diff options
Diffstat (limited to 'share/FAQ/Text/ESDI.FAQ')
| -rw-r--r-- | share/FAQ/Text/ESDI.FAQ | 187 |
1 files changed, 187 insertions, 0 deletions
diff --git a/share/FAQ/Text/ESDI.FAQ b/share/FAQ/Text/ESDI.FAQ new file mode 100644 index 000000000000..8d869240d6f3 --- /dev/null +++ b/share/FAQ/Text/ESDI.FAQ @@ -0,0 +1,187 @@ + 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 |