Why is ESDI such a pain to get working in the first place?
People who tried ESDI disks with FreeBSD are known to have developed a profound sense of frustration. A combination of factors works against you to produce effects that are hard to understand when you have never seen them before.
This has also led to the popular legend ESDI and FreeBSD is a plain NO-GO. The following sections try to list all the pitfalls and solutions.
As briefly mentioned before, ESDI comes in two speed flavours. The older drives and controllers use a 10 Mbits/second data transfer rate. Newer stuff uses 15 Mbits/second.
It is not hard to imagine that 15 Mbits/second drive cause problems on controllers laid out for 10 Mbits/second. As always, consult your controller and drive documentation to see if things match.
Mainstream ESDI drives use 34 to 36 sectors per track. Most (older) controllers cannot handle more than this number of sectors. Newer, higher capacity, drives use higher numbers of sectors per track. For instance, I own a 670 Mb drive that has 54 sectors per track.
In my case, the controller could not handle this number of sectors. It proved to work well except that it only used 35 sectors on each track. This meant losing a lot of diskspace.
Once again, check the documentation of your hardware for more info. Going out-of-spec like in the example might or might not work. Give it a try or get another more capable controller.
Most ESDI drives allow hard or soft sectoring to be selected using a jumper. Hard sectoring means that the drive will produce a sector pulse on the start of each new sector. The controller uses this pulse to tell when it should start to write or read.
Hard sectoring allows a selection of sector size (normally 256, 512 or 1024 bytes per formatted sector). FreeBSD uses 512 byte sectors. The number of sectors per track also varies while still using the same number of bytes per formatted sector. The number of unformatted bytes per sector varies, dependent on your controller it needs more or less overhead bytes to work correctly. Pushing more sectors on a track of course gives you more usable space, but might give problems if your controller needs more bytes than the drive offers.
In case of soft sectoring, the controller itself determines where to start/stop reading or writing. For ESDI hard sectoring is the default (at least on everything I came across). I never felt the urge to try soft sectoring.
In general, experiment with sector settings before you install FreeBSD because you need to re-run the low-level format after each change.
ESDI drives need to be low level formatted before they are usable. A reformat is needed whenever you figgle with the number of sectors/track jumpers or the physical orientation of the drive (horizontal, vertical). So, first think, then format. The format time must not be underestimated, for big disks it can take hours.
After a low level format, a surface scan is done to find and flag bad sectors. Most disks have a manufacturer bad block list listed on a piece of paper or adhesive sticker. In addition, on most disks the list is also written onto the disk. Please use the manufacturer's list. It is much easier to remap a defect now than after FreeBSD is installed.
Stay away from low-level formatters that mark all sectors of a track as bad as soon as they find one bad sector. Not only does this waste space, it also and more importantly causes you grief with bad144 (see the section on bad144).
Translations, although not exclusively a ESDI-only problem, might give you real trouble. Translations come in multiple flavours. Most of them have in common that they attempt to work around the limitations posed upon disk geometries by the original IBM PC/AT design (thanks IBM!).
First of all there is the (in)famous 1024 cylinder limit. For a system to be able to boot, the stuff (whatever operating system) must be in the first 1024 cylinders of a disk. Only 10 bits are available to encode the cylinder number. For the number of sectors the limit is 64 (0-63). When you combine the 1024 cylinder limit with the 16 head limit (also a design feature) you max out at fairly limited disk sizes.
To work around this problem, the manufacturers of ESDI PC controllers added a BIOS prom extension on their boards. This BIOS extension handles disk I/O for booting (and for some operating systems all disk I/O) by using translation. For instance, a big drive might be presented to the system as having 32 heads and 64 sectors/track. The result is that the number of cylinders is reduced to something below 1024 and is therefore usable by the system without problems. It is noteworthy to know that FreeBSD after it's kernel has started no longer uses the BIOS. More on this later.
A second reason for translations is the fact that most older system BIOSes could only handle drives with 17 sectors per track (the old ST412 standard). Newer system BIOSes usually have a user-defined drive type (in most cases this is drive type 47).
Whatever you do to translations after reading this document, keep in mind that if you have multiple operating systems on the same disk, all must use the same translation
While on the subject of translations, I've seen one controller type (but there are probably more like this) offer the option to logically split a drive in multiple partitions as a BIOS option. I had select 1 drive == 1 partition because this controller wrote this info onto the disk. On powerup it read the info and presented itself to the system based on the info from the disk.
Most ESDI controllers offer the possibility to remap bad sectors. During/after the low-level format of the disk bad sectors are marked as such, and a replacement sector is put in place (logically of course) of the bad one.
In most cases the remapping is done by using N-1 sectors on each track for actual datastorage, and sector N itself is the spare sector. N is the total number of sectors physically available on the track. The idea behind this is that the operating system sees a 'perfect' disk without bad sectors. In the case of FreeBSD this concept is not usable.
The problem is that the translation from bad to good is performed by the BIOS of the ESDI controller. FreeBSD, being a true 32 bit operating system, does not use the BIOS after it has been booted. Instead, it has device drivers that talk directly to the hardware.
So: don't use spare sectoring, bad block remapping or whatever it may be called by the controller manufacturer when you want to use the disk for FreeBSD.
The preceding section leaves us with a problem. The controller's bad block handling is not usable and still FreeBSD's filesystems assume perfect media without any flaws. To solve this problem, FreeBSD use the bad144 tool. Bad144 (named after a Digital Equipment standard for bad block handling) scans a FreeBSD slice for bad blocks. Having found these bad blocks, it writes a table with the offending block numbers to the end of the FreeBSD slice.
When the disk is in operation, the diskaccesses are checked against the table read from the disk. Whenever a blocknumber is requested that is in the bad144 list, a replacement block (also from the end of the FreeBSD slice) is used. In this way, the bad144 replacement scheme presents 'perfect' media to the FreeBSD filesystems.
There are a number of potential pitfalls associated with the use of bad144. First of all, the slice cannot have more than 126 bad sectors. If your drive has a high number of bad sectors, you might need to divide it into multiple FreeBSD slices each containing less than 126 bad sectors. Stay away from low-level format programs that mark every sector of a track as bad when they find a flaw on the track. As you can imagine, the 126 limit is quickly reached when the low-level format is done this way.
Second, if the slice contains the root filesystem, the slice should be within the 1024 cylinder BIOS limit. During the boot process the bad144 list is read using the BIOS and this only succeeds when the list is within the 1024 cylinder limit. Note that the restriction is not that only the root filesystem must be within the 1024 cylinder limit, but rather the entire slice that contains the root filesystem.
ESDI disks are handled by the same wddriver as IDE and ST412 MFM disks. The wd driver should work for all WD1003 compatible interfaces.
Most hardware is jumperable for one of two different I/O address ranges and IRQ lines. This allows you to have two wd type controllers in one system.
When your hardware allows non-standard strappings, you
can use these with FreeBSD as long as you enter the
correct info into the kernel config file.
An example from the kernel config file (they live in
/sys/i386/conf BTW).
# First WD compatible controller
controller wdc0 at isa? port "IO_WD1" bio irq 14 vector wdintr
disk wd0 at wdc0 drive 0
disk wd1 at wdc0 drive 1
# Second WD compatible controller
controller wdc1 at isa? port "IO_WD2" bio irq 15 vector wdintr
disk wd2 at wdc1 drive 0
disk wd3 at wdc1 drive 1