Hard Disk

Many Linux users love to tinker under the hood to increase the performance of their computers, and Linux gives you some great tools to do just that. Whereas mothers tell us, "Don't fix what's not broken," fathers often say, "Fix it until it breaks." In this section, you learn about many of the commands used to tune, or "tweak," your file system.

Before you undertake any "under the hood" work with Linux, however, keep a few points in mind. First, perform a benchmark on your system before you begin. Linux does not offer a well-developed benchmarking application, but availability changes rapidly. You can search online for the most up-to-date information for benchmarking applications for Linux. If you are a system administrator, you might choose to create your own benchmarking tests. Second, tweak only one thing at a time so that you can tell what works, what does not work, and what breaks. Some of these tweaks might not work or might lock up your machine.

Always have a working boot disk handy and remember that you are personally assuming all risks for attempting any of these tweaks.

Using the BIOS and Kernel to Tune the Disk Drives

One method of tuning involves adjusting the settings in your BIOS. Because the BIOS is not Linux and every BIOS seems different, always read your motherboard manual for better possible settings and make certain that all the drives are detected correctly by the BIOS. Change only one setting at a time.

Linux does provide a limited means to interact with BIOS settings during the boot process (mostly overriding them). In this section, you will learn about those commands.

Other options are in the following list, and are more fully outlined in the BOOTPROMPT HOWTO and the kernel documentation. These commands can be used to force the IDE controllers and drives to be optimally configured. Of course, YMMV (Your Mileage May Vary) because these do not work for everyone.

idex=dma This will force DMA support to be turned on for the primary IDE bus, where x=0, or the secondary bus, where x=1.

idex=autotune This command will attempt to tune the interface for optimal performance.

idex=ata66 If you have ATA66 drives and controllers, this command will enable support for it.

hdx=ide-scsi This command will enable SCSI emulation of an IDE drive. This is required for some CD-RW drives to work properly in write mode and it might provide some performance improvements for regular CD-R drives as well.

idebus=xx This can be any number from 20 to 66; autodetection is attempted, but this can set it manually if dmesg says that it isn't autodetected correctly or if you have it set in the BIOS to a different value (overclocked). Most PCI controllers will be happy with 33.

pci=biosirq Some motherboards might cause Linux to generate an error message saying that you should use this. Look in dmesg for it; if you do not see it, you don't need to use it.

These options can be entered into /etc/lilo.conf or /boot/grub/grub.conf in the same way as other options are appended.

The hdparm Command

The hdparm utility can be used by root to set and tune the settings for IDE hard drives. You would do this to tune the drives for optimal performance.

Once a kernel patch and associated support programs, the hdparm program is now included with Fedora Core. You should experiment only with the drives mounted readonly because some settings can damage some file systems when used improperly. The hdparm command also works with CD-ROM drives and some SCSI drives.

The general format of the command is this:

# hdparm command device

This command runs a hard disk test:

hdparm -tT /dev/hda

You will need to replace /dev/hda with the location of your hard disk. hdparm will then run two tests: cached reads and buffered disk reads. A good IDE hard disk should be getting 400-500MB/sec for the first test, and 20-30MB/sec for the second. Note your scores, and then try this command:

hdparm -m16 -d1 -u1 -c1 /dev/hda

That enables various performance-enhancing settings. Now try executing the original command againif you see an increase, you should run this command:

hdparm -m16 -d1 -u1 -c1 -k1 /dev/hda

The extra parameter tells hdparm to write the settings to disk so that they will be used each time you boot upensuring optimal disk performance in the future.

The man entry for hdparm is extensive and contains useful detailed information, but because the kernel configuration selected by Fedora Core already attempts to optimize the drives, it might be that little can be gained through tweaking. Because not all hardware combinations can be anticipated by Fedora Core or by Linux and performance gains are always useful, you're encouraged to try.

Tip

You can use the hdparm command to produce a disk transfer speed result with

# hdparm -tT device

Be aware, however, that although the resulting numbers appear quantitative, they are subject to several technical qualifications beyond the scope of what is discussed and explained in this chapter. Simply put, do not accept values generated by hdparm as absolute numbers, but only as a relative measure of performance.

Systemwide tweaks to hdparm are formally handled through the /etc/sysconfig/harddisks files, but this file's use is poorly documented and, therefore, of little use.

Tip

You might find that your system turns off DMA for any CD-RW drives detected via a shell script command in rc.sysinit. This might not be appropriate for your hardware, so you can turn it back on simply by adding the line options ide-cd dma=1 to /etc/modules.conf.

File System Tuning

Never content to leave things alone, Linux provides several tools to adjust and customize the file system settings. The belief is that hardware manufacturers and distribution creators tend to select conservative settings that will work well all the time, leaving some of the potential of your system leashedthat's why you have chosen Red Hat Fedora 5 Unleashed to help you.

The Linux file system designers have done an excellent job of selecting default values used for file system creation and the 2.6 version of the Linux kernel now contains new code for the IDE subsystem that significantly improves I/O (input/output) transfer speeds over older versions, obviating much of the need for special tweaking of the file system and drive parameters if you use IDE disks. Although these values work well for most users, some server applications of Linux benefit from file system tuning. As always, observe and benchmark your changes.

Synchronizing the File System with sync

Because Linux uses buffers when writing to devices, the write will not occur until the buffer is full, until the kernel tells it to, or if you tell it to by using the sync command. Traditionally, the command is given twice, as in the following:

# sync ; sync

It is really overkill to do it twice. Still, it can be helpful prior to the unmounting of certain types of media with slow write speeds (such as some USB hard drives or PCMCIA storage media), but only because it delays the user from attempting to remove the media too soon, not because two syncs are better than one.

The mke2fs Command

mke2fs -O sparse_super will create a file system with sparse superblocks, reducing the space allocated to root for file system repair. This is not a performance enhancement per se, but it frees up additional space on a drive. This command works only on pre-2.2 kernels and is included here because a number of existing references don't make that distinction. With kernel version 2.2, the sparse superblock option has been moved to tune2fs. This feature is turned on by default, so you will always create any new file systems with sparse superblocks.

mke2fs -b blocksize sets the block size. The block size chosen can also have an effect on the performance of the file system. A larger block size works better with large files and vice versa. There doesn't seem to be any hard and fast rule about this, and most advice is to accept the default block size of 1024KB unless you want to spend some time running benchmarks.

The tune2fs Command

With tune2fs, you can adjust the tunable file system parameters on an ext2 or ext3 file system. A few performance-related items of note are as follows:

To disable file system checking, the -c 0 option sets the maximum mount count to zero.

The interval between forced checks can be adjusted with the -I option.

The -m option will set the reserved blocks percentage with a lower value, freeing more space at the expense of fsck having less space to write any recovered files.

Decrease the number of superblocks to save space with the -O sparse_super option. (Modern file systems use this by default.) Always run e2fsck after you change this value.

More space can be freed with the -r option that sets the number of reserved (for root) blocks.

Note that most of these uses of tune2fs free up space on the drive at the expense of the capability of fsck to recover data. Unless you really need the space and can deal with the consequences, just accept the defaults; large drives are now relatively inexpensive.

The e2fsck Command

This utility checks an ext2/ext3 file system. Some useful arguments taken from man e2fsck are as follows:

-c Checks for bad blocks and then marks them as bad.

-f Forces checking on a clean file system.

-v Verbose mode.

The badblocks Command

Although not a performance tuning program per se, the badblocks utility checks an (preferably) unmounted partition for bad blocks. It is not recommended that you run this command by itself, but rather allow it to be called by fsck. It should be used directly only if you specify the block size accuratelydon't guess or assume anything.

The options available for badblocks are detailed in the man page. They allow for very low-level manipulation of the file system that is useful for data recovery by file system experts or for file system hacking, but is beyond the scope of this chapter and the average user.

Disabling File Access Time

Whenever Linux reads a file, it changes the last access timeknown as the atime. This is also true for your web server: If you are getting hit by 50 requests a second, your hard disk will be updating the atime 50 times a second. Do you really need to know the last time a file was accessed? If not, you can disable the atime setting for a directory by typing this:

chattr -R +A /path/to/directory

The chattr command changes file system attributes, of which "don't update atime" is one. To set that attribute, use +A and specify -R so that it is recursively set. /path/to/directory gets changed, and so do all the files and subdirectories it contains.