268 lines
12 KiB
Plaintext
268 lines
12 KiB
Plaintext
How to assign disk space to FreeBSD.
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$Id$
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1.0 Getting started.
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---------------------
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After a general introduction, you will find some explanation on what you
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need to do to assign space to FreeBSD on your disk(s). This is done
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through the "sysinstall" program, which lives on the inital boot floppy.
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Those already expert with PCs may wish to skip ahead to section 1.2, the
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rest of you may (or may not) enjoy the brief history lesson.
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1.1 The ins and outs of allocating disk storage on your PC.
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------------------------------------------------------------
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Modern hard disk drives are now getting big enough that people don't want
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to allocate all of one to just one operating system anymore, especially
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given the increasing size of disk drives (the latest 9.0 Gbyte models
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holding the equivalent of some six thousand 1.44MB floppies!) and the
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virtual explosion of operating system options available for the PC. To
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solve this problem, IBM came up with a scheme for "slicing" the disks
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into more manageable chunks, or partitions. It works, but only just.
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To better understand why, first a brief bit of history:
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MS-DOS, when hard disk support was unceremoniously grafted on back in the
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late eighties, didn't have such "slices". What it had was a way to install
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Xenix and MS-DOS on the same disk (Remember when Microsoft were in the UNIX
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business?).
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In the first sector on the disk was a piece of "primary boot code" and a
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table with four entries. Each of those entries pointed at an arbitrary
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slice of the disk, with one of them was marked "active". The machine would
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boot by reading the first sector containing the boot code into RAM and then
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jumping to it. The job of this small piece of boot code was to look at
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the 4 entry table and decide which OS was to be booted by looking
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for the "active" flag. It would go and load the first sector of that slice
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of the disk into RAM and then and jump to it in turn. This bit of boot
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code was called the "secondary boot", and could be specific to a given
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operating system. The primary boot code and 4-entry table is known
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as the Master Boot Record, or MBR, and is very important to the proper
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operation of your PC! We will discuss the MBR in more detail later.
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It was later realized, with the hindsight that IBM is famous for, that disks
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could be bigger than the 32Mb that the early DOS FAT-12 file system could
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handle, so they added a kludge: They had two MSDOS slices, a "Primary" and
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a "Secondary". The primary could still only be 32Mb, but the Secondary had
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no size limit. And the trick was that the secondary had ANOTHER "table
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entry" so that now suddenly up to 5 slices could be available to MS-DOS.
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The Secondary boot record was later made recursive, thus effectively
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avoiding any fixed limit. Of course, they were still stuck with a maximum
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of 26 slices given the use of "drive letters" in DOS. They also reserved
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only 10 bits for cylinder addressing, limiting DOS to being able to address
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a maximum of 1024 cylinders (and cause of the dreaded "cylinder translation"
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kludges, the misconfiguration of which many users have seen as the notorious
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"Missing Operating System" message). Yes, truly DOS was and is an utterly
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terrible operating system, which of course explains its amazing degree of
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success. Anyway, this all brings us up to today, which is where FreeBSD
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comes in:
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1.2 What FreeBSD does
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----------------------
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FreeBSD has, like any other UNIX-like operating system, the concept of
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"partitions." Partitions are used to implement its own "slicing"
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abstraction, and although there is no real difference between a slice and a
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partition as such, we use the two words to distinguish between these two
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different levels of slicing.
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The result is that we have a two-tier structure on the disk:
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+-----------+
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| MBR-table |
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+-----------+ +---------+
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| Slice 1 | -----> | MSDOS |
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+-----------+ +---------+
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| Slice 2 |
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+-----------+ +-------------------+
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| Slice 3 | -----> | FreeBSD-disklabel |
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+-----------+ +-------------------+ +-----------------+
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| Slice 4 | | Partition A | -----> | Root-filesystem |
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+-----------+ +-------------------+ +-----------------+
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| Partition B | ---
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+-------------------+ \ +----------------+
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| Partition C | --> | swap-partition |
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+-------------------+ +----------------+
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| ... |
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Here are the rules that FreeBSD plays by:
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A: FreeBSD always has an MBR slice with type 0xa5 (each of the 4 slices can
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also have a unique integer identifier so you can tell your DOS slices
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from your FreeBSD slices from your Linux slices, etc). This means that
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there should always be an MBR record, even in the case where FreeBSD
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occupies the entire disk.
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B: The FreeBSD slice contains the FreeBSD disklabel in the second sector
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(remember, the first sector contains the secondary boot code for FreeBSD,
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which is what prints that FreeBSD prompt at you when you first boot
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FreeBSD from a floppy or hard disk).
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C: The 'C' partition in the FreeBSD disklabel corresponds to the entire
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FreeBSD slice.
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D: The 'D' partition corresponds to the entire physical disk.
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E: Should a disk not have a FreeBSD slice (because there simply is no
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FreeBSD on it anywhere), then the MBR slices are mapped into partitions
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'E' to 'H' of an artificially created FreeBSD disklabel. This is useful
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for getting at DOS-only disks.
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Therefore, to get FreeBSD onto your disk, you need to do the following:
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Step FreeBSD utility
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------------------------------------------------------------ ---------------
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1. Make an MBR slice for FreeBSD (FDISK)
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2. Partition the diskspace in the MBR slice into partitions (DISKLABEL)
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3. Assign mountpoints to the partitions. (DISKLABEL)
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2. The sysinstall utility
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--------------------------
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The sysinstall utility is the program you first see when you boot
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FreeBSD's install floppy. It is responsible for partitioning your
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disk, creating an MBR slice for FreeBSD, setting up the disklabel
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within that slice and creating filesystems for each FreeBSD partition
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you create within that slice. It is composed of a number of screens.
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These are described below.
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2.1 The main screen
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--------------------
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The main screen shows you the current status, It shows you which disks
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FreeBSD has found, how big they are and how much of it is assigned to
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FreeBSD in a FreeBSD MBR slice. It also shows the partitions which have
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had a mountpoint assigned to them (not necessarily FreeBSD partitions;
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FreeBSD is perfectly capable of mounting DOS disks directly).
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(H)elp -- shows you this file.
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(F)disk -- enters the Fdisk editor, where you can change the MBR record.
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This is what you want to use to assign some part of the disk to FreeBSD.
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(D)isklabel -- enters the Disklabel editor, here you can change how the
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FreeBSD slice is partitioned for FreeBSD.
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(P)rocede -- will continue the installation process.
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(Q)uit -- Go back to the entry screen.
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2.2 FDISK - how to make an MBR slice
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-------------------------------------
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There are some rules to follow here since altering your MBR is a potential
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minefield. There is really no way for the sysinstall program to genuinely
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know that you have a valid MBR, so you have to be extra careful in what
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you edit. Failure to do this properly can and will destroy your other
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operating system entries!
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Even if you don't plan to have MSDOS on a disk, make an MSDOS slice
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using the MSDOS's FDISK.COM program. The reason for this is that if you
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do it that way, you are 100% sure that FreeBSD will use the same number
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of heads, sectors and cylinders as MSDOS would use. If you really don't
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plan to have MSDOS on the disk, just (D)elete the slice in the FreeBSD's
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(F)disk editor.
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From the main screen press 'F' to enter the MBR editor. You have five
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commands available:
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(H)elp -- Shows you this file.
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(D)elete -- Deletes a slice entirely.
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(E)dit -- Allows you to edit a slice. It will ask how many megabytes
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you want to assign to the slice, and will suggest the maximum possible
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as a default. It might say zero, even though there is disk space
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available, in which case you will probably need to delete and recreate the
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other partitions to get it to see where the free space is.
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It will then ask you what type to give the slice, for which the default is
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0xa5 (a FreeBSD slice). You can enter any other number here too, which
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can be useful as a placeholder for some other OS you plan to install
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later. Finally, it will ask you about the "boot flag". 0x80 means "boot
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from this" slice by default, and anything else means "don't".
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If you specified a FreeBSD slice, any existing slices with the 0xa5
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type will be reset to 0x00 "unused". FreeBSD only supports one slice
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per disk for FreeBSD.
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(R)eread -- This is your "undo" function. It will read the data of the
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disk again, disposing of any changes you may have made.
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(W)rite -- When you are satisfied with the data, this function will write
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the new MBR to the disk.
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(Q)uit -- Go back to the main screen.
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2.3 Disklabel - How to divide up the FreeBSD slice.
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----------------------------------------------------
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The disklabel screen provides the following commands:
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(H)elp -- Shows you this file.
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(S)ize -- Resizes a partition for you, it will suggest as a default the
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maximum amount of diskspace it can find. This algorithm isn't too smart
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and may say zero, even though there is diskspace available. If it
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does, delete and resize the other partitions.
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(A)ssign -- Here you assign where the filesystem in a partition is to
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be mounted. `b' partitions will always be made into "swap" partitions.
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(D)elete -- Delete a partition.
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(R)eread -- The undo function. It will reread the current disklabel from
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the kernel.
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(W)rite -- This will write the disklabel to the disk. You must always write
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before you quit, otherwise your changes will be lost.
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(Q)uit -- Exit back to the main screen.
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2.4. Hints on partition sizing
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-------------------------------
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While it's impossible to say how much space you're going to want to
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make your various partitions without knowing more about your intended
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applicatins, here are some good rules of thumb to follow:
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1. Root (/) should be at least 18MB, and probably no more than 50MB unless
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you have some special reason for making your root partition really
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large. Remember that the root filesystem is only supposed to contain
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vital system files and little else.
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2. Swap should be at least 2*memory. That is to say if you have 8MB of
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memory, then you probably want 16MB of swap. Even more swap space
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certainly doesn't hurt, if you can afford to allocate it, and you should
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also think ahead a little to any planned memory upgrades you may have
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in mind since increasing this later can be very painful!
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If you're going to run the X Window System (XFree86), you should also
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consider having a *minimum* of 16MB of swap, since X tends to really
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use it up.
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3. /usr can take up the rest of your disk, though some people like to create
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extra partitions for user home directories and the like. Be sure to make
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your /usr big enough to contain the system software (about 50MB) and
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perhaps some of your own, unless you're going to use symbolic links to
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point things like /usr/local (or /usr/src) somewhere else.
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Here are some suggested filesystem names and sizes, just for reference:
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Mountpoint Filesystem size
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-------------------------------
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/var 10Mb
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/usr 50Mb
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/ 16Mb
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/usr/src 120Mb If you want to have the sources online
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/usr/obj 100Mb If you want to compile all of them at one time
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/usr/X11R6 50Mb If you load the entire XFree86 binary kit.
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$Id: diskspace.FAQ,v 1.1 1995/01/03 15:48:36 gclarkii Exp $
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