What are the performance measures of Disks?

Performance Measures of Disks

The main measures of the qualities of a disk are capacity, access time, data transfer rate, and reliability.

Access time: the time from when a read or write request is issued to when data transfer begins. To access data on a given sector of a disk, the arm first must move so that it is positioned over the correct track, and then must wait for the sector to appear under it as the disk rotates. The time for repositioning the arm is called seek time, and it increases with the distance the arm must move. Typical seek time range from 2 to 30 milliseconds.

Average seek time is the average of the seek time, measured over a sequence of (uniformly distributed) random requests, and it is about one third of the worst-case seek time.

Once the seek has occurred, the time spent waiting for the sector to be accesses to appear under the head is called rotational latency time. Average rotational latency time is about half of the time for a full rotation of the disk. (Typical rotational speeds of disks ranges from 60 to 120 rotations per second).

The access time is then the sum of the seek time and the latency and ranges from 10 to 40 milli-sec.

Data transfer rate, the rate at which data can be retrieved from or stored to the disk. Current disk systems support transfer rate from 1 to 5 megabytes per second.

Reliability, measured by the mean time to failure. The typical mean time to failure of disks today ranges from 30,000 to 800,000 hours (about 3.4 to 91 years).

What is the maximum length of a table name, database name, and fieldname in MySQL?

MySQL is an open source database management system. It is highly popular because of its high reliability, ease of use and high performance. MySQL is used for many latest applications.

The following table describes the maximum length and allowable characters for each type of identifier.

For MySql:

tablename=64characters long (maximum)

databasename=64characters long (maximum)

fieldname=64characters long (maximum)

In addition to the restrictions noted in the table, no identifier can contain ASCII 0 or a byte with a value of 255. Database, table, and column names should not end with space characters. The use of identifier quote characters in identifiers is permitted, although it is best to avoid doing so if possible.

Identifiers are stored using Unicode. This applies to identifiers in table definitions that stored in.frm files and to identifiers stored in the grant tables in the mysql database. The sizes of the string columns in the grant tables (and in any other tables) in MySQL 5.1 are given as number of characters. This means that (unlike some earlier versions of MySQL) we can use multi-byte characters without reducing the number of characters allowed for values stored in these columns.