name='Innodb_page_size') A,
    -&gt;             (SELECT variable_value num FROM information_schema.global_status
    -&gt;             WHERE variable_name='Innodb_buffer_pool_pages_data') B,
    -&gt;             (SELECT variable_value num FROM information_schema.global_status
    -&gt;             WHERE variable_name='Innodb_buffer_pool_pages_total') C
    -&gt;     ) AA
    -&gt; ;
+-----------+--------+----------+--------------------+---------------+------------+------------+------------+
| BytesUsed | UsedKB | UsedMB   | UsedGB             | BytesReserved | ReservedKB | ReservedMB | ReservedGB |
+-----------+--------+----------+--------------------+---------------+------------+------------+------------+
|   6864896 |   6704 | 6.546875 | 0.0063934326171875 |     268435456 |     262144 |        256 |       0.25 |
+-----------+--------+----------+--------------------+---------------+------------+------------+------------+
1 row in set (0.00 sec)

mysql&gt;


You can set this value when MySQL starts up. For example:

To set it to use 1G of RAM

[mysqld]
innodb_buffer_pool_size=1G


To set it to use 256MB of RAM

[mysqld]
innodb_buffer_pool_size=256M


After changing it in my.cnf, you must restart mysql for it to take effect.