ハンドブック:PPC64/ブロック/ディスク

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This page is a translated version of the page Handbook:PPC64/Blocks/Disks and the translation is 37% complete.


Partitions and slices

Although it is theoretically possible to use a full disk to house a Linux system, this is almost never done in practice. Instead, full disk block devices are split up in smaller, more manageable block devices. On most systems, these are called partitions. Other architectures use a similar technique, called slices.

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 /var/db/repos/gentoo) which already takes around 650 MiB. This disk space estimate excludes the binpkgs/ and distfiles/ directories that are stored under /var/cache/ by default.

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.


Default: Using mac-fdisk

Important
These instructions are for the Apple G5 system.

Start mac-fdisk:

root #mac-fdisk /dev/sda

First delete the partitions that have been cleared previously to make room for Linux partitions. Use the d key in mac-fdisk to delete those partition(s). It will ask for the partition number to delete.

Second, create an Apple_Bootstrap partition by pressing the b key. It will ask for a block from which to start. Enter the number of the first free partition, followed by entering a p. For instance this is 2p.

Note
This partition is not a "boot" partition. It is not used by Linux at all; there is no need to place any filesystem on it and it should never be mounted. PPC users do not need an extra partition for /boot.

Now create a swap partition by pressing the c key. Again mac-fdisk will ask what block to start from. As we used 2 before to create the Apple_Bootstrap partition, enter 3p. When asked for the size, enter 512M (or whatever size needed). When asked for a name, enter swap (mandatory).

To create the root partition, enter c, followed by 4p to select from what block the root partition should start. When asked for the size, enter 4p again. mac-fdisk will interpret this as "Use all available space". When asked for the name, enter root (mandatory).

To finish up, write the partition to the disk using w and q to quit mac-fdisk.

Note
To make sure everything is okay, run mac-fdisk once more to verify all the partitions are present. If a partition is absent, or it is missing some of the changes that were made, then reinitialize the partitions by pressing i in mac-fdisk. Note that this will recreate the partition map and thus remove all the partitions.

代替案: fdisk を用いる

Important
The following instructions are for IBM pSeries, iSeries, and OpenPower systems.
Note
When planning to use a RAID disk array for the Gentoo installation on POWER5-based hardware, first run iprconfig to format the disks to Advanced Function format and create the disk array. Emerge sys-fs/iprutils after the installation is complete.

If the system has an ipr-based SCSI adapter, start the ipr utilities now.

root #/etc/init.d/iprinit start

The following parts explain how to create the example partition layout described previously, namely:

Partition Description
/dev/sda1 PPC PReP Boot partition
/dev/sda2 Swap partition
/dev/sda3 Root partition

Change or modify the partition layout according to personal preference.

Viewing current partition layout

fdisk is a popular and powerful tool to split a disk into partitions. Fire up fdisk on the current disk (in our example, we use /dev/sda):

root #fdisk /dev/sda
Command (m for help)

If there is still an AIX partition layout on the system, then the following error message will be displayed:

root #fdisk /dev/sda
  There is a valid AIX label on this disk.
  Unfortunately Linux cannot handle these
  disks at the moment.  Nevertheless some
  advice:
  1. fdisk will destroy its contents on write.
  2. Be sure that this disk is NOT a still vital
     part of a volume group. (Otherwise you may
     erase the other disks as well, if unmirrored.)
  3. Before deleting this physical volume be sure
     to remove the disk logically from your AIX
     machine.  (Otherwise you become an AIXpert).

Don't worry, new empty DOS partition table can be created by pressing o.

Warning
This will destroy any installed AIX version!

Type p to display the disk current partition configuration:

Command (m for help):p
Disk /dev/sda: 30.7 GB, 30750031872 bytes
141 heads, 63 sectors/track, 6761 cylinders
Units = cylinders of 8883 * 512 = 4548096 bytes
  
   Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1          12       53266+  83  Linux
/dev/sda2              13         233      981571+  82  Linux swap
/dev/sda3             234         674     1958701+  83  Linux
/dev/sda4             675        6761    27035410+   5  Extended
/dev/sda5             675        2874     9771268+  83  Linux
/dev/sda6            2875        2919      199836   83  Linux
/dev/sda7            2920        3008      395262   83  Linux
/dev/sda8            3009        6761    16668918   83  Linux

This particular disk is configured to house six Linux filesystems (each with a corresponding partition listed as "Linux") as well as a swap partition (listed as "Linux swap").

Removing all partitions

First remove all existing partitions from the disk. Type d to delete a partition. For instance, to delete an existing /dev/sda1:

Command (m for help):d
Partition number (1-4): 1

The partition has been scheduled for deletion. It will no longer show up when typing p, but it will not be erased until the changes have been saved. If a mistake was made and the session needs to be aborted, then type q immediately and hit Enter and none of the partitions will be deleted or modified.

Now, assuming that indeed all partitions need to be wiped out, repeatedly type p to print out a partition listing and then type d and the number of the partition to delete it. Eventually, the partition table will show no more partitions:

Command (m for help):p
Disk /dev/sda: 30.7 GB, 30750031872 bytes
141 heads, 63 sectors/track, 6761 cylinders
Units = cylinders of 8883 * 512 = 4548096 bytes
  
Device Boot    Start       End    Blocks   Id  System

Now that the in-memory partition table is empty, let's create the partitions. We will use a default partitioning scheme as discussed previously. Of course, don't follow these instructions to the letter but adjust to personal preference.

Creating the PPC PReP boot partition

First create a small PReP boot partition. Type n to create a new partition, then p to select a primary partition, followed by 1 to select the first primary partition. When prompted for the first cylinder, hit Enter. When prompted for the last cylinder, type +7M to create a partition 7 MB in size. After this, type t to set the partition type, 1 to select the partition just created and then type in 41 to set the partition type to "PPC PReP Boot". Finally, mark the PReP partition as bootable.

Note
The PReP partition has to be smaller than 8 MB!
Command (m for help):p
Disk /dev/sda: 30.7 GB, 30750031872 bytes
141 heads, 63 sectors/track, 6761 cylinders
Units = cylinders of 8883 * 512 = 4548096 bytes
  
   Device Boot      Start         End      Blocks   Id  System
Command (m for help):n
Command action
      e   extended
      p   primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-6761, default 1): 
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-6761, default
6761): +8M
Command (m for help):t
Selected partition 1
Hex code (type L to list codes): 41
Changed system type of partition 1 to 41 (PPC PReP Boot)
Command (m for help):a
Partition number (1-4): 1
Command (m for help):

Now, when looking at the partition table again (through p), the following partition information should be shown:

Command (m for help):p
Disk /dev/sda: 30.7 GB, 30750031872 bytes
141 heads, 63 sectors/track, 6761 cylinders
Units = cylinders of 8883 * 512 = 4548096 bytes
  
   Device Boot      Start         End      Blocks   Id  System
/dev/sda1  *            1           3       13293   41  PPC PReP Boot

スワップパーティションを作成する

Now create the swap partition. To do this, type n to create a new partition, then p to tell fdisk to create a primary partition. Then type 2 to create the second primary partition, /dev/sda2 in our case. When prompted for the first cylinder, hit Enter. When prompted for the last cylinder, type +512M to create a partition 512MB in size. After this, type t to set the partition type, 2 to select the partition just created and then type in 82 to set the partition type to "Linux Swap". After completing these steps, typing p should display a partition table that looks similar to this:

Command (m for help):p
Disk /dev/sda: 30.7 GB, 30750031872 bytes
141 heads, 63 sectors/track, 6761 cylinders
Units = cylinders of 8883 * 512 = 4548096 bytes
  
   Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1           3       13293   41  PPC PReP Boot
/dev/sda2               4         117      506331   82  Linux swap

ルートパーティションを作成する

Finally, create the root partition. To do this, type n to create a new partition, then p to tell fdisk to create a primary partition. Then type 3 to create the third primary partition, /dev/sda3 in our case. When prompted for the first cylinder, hit Enter. When prompted for the last cylinder, hit enter to create a partition that takes up the rest of the remaining space on the disk. After completing these steps, typing p should display a partition table that looks similar to this:

Command (m for help):p
Disk /dev/sda: 30.7 GB, 30750031872 bytes
141 heads, 63 sectors/track, 6761 cylinders
Units = cylinders of 8883 * 512 = 4548096 bytes
  
   Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1           3       13293   41  PPC PReP Boot
/dev/sda2               4         117      506331   82  Linux swap
/dev/sda3             118        6761    29509326   83  Linux

パーティションのレイアウトを保存する

To save the partition layout and exit fdisk, type w.

Command (m for help):w