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(Continued from previous question...)
MySQL - Disk Issues
As mentioned before, disks seeks are a big performance bottleneck. This problems gets more and more apparent when the data
starts to grow so large that effective caching becomes impossible. For large databases, where you access data more or less
randomly, you can be sure that you will need at least one disk seek to read and a couple of disk seeks to write things. To
minimize this problem, use disks with low seek times.
Increase the number of available disk spindles (and thereby reduce the seek overhead) by either symlink files to different
disks or striping the disks.
Using symbolic links
This means that you symlink the index and/or data file(s) from the normal data directory to another disk (that may also be
striped). This makes both the seek and read times better (if the disks are not used for other things).
Striping
Striping means that you have many disks and put the first block on the first disk, the second block on the second disk, and
the Nth on the (N mod number_of_disks) disk, and so on. This means if your normal data size is less than the stripe size (or
perfectly aligned) you will get much better performance. Note that striping is very dependent on the OS and stripe-size. So
benchmark your application with different stripe-sizes.
Note that the speed
difference for striping is very dependent on the parameters. Depending on how you set the striping parameters and number of
disks you may get a difference in orders of magnitude. Note that you have to choose to optimize for random or sequential
access.
For reliability you may want to use RAID 0+1 (striping + mirroring), but in this case you will need 2*N drives to hold N
drives of data. This is probably the best option if you have the money for it! You may, however, also have to invest in some
volume-management software to handle it efficiently.
A good option is to have semi-important data (that can be regenerated) on RAID 0 disk while storing really important data
(like host information and logs) on a RAID 0+1 or RAID N disk. RAID N can be a problem if you have many writes because of
the time to update the parity bits.
You may also set the parameters for the file system that the database uses. One easy change is to mount the file system with
the noatime option. That makes it skip the updating of the last access time in the inode and by this will avoid some disk
seeks.
On Linux, you can get much more performance (up to 100 % under load is not uncommon) by using hdpram to configure your
disk's interface! The following should be quite good hdparm options for MySQL (and probably many other applications):
hdparm -m 16 -d 1
Note that the performance/reliability when using the above depends on your hardware, so we strongly suggest that you test
your system throughly after using hdparm! Please consult the hdparm man page for more information! If hdparm is not used
wisely, filesystem corruption may result. Backup everything before experimenting!
On many operating systems you can mount the disks with the 'async' flag to set the file system to be updated asynchronously.
If your computer is reasonable stable, this should give you more performance without sacrificing too much reliability. (This
flag is on by default on Linux.)
If you don't need to know when a file was last accessed (which is not really useful on a databasa server), you can mount
your file systems with the noatime flag.
(Continued on next question...)
