original in en Mark Nielsen
The author works at The Computer Underground, Inc. as a Linux geek and enjoys doing silly things and making up silly projects, because hey, computers are SUPPOSED to be fun. Mark also works at ZING, www.genericbooks.com as a volunteer to help and promote free and open software and literature.
Ramdisk is basically memory that you pretend to use as a hard disk drive. Anytime you use a ramdisk, you basically are using memory and not your hard drive. There are advantages and disadvantages to this. Basically, the big advantage is that since you are using memory, whatever you are doing will be much faster since your hard drive is slower than your memory. The big disadvantage is that if you make changes to the database server and you reboot your computer, you will loose all your changes.
For the database server PostgreSQL, if you load all the databases in memory, you will increase its speed.
mkdir -p /Test mkfs -t ext2 /dev/ram0 mount /dev/ram0 /TestIf this fails then you might have no ramdisk support compiled into the Kernel. CONFIG_BLK_DEV_RAM is the Kernel configuration option which you need to enable it.
The above gives you a ramdisk with available space of just under 4Mb. Have a look at the Ramdisk Article to see how you can change this to e.g 50Mb.
NOTE: We assume you have set the ramdisk option in your lilo.conf file to be larger than the size of your database server. If you need to find the approximate size, issue this command "cd /var/lib/pgsql; du ".
However, to take your current postgresql server installed at "/var/lib/pgsql" and to put it into memory, do this,
### Stop the current postgresql server /etc/rc.d/init.d/postgres stop ### rename the current directory mv /var/lib/pgsql /var/lib/pgsql_main #### Create a directory to have our ramdisk on mkdir -p /var/lib/pgsql_memory #### change the ownership of the new directory to postgres or whatever #### the actual owner is. chown postgres /var/lib/pgsql_memory #### Make an alias or link to the original name, /var/lib/pgsql ln -s /var/lib/pgsql_memory /var/lib/pgsql #### Format the ramdisk mkfs -t ext2 /dev/ram0 #### Mount the ramdisk to the postgresql directory mount /dev/ram0 /var/lib/pgsql_memory #### Copy everything from the main directory into the ramdisk tar -C /var/lib/pgsql_main -cp . | tar -C /var/lib/pgsql_memory -xp ### Start the current postgresql server /etc/rc.d/init.d/postgres start
What I noticed is that when the data is cached by the system, the performance gain is not very much, about 10% or 20%. When there is lots of unique data being pumped into the database, the performance goes up dramatically.
The biggest advantage with the ramdisk trick is that you force the database to remain in memory and you avoid using your hard drive. If you don't use ramdisk, your database will probably get swapped out of cache eventually, and then when it gets used again, it will have to reload from the hard drive.