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Handbook:MIPS/Blocks/Disks
Partitions
Although it is theoretically possible to use a full disk to house your Linux system, this is almost never done in practice. Instead, full disk block devices are split up in smaller, more manageable block devices. These are called partitions.
Designing a partition scheme
How many partitions and how big?
The number of partitions is highly dependent on the environment. For instance, if there are lots of users, then it is advised to have /home/ separate as it increases security and makes backups easier. If Gentoo is being installed to perform as a mail server, then /var/ should be separate as all mails are stored inside /var/. A good choice of filesystem will then maximize the performance. Game servers will have a separate /opt/ as most gaming servers are installed there. The reason is similar for the /home/ directory: security and backups. In most situations, /usr/ is to be kept big: not only will it contain the majority of applications, it typically also hosts the Gentoo ebuild repository (by default located at /usr/portage) which already takes around 650 MiB. This disk space estimate excludes the packages/ and distfiles/ directories that are generally stored within this ebuild repository.
It very much depends on what the administrator wants to achieve. Separate partitions or volumes have the following advantages:
- Choose the best performing filesystem for each partition or volume.
- The entire system cannot run out of free space if one defunct tool is continuously writing files to a partition or volume.
- If necessary, file system checks are reduced in time, as multiple checks can be done in parallel (although this advantage is more with multiple disks than it is with multiple partitions).
- Security can be enhanced by mounting some partitions or volumes read-only,
nosuid(setuid bits are ignored),noexec(executable bits are ignored) etc.
However, multiple partitions have disadvantages as well. If not configured properly, the system might have lots of free space on one partition and none on another. Another nuisance is that separate partitions - especially for important mount points like /usr/ or /var/ - often require the administrator to boot with an initramfs to mount the partition before other boot scripts start. This isn't always the case though, so results may vary.
There is also a 15-partition limit for SCSI and SATA unless the disk uses GPT labels.
What about swap space?
There is no perfect value for the swap partition. The purpose of swap space is to provide disk storage to the kernel when internal memory (RAM) is under pressure. A swap space allows for the kernel to move memory pages that are not likely to be accessed soon to disk (swap or page-out), freeing memory. Of course, if that memory is suddenly needed, these pages need to be put back in memory (page-in) which will take a while (as disks are very slow compared to internal memory).
When the system is not going to run memory intensive applications or the system has lots of memory available, then it probably does not need much swap space. However, swap space is also used to store the entire memory in case of hibernation. If the system is going to need hibernation, then a bigger swap space is necessary, often at least the amount of memory installed in the system.
Using fdisk
SGI machines: Creating an SGI disk label
All disks in an SGI System require an SGI Disk Label, which serves a similar function as Sun & MS-DOS disklabels -- It stores information about the disk partitions. Creating a new SGI Disk Label will create two special partitions on the disk:
- SGI Volume Header (9th partition): This partition is important. It is where the bootloader will reside, and in some cases, it will also contain the kernel images.
- SGI Volume (11th partition): This partition is similar in purpose to the Sun Disklabel's third partition of "Whole Disk". This partition spans the entire disk, and should be left untouched. It serves no special purpose other than to assist the PROM in some undocumented fashion (or it is used by IRIX in some way).
The SGI Volume Header must begin at cylinder 0. Failure to do so means a failure to boot from the disk.
The following is an example excerpt from an fdisk session. Read and tailor it to personal preference...
root #fdisk /dev/sdaSwitch to expert mode:
Command (m for help):xWith m the full menu of options is displayed:
Expert command (m for help):mCommand action b move beginning of data in a partition c change number of cylinders d print the raw data in the partition table e list extended partitions f fix partition order g create an IRIX (SGI) partition table h change number of heads m print this menu p print the partition table q quit without saving changes r return to main menu s change number of sectors/track v verify the partition table w write table to disk and exit
Build an SGI disk label:
Expert command (m for help):gBuilding a new SGI disklabel. Changes will remain in memory only, until you decide to write them. After that, of course, the previous content will be irrecoverably lost.
Return to the main menu:
Expert command (m for help):rTake a look at the current partition layout:
Command (m for help):pDisk /dev/sda (SGI disk label): 64 heads, 32 sectors, 17482 cylinders Units = cylinders of 2048 * 512 bytes ----- partitions ----- Pt# Device Info Start End Sectors Id System 9: /dev/sda1 0 4 10240 0 SGI volhdr 11: /dev/sda2 0 17481 35803136 6 SGI volume ----- Bootinfo ----- Bootfile: /unix ----- Directory Entries -----
If the disk already has an existing SGI Disklabel, then fdisk will not allow the creation of a new label. There are two ways around this. One is to create a Sun or MS-DOS disklabel, write the changes to disk, and restart fdisk. The second is to overwrite the partition table with null data via the following command:
dd if=/dev/zero of=/dev/sda bs=512 count=1Resizing the SGI volume header
This step is often needed, due to a bug in fdisk. For some reason, the volume header isn't created correctly, the end result being it starts and ends on cylinder 0. This prevents multiple partitions from being created. To get around this issue... read on.
Now that an SGI Disklabel is created, partitions may now be defined. In the above example, there are already two partitions defined. These are the special partitions mentioned above and should not normally be altered. However, for installing Gentoo, we'll need to load a bootloader, and possibly multiple kernel images (depending on system type) directly into the volume header. The volume header itself can hold up to eight images of any size, with each image allowed eight-character names.
The process of making the volume header larger isn't exactly straight-forward; there's a bit of a trick to it. One cannot simply delete and re-add the volume header due to odd fdisk behavior. In the example provided below, we'll create a 50MB Volume header in conjunction with a 50MB /boot/ partition. The actual layout of a disk may vary, but this is for illustrative purposes only.
Create a new partition:
Command (m for help):nPartition number (1-16): 1 First cylinder (5-8682, default 5): 51 Last cylinder (51-8682, default 8682): 101
Notice how fdisk only allows Partition #1 to be re-created starting at a minimum of cylinder 5? If we attempted to delete & re-create the SGI Volume Header this way, this is the same issue we would have encountered. In our example, we want /boot/ to be 50MB, so we start it at cylinder 51 (the Volume Header needs to start at cylinder 0, remember?), and set its ending cylinder to 101, which will roughly be 50MB (+/- 1-5MB).
Delete the partition:
Command (m for help):dPartition number (1-16): 9
Now recreate it:
Command (m for help):nPartition number (1-16): 9 First cylinder (0-50, default 0): 0 Last cylinder (0-50, default 50): 50
If unsure how to use fdisk have a look down further at the instructions for partitioning on Cobalts. The concepts are exactly the same -- just remember to leave the volume header and whole disk partitions alone.
Once this is done, create the rest of your partitions as needed. After all the partitions are laid out, make sure to set the partition ID of the swap partition to 82, which is Linux Swap. By default, it will be 83, Linux Native.
Partitioning Cobalt drives
On Cobalt machines, the BOOTROM expects to see a MS-DOS MBR, so partitioning the drive is relatively straightforward -- in fact, it's done the same way as done for an Intel x86 machine. However there are some things you need to bear in mind.
- Cobalt firmware will expect /dev/sda1 to be a Linux partition formatted EXT2 Revision 0. EXT2 Revision 1 partitions will NOT WORK! (The Cobalt BOOTROM only understands EXT2r0)
- The above said partition must contain a gzipped ELF image, vmlinux.gz in the root of that partition, which it loads as the kernel
For that reason, it is recommended to create a ~20MB /boot/ partition formatted EXT2r0 upon which to install CoLo & kernels. This allows the user to run a modern filesystem (EXT3 or ReiserFS) for the root filesystem.
In the example, it is assumed that /dev/sda1 is created to mount later as a /boot/ partition. To make this /, keep the PROM's expectations in mind.
So, continuing on... To create the partitions type fdisk /dev/sda at the prompt. The main commands to know are these:
o: Wipe out old partition table, starting with an empty MS-DOS partition table
n: New Partition
t: Change Partition Type
Use type 82 for Linux Swap, 83 for Linux FS
d: Delete a partition
p: Display (print) Partition Table
q: Quit -- leaving old partition table as is.
w: Quit -- writing partition table in the process.
root #fdisk /dev/sdaThe number of cylinders for this disk is set to 19870. There is nothing wrong with that, but this is larger than 1024, and could in certain setups cause problems with: 1) software that runs at boot time (e.g., old versions of LILO) 2) booting and partitioning software from other OSs (e.g., DOS FDISK, OS/2 FDISK)
Start by clearing out any existing partitions:
Command (m for help):oBuilding a new DOS disklabel. Changes will remain in memory only, until you decide to write them. After that, of course, the previous content won't be recoverable. The number of cylinders for this disk is set to 19870. There is nothing wrong with that, but this is larger than 1024, and could in certain setups cause problems with: 1) software that runs at boot time (e.g., old versions of LILO) 2) booting and partitioning software from other OSs (e.g., DOS FDISK, OS/2 FDISK) Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)
Now verify the partition table is empty using the p command:
Command (m for help):pDisk /dev/sda: 10.2 GB, 10254827520 bytes 16 heads, 63 sectors/track, 19870 cylinders Units = cylinders of 1008 * 512 = 516096 bytes Device Boot Start End Blocks Id System
Create the /boot partition:
Command (m for help):nCommand action e extended p primary partition (1-4) p Partition number (1-4): 1 First cylinder (1-19870, default 1): Last cylinder or +size or +sizeM or +sizeK (1-19870, default 19870): +20M
When printing the partitions, notice the newly created one:
Command (m for help):pDisk /dev/sda: 10.2 GB, 10254827520 bytes 16 heads, 63 sectors/track, 19870 cylinders Units = cylinders of 1008 * 512 = 516096 bytes Device Boot Start End Blocks Id System /dev/sda1 1 40 20128+ 83 Linux
Let's now create an extended partition that covers the remainder of the disk. In that extended partition, we'll create the rest (logical partitions):
Command (m for help):nCommand action e extended p primary partition (1-4) e Partition number (1-4): 2 First cylinder (41-19870, default 41): Using default value 41 Last cylinder or +size or +sizeM or +sizeK (41-19870, default 19870): Using default value 19870
Now we create the / partition, /usr, /var, et.
Command (m for help):nCommand action l logical (5 or over) p primary partition (1-4) l First cylinder (41-19870, default 41):<Press ENTER> Using default value 41 Last cylinder or +size or +sizeM or +sizeK (41-19870, default 19870): +500M
Repeat this as needed.
Last but not least, the swap space. It is recommended to have at least 250MB swap, preferrably 1GB:
Command (m for help):nCommand action l logical (5 or over) p primary partition (1-4) l First cylinder (17294-19870, default 17294): <Press ENTER> Using default value 17294 Last cylinder or +size or +sizeM or +sizeK (1011-19870, default 19870): <Press ENTER> Using default value 19870
When checking the partition table, everything should be ready - one thing notwithstanding.
Command (m for help):pDisk /dev/sda: 10.2 GB, 10254827520 bytes 16 heads, 63 sectors/track, 19870 cylinders Units = cylinders of 1008 * 512 = 516096 bytes Device Boot Start End Blocks ID System /dev/sda1 1 21 10552+ 83 Linux /dev/sda2 22 19870 10003896 5 Extended /dev/sda5 22 1037 512032+ 83 Linux /dev/sda6 1038 5101 2048224+ 83 Linux /dev/sda7 5102 9165 2048224+ 83 Linux /dev/sda8 9166 13229 2048224+ 83 Linux /dev/sda9 13230 17293 2048224+ 83 Linux /dev/sda10 17294 19870 1298776+ 83 Linux
Notice how #10, the swap partition is still type 83? Let's change that to the proper type:
Command (m for help):tPartition number (1-10): 10 Hex code (type L to list codes): 82 Changed system type of partition 10 to 82 (Linux swap)
Now verify:
Command (m for help):pDisk /dev/sda: 10.2 GB, 10254827520 bytes 16 heads, 63 sectors/track, 19870 cylinders Units = cylinders of 1008 * 512 = 516096 bytes Device Boot Start End Blocks ID System /dev/sda1 1 21 10552+ 83 Linux /dev/sda2 22 19870 10003896 5 Extended /dev/sda5 22 1037 512032+ 83 Linux /dev/sda6 1038 5101 2048224+ 83 Linux /dev/sda7 5102 9165 2048224+ 83 Linux /dev/sda8 9166 13229 2048224+ 83 Linux /dev/sda9 13230 17293 2048224+ 83 Linux /dev/sda10 17294 19870 1298776+ 82 Linux Swap
We write out the new partition table:
Command (m for help):wThe partition table has been altered! Calling ioctl() to re-read partition table. Syncing disks.