Add John Lind's instructions on dealing with ESDI/MFM drives.

Submitted by:	John Lind <john@starfire.MN.ORG>

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+                                  FreeBSD
+             Using "Mature Technology" (MFM, ESDI) hard drives
+
+First, please read and make sure that you understand the "diskspace" FAQ.
+
+The term "partition" has become overloaded when referring to an area of a
+hard disk drive.  I will use "slice" in this document to refer to an area
+which is a "partition" to DOS FDISK.  This usage is consistent with most of
+current FreeBSD.  I will use the word "partition" to refer to an area de-
+fined by a FreeBSD disklabel, of which there is one per FreeBSD slice.  The
+FreeBSD partitions may contain filesystems, swap space, or be available as
+raw disk devices to applications.
+
+This document covers the steps you will need to perform before starting the
+FreeBSD installation and which are specific to these types of disk drives.
+
+1 Disk layout planning
+2 Disk installation (may already be done)
+3 Low-level formatting (may or may not be required)
+
+If your drive is installed and formatted properly, careful planning is all
+that you need to do.
+
+During the installation, there is only one special step that is required.
+This is also one place where the FreeBSD taxonomic convention breaks down
+-- the assignment of slices is called "Partitioning" in the FreeBSD proce-
+dures at the time of this writing.  During that step, when you assign the
+FDISK slices, be sure to specify that the bad144 lists should be created
+(the "B" command).
+
+1 Disk layout planning
+----------------------
+
+To be able to make the right decisions regarding the setup of slices for
+FreeBSD, you need to understand that the initial boot stages for FreeBSD
+rely on the ROM BIOS, but that the ROM BIOS is not used in any way once the
+FreeBSD kernel is loaded.  After the kernel is loaded, it uses its own
+driver instead of the BIOS to access the disk.
+
+These older disks do not do automatic bad block management.  Some con-
+trollers seem to do so, but this is a feature of the ROM BIOS on that con-
+troller, and therefore is not available once FreeBSD is running.  Other
+controllers use a different method of bad sector handling (slipping), but
+this feature can also induce translations in the sectoring which will pre-
+vent successful installation.  Do not use automatic sector mapping or sec-
+tor slipping, even if it is supported on the controller, for the same rea-
+sons that you cannot use a translated disk geometry.
+
+To be able to use these drives, the driver has to be able to substitute
+good sectors for the bad ones.  The FreeBSD filesystems assume "perfect"
+disks, so the bad sector handling is done in the driver.  The way this is
+done is that a few copies of the list of bad sectors is kept at the end of
+the slice.  When a slice is opened, this list is read and kept in the driv-
+er.  On every access to the drive, the list is consulted to see if a sub-
+stitute sector, also from the end of the slice, must be used rather than
+the sector that the filesystem or application is actually asking for.  This
+list is called the bad144 list, which name comes from a Digital Equipment
+Corporation standard for keeping this information.
+
+There are three reasons that you would be required to use more than one
+FreeBSD slice on your disk, and all of them are more probable the larger
+your drive is.
+
+1)  The FreeBSD portion of your drive will extend beyond cylinder 1023.
+2)  The FreeBSD portion of your drive has (or is likely to have) more than
+    126 bad sectors.
+3)  You need more than 7 filesystems for FreeBSD.
+
+It is not sufficient to make sure that the entire boot filesystem is inside
+of cylinder 1024, unless it just so happens that that filesystem occupies a
+flawless part of the disk.  To be able to read the bad144 list during the
+boot process (via the ROM BIOS), the end of the slice must be within the
+first 1024 cylinders.  There are also some boot managers, e.g. the OS/2
+boot manager, that require a bootable slice to be entirely below cylinder
+1024.
+
+The bad144 data format only allows for 126 sectors to be mapped.  If your
+drive is large, it could easily have more than this many over its entire
+size.  I have a 320 Mb drive which is unusually error-filled, but still
+within acceptable tolerances, and it has this problem (but it also has more
+than 1024 cylinders, so I'd have had to split it, anyway).
+
+The FreeBSD disk label has room for 8 partitions.  It is not recommended
+that filesystems be placed on partitions b or c, even on non-booted slices,
+so that leaves 6 partitions within each slice which may contain filesys-
+tems.  Older versions of FreeBSD did not support filesystems on partition d
+either, but the slice-handling capability has eliminated that restriction.
+It may be best to avoid using partition d if you can for compatibility pur-
+poses.  The installation procedures skip from a to e for this reason.
+
+2 Disk installation
+-------------------
+
+Physical installation is outside the scope of this document.  Consult the
+documentation provided by your computer, controller, and disk drive ven-
+dor(s).
+
+3 Low-level formatting
+----------------------
+
+If you are starting from scratch, you may need to low-level format your
+drive(s).  If you have been using your drive with the intended controller
+for DOS or some other system, and will not be changing the physical orien-
+tation of the drive, then you can skip this step.  If the drive is new, if
+you are changing the physical orientation of the drive, or it has been used
+with a different controller, you may need to do the low-level format.  Be
+aware that some drives have jumpers on them to help compensate for changes
+in physical orientation (horizontal, right side, or left side), but it is
+highly recommended that, if you are changing the physical orientation of
+the drive, you redo the low-level format.  The "sag" of the disk head arma-
+ture and other effects of gravity are quite significant at the sizes of the
+bits and tracks on these drives.  The greater the capacity of your drive,
+the more critical this becomes.  With 10-40Mb MFM drives, you may get away
+with it.  Beyond that, you are definitely rolling dice.
+
+The MFM drive format is standard, and a drive formatted on one manufacturer
+and model of controller should work just fine on another, but ESDI drive
+formats vary between manufacturers and sometimes even between models.  A
+new ESDI drive (yes, they can still be found new in the box, years old), or
+one that has been in use on a different controller or in a different physi-
+cal orientation will definitely require reformatting.
+
+As the ESDI specification developed, the ability to put the error map in-
+formation (Manufacturer's Defect List, or MDL) on the drive was added.
+Since it was not known how the drive would be formatted, or even what the
+size of the data part of the sectors would be, each bad spot is expressed
+as bytes from the index and length of the bad area in bits.  This informa-
+tion is recorded in a few different cylinders on each track and contains
+only information pertinent to the corresponding head.  The most universal
+place for this information is in the last cylinder.  Later ESDI drives sup-
+port a "phantom" cylinder at 0xfff (4095) where this information is kept --
+the actual location of this cylinder is beyond the "last" cylinder reported
+for data use.  If your drive does not support cylinder 0xfff, or if your
+controller doesn't know how to use it, and if you wish to preserve the man-
+ufacturer's defect list, do not format the last cylinder of your drive.
+The format of the MDL is such that regular data operations will not work on
+a track containing that information.
+
+As a further caveat, it as been observed that some controllers hang if the
+MDL area is accessed for data use, while others simply report an error and
+go on with life.  You will want to be careful to not include the MDL area
+in any FreeBSD slice, but you will want to be especially careful in case
+your controller is one of those that hangs if you miss.
+
+Now that you have decided how much of the drive to format, you can proceed
+with the actual format process.  How this is done varies widely from con-
+troller to controller.  For most of them, you need to jump into a special
+location in the controller ROM using the DOS DEBUG program.  For a few,
+special software is provided on diskette for the controller.  Because these
+procedures and the ways to initiate them vary so much, it is outside of the
+scope of this document to describe them.  Consult the manufacturer's docu-
+mentation for this procedure.
+
+Many of the controllers have the ability to read and use the MDL.  Even
+though you cannot use the controller's bad block mapping capability, which
+is supported through the BIOS, it may be beneficial to allow the controller
+to use this information during the format process.  When the drive was
+tested at the factory, it was tested at the operating margins, not just op-
+timal conditions.  Therefore, there may be entries in the MDL that would be
+missed by a run-of-the-mill data scan.  If the controller is permitted to
+use the MDL during formatting, many of them will format the sector with a
+special flag set in the sector preamble to guarantee that that sector will
+show up as bad on a read.  This is, in fact, the mechanism that some con-
+trollers use to handle bad sector mapping, though FreeBSD does not use the
+same mechanism.  We can take advantage of this feature as a 'round about
+way to get the MDL represented in the bad144 list.  Having the sectors
+which contain a bad spot formatted as bad will make certain that you don't
+use a sector where some data patterns may fail even though the initial scan
+passed that sector as OK.  Even if the sector doesn't produce hard errors,
+it may cause soft (correctable) errors and time-consuming retries.
+
+Finally, FreeBSD
+----------------
+
+Having made your careful plans and preparations, you are ready to use
+FreeBSD on your MFM or ESDI disk drive.  Don't forget to request bad block
+scanning during the "Partition" slice assignment, and you should be on your
+way to satisfying computing.  Be prepared to allow time for the bad block
+scan to take place.  Depending on a variety of system parameters, such as
+CPU speed, controller type, disk rotational and seek speeds, and so forth,
+this process will take anywhere from several minutes to hours.  If you for-
+get to do the scan, it is likely that the installation will fail trying to
+make the filesystems, and if it should make the filesystems, it will surely
+fail when you start using them.
+
+		   John Lind, Starfire Consulting Services
+E-mail: john@starfire.MN.ORG		USnail: PO Box 17247, Mpls MN  55417