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netbsd_partitioning [2013/06/21 13:40]
 netbsd_partitioning [2013/06/21 13:40] (current)
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 +====== NetBSD Partitioning ======
  
 +  * [[NetBSD]]
 +  * [[disklabel]]
 +  * [[fdisk]]
 +  * [[sysctl]]
 +
 +==== MBR Partitions ====
 +
 +Coming from Linux, setting up partitions in NetBSD is quite different. ​ The two operating systems can co-exist happily, though, because of how the two share partitions. ​ The short version of this document is this: Linux uses an MBR entry for each of it's partitions, while NetBSD uses one MBR entry for all of its partitions, and then uses a subset to divide that entry as well into smaller entries.
 +
 +For example, in a Linux setup with MBR, you may have something that looks like this:
 +
 +^ Device ^ Type ^ Mount ^ System ^
 +| /dev/sda1 | ext2 | /boot | Linux ^
 +| /dev/sda2 | swap | | Linux swap ^
 +| /dev/sda3 | ext4 | / | Linux ^
 +
 +In this example, ''/​dev/​sda''​ is the actual device (presumably a hard drive) that is divided into MBR partitions.
 +
 +Using ''​fdisk -l /​dev/​sda''​ in Linux would display the MBR table.
 +
 +=== Adding a NetBSD partition to MBR ===
 +
 +If you wanted to add a NetBSD partition to this table, you would create a new entry with fdisk, and end up with a table similar to this:
 +
 +^ Device ^ Type ^ Mount ^ System ^
 +| /dev/sda1 | ext2 | /boot | Linux ^
 +| /dev/sda2 | swap | | Linux swap ^
 +| /dev/sda3 | ext4 | / | Linux ^
 +| /dev/sda4 | | | NetBSD ^
 +
 +The partition solely for NetBSD resides alongside the other MBR entries.
 +
 +=== NetBSD and disklabel ===
 +
 +NetBSD occupies an entry in the MBR, but also creates its own partition table under that, using ''​disklabel''​. ​ These partitions are specific to NetBSD only -- they will not be displayed in the MBR partition list.
 +
 +NetBSD uses DOS partitioning naming schemes, with devices ''​a:'',​ ''​b:'',​ ''​c:''​ and ''​d:''​.
 +
 +Using this naming scheme, ''​a:''​ and ''​b:''​ would refer to the first two disklabels. ​ However, ''​c:''​ would refer to the entire MBR partition that NetBSD is using. ​ Using the example above, that would be the same device as ''/​dev/​sda4''​.
 +
 +NetBSD also reserves the ''​d:''​ partition to represent the entire device. ​ Comparatively,​ ''/​dev/​sda''​ for Linux.
 +
 +^ Linux Device ^ NetBSD Device ^ NetBSD disklabel ^ Disk ^
 +| /dev/sda | /dev/sd0d | d: | Entire disk |
 +| /dev/sda4 | /dev/sd0c | c: | Fourth MBR entry |
 +
 +For the record, NetBSD also uses ''/​dev/​sd0''​ to represent ''/​dev/​sd0d''​ as well; that is, the whole disk.  So if we modified our table to include all the entries, here is how it would look:
 +
 +^ Linux Device ^ NetBSD Device ^ NetBSD disklabel ^ Disk ^
 +| /dev/sda | /dev/sd0 | d: | Entire disk |
 +| /dev/sda | /dev/sd0d | d: | Entire disk |
 +| /dev/sda4 | /dev/sd0c | c: | Fourth MBR entry |
 +
 +This graphic from the NetBSD guide is also helpful:
 +
 +{{:​part.gif|}}
 +
 +See [[http://​www.netbsd.org/​docs/​guide/​en/​chap-inst.html#​chap-inst-install-partition|this document]] from the NetBSD guide.
 +
 +=== Partitioning an MBR in NetBSD ===
 +
 +If, for any reason, you want to clear out the MBR completely of an existing device, you can use ''​dd''​ to write over it:
 +
 +<​code>​
 +dd if=/​dev/​zero bs=512 count=1 of=/dev/sd0
 +</​code>​
 +
 +== Using fdisk ==
 +
 +When you have a new device that needs to create an MBR table, use ''​fdisk''​ in interactive mode.
 +
 +Just running ''​fdisk''​ on a device will print out it's partition table. ​ For example: ''​fdisk sd0''​.
 +
 +Run ''​fdisk -u sd0''​ to run fdisk in interactive mode.  **Please note:** that the syntax is ''​sd0''​ and not ''/​dev/​sd0''​.
 +
 +The first question it will ask you is if you want the change the disk geometry. ​ The defaults are fine, so answer no.
 +
 +<​code>​
 +Do you want to change our idea of what BIOS thinks? [n]
 +</​code>​
 +
 +Next, fdisk will display the current MBR table layout. ​ If the MBR is empty, then they will all be marked as unused:
 +
 +<​code>​
 +Partition table:
 +0: <​UNUSED>​
 +1: <​UNUSED>​
 +2: <​UNUSED>​
 +3: <​UNUSED>​
 +</​code>​
 +
 +It's worth pointing out here that the MBR table lists four devices. ​ This is consistent with the original MBR limitations,​ where only four partitions could be created. ​ Extended partitions works around this limitation, which we aren't going to cover here.
 +
 +== Creating a partition ==
 +
 +Now, create a partition. ​ Since this one is empty, start with the first, numbered zero.
 +
 +<​code>​
 +Which partition do you want to change?: [none] 0
 +</​code>​
 +
 +The first value, the ''​sysid''​ is the type of partition. ​ The equivalent in ''​fdisk''​ for Linux would be the hex code, such as 82 for Linux and 83 for Linux swap.  Use the default value of 169 to create a NetBSD partition.
 +
 +The three following defaults are fine as well:
 +
 +<​code>​
 +sysid: [0..255 default: 169] 
 +start: [0..1cyl default: 63, 0cyl, 0MB] 
 +size: [0..1cyl default: 16321, 1cyl, 8MB] 
 +bootmenu: []
 +</​code>​
 +
 +''​fdisk''​ will then print out the new table layout with the propsed changes.
 +
 +<​code>​
 +Partition table:
 +0: NetBSD (sysid 169)
 +    start 63, size 16321 (8 MB, Cyls 0-1/5/4)
 +        PBR is not bootable: All bytes are identical (0x00)
 +1: <​UNUSED>​
 +2: <​UNUSED>​
 +3: <​UNUSED>​
 +</​code>​
 +
 +If you have no more partitions to create, then select '​none'​ and write out the new partition table.
 +
 +<​code>​
 +Which partition do you want to change?: [none]
 +Should we write new partition table? [n] y
 +</​code>​
 +
 +=== Create NetBSD slices ===
 +
 +Short version: run ''​disklabel sd0''​ (same syntax, sd0, sd1, sd2, etc.) to see stuff, ''​disklabel -i -I sd0''​ to create slices. ​ Use ''​a:''​ ''​b:''​ and ''​e:'',​ skipping ''​c:''​ and ''​d:''​.
 +
 +To remove a slice, set the size to 0 on both ends.  To create one to fill the remaining space, use $.
 +
 +Then use [[newfs]] to create new filesystems. :)