4.3. Tuning your drives

4.3.1.1. Tuning IDE from within the BIOS

Typically, most users will just leave the BIOS to automatically configure itself based off a negotiation between the hard drive and the controller in the system. There are, however, a few options within the BIOS that users can examine to get either improved performance, or debug tricky issues.

There are three methods for IDE to address a drive: Normal, CHS, and LBA. Normal mode works only with drives smaller than 500MB. Since no drives since 1996 have been made this small, we can forget this mode. CHS stands for Cylinder, Head, and Sector, while LBA stands for Logical Block Address. CHS and LBA are fairly similar in performance, but be aware that taking a drive that is configured in CHS and moving it to a system that use LBA will destroy all the data on that drive.

Now that the drive can be accessed, there are two methods for that drive to get its data from the drive to the CPU: PIO, and DMA. PIO requires the CPU to shuttle data from the drive to memory so the CPU can use it later, but is the most compatible way for Linux to talk to a drive due to the large number of IDE controller chips. DMA mode seems to be much faster, since the CPU is not involved in moving the data between the drive and memory, freeing the CPU to do other things. Until the later Linux 2.2 kernels, DMA mode was not selected by default for access to drives due to compatability with interface chips. You can now select a specific IDE controller and select DMA access to make sure your combination is correct.

4.3.1.2. Tuning IDE from within Linux

As mentioned in Section 4.3.1.1, Linux has drivers for some specific IDE controllers, plus a generic IDE interface. Once the OS boots, Linux will ignore the state of the BIOS when it accesses hardware, choosing instead to use its own drivers and options. By default, the Linux kernel will have DMA access available, but not activated. This can be changed if you choose to compile your own kernel (see Section 6.2 for more information).

Once Linux is started, all IDE tuning occurs using hdparm. Using hdparm followed by a drive (hdparm /dev/hda) will return the current settings for that drive.

hdparm [-c iovalue] [-d] [-m multimode] {device}

# hdparm /dev/hda

/dev/hda:
multcount    =  0 (off)
I/O support  =  0 (default 16-bit)
unmaskirq    =  0 (off)
using_dma    =  1 (on)
keepsettings =  0 (off)
nowerr       =  0 (off)
readonly     =  0 (off)
readahead    =  8 (on)
geometry     = 1559/240/63, sectors = 23572080, start = 0
#

As you can see, there are a few options for tuning that are available for this drive. The biggest boost for IDE drive performance is to use DMA access, and this is already turned on. If DMA access is not turned on by default in the kernel, you can use -d1 to turn DMA access on, and -d0 to turn it off. Note that DMA access will not necessarily increase performance of getting data to and from the drive, but will free up the CPU during data transfers.

Another item to take a look at is the I/O support, which on our sample drive is set to 16-bit access instead of 32. The -c option to hdparm will check the I/O support for the specified device, there are a total of three options that can be given to -c to set the I/O support. The two most stable for systems are 0, used to set 16-bit access, and 3, used to set 32-bit synchronous access. Using an iovalue of 1 sets 32-bit access but may not work on all chipsets, causing a system hang if the chipset does not support regular 32-bit access.

Also test and check out multiple sector I/O with the -m option. Many newer IDE drives support transferrint multiple sectors worth of data per I/O cycle, reducing the numer of I/O cycles required to tranfer data. The -i option to hdparm will tell you what the maximum multimode is in the MaxMultSect section. By setting multimode to its highest, large transfers of data happen even quicker.

The hdparm manual page indicates that with some drives, using multiple sector I/O can result in massive filsystem corruption. As always, be sure to test your systems thoroughly before using them in a production environment.

Unfortunately, the correct setting for DMA, multiple sector I/O, and I/O support varies by drive and controller. You will have to do your own testing to find the correct combination for your system. The right combination should be able to:

• Have a high throughput of data from the drive to the system.

• Maintain that high throughput under heavy CPU load.

• Prevent the drive or the IDE driver from corrupting data.

Fortunately, testing throughput is easy with hdparm. The -t option tests the throughput of data directly from the drive, while -T tests throughput of data directly from the Linux drive buffer. To ensure the tests run properly, they should be run when the system is quiet and few other processes are running. Usually, runlevel 1, also known as single user mode, provides this environment. Tests should be run two or three times.

Once an optimum setting is found, you can have hdparm make the setting at boot time by including the relevant commands in /etc/rc.local

4.3.2.1. SCSI Options

In many cases, SCSI support is an add-in card to the system, giving you many options in vendors and card types. Some server motherboards include SCSI chipset and support onboard as well. In addition, there are dedicated RAID cards that implement various RAID levels in hardware, offloading much of the logic from the CPU.

SCSI has three major protocol varieties: SCSI-1, SCSI-2, and SCSI-3. Fortunately, the protocols are backwards and forwards compatible, meaning that a SCSI-3 controller card can talk to a SCSI-1 drive, albeit at a slower speed. For best performance, make sure that the protocols of the controller card and devices match.

On the physical side, cable types range from a DB-25 pin connector on older Apple Macintoshes to 80-pin SCA connectors that not only include the SCSI pins, but power and ID information.

Here's a quick rundown on the various connector types available.

INSERT TABLE OF CONNECTORS