Advanced Disk Setup

Contents

2.1. Using the YaST Partitioner
2.2. LVM Configuration
2.3. Soft RAID Configuration

Sophisticated system configurations require specific disk setups. All common partitioning tasks can be done with YaST. To get persistent device naming with block devices, use the block devices below /dev/disk/by-id or /dev/disk/by-uuid. Logical Volume Management (LVM) is a disk partitioning scheme that is designed to be much more flexible than the physical partitioning used in standard setups. Its snapshot functionality enables easy creation of data backups. Redundant Array of Independent Disks (RAID) offers increased data integrity, performance, and fault tolerance.

Using the YaST Partitioner

With the expert partitioner, shown in Figure 2.1, “The YaST Partitioner”, manually modify the partitioning of one or several hard disks. Partitions can be added, deleted, resized, and edited. Also access the soft RAID and LVM configuration from this YaST module.

	Repartitioning the Running System
Although it is possible to repartition your system while it is running, the risk of making a mistake that causes data loss is very high. Try to avoid repartitioning your installed system and always do a complete backup of your data before attempting to do so.

Figure 2.1. The YaST Partitioner

All existing or suggested partitions on all connected hard disks are displayed in the list of Available Storage in the YaST Expert Partitioner dialog. Entire hard disks are listed as devices without numbers, such as /dev/sda. Partitions are listed as parts of these devices, such as /dev/sda1. The size, type, file system, and mount point of the hard disks and their partitions are also displayed. The mount point describes where the partition appears in the Linux file system tree.

Several functional views are available on the left System View menu. Use these views to gather information about existing storage configurations, or to configure functions like RAID, Volume Management, Crypt Files, or NFS.

If you run the expert dialog during installation, any free hard disk space is also listed and automatically selected. To provide more disk space to openSUSE®, free the needed space starting from the bottom toward the top of the list (starting from the last partition of a hard disk toward the first). For example, if you have three partitions, you cannot use the second exclusively for openSUSE and retain the third and first for other operating systems.

Partition Types

Every hard disk has a partition table with space for four entries. Every entry in the partition table corresponds to a primary partition or an extended partition. Only one extended partition entry is allowed, however.

A primary partition simply consists of a continuous range of cylinders (physical disk areas) assigned to a particular operating system. With primary partitions you would be limited to four partitions per hard disk, because more do not fit in the partition table. This is why extended partitions are used. Extended partitions are also continuous ranges of disk cylinders, but an extended partition can be divided into logical partitions itself. Logical partitions do not require entries in the partition table. In other words, an extended partition is a container for logical partitions.

If you need more than four partitions, create an extended partition as the fourth partition (or earlier). This extended partition should span the entire remaining free cylinder range. Then create multiple logical partitions within the extended partition. The maximum number of logical partitions is 15 on SCSI, SATA, and Firewire disks and 63 on (E)IDE disks. It does not matter which types of partitions are used for Linux. Primary and logical partitions both function normally.

Creating a Partition

To create a partition from scratch select Hard Disks and then a hard disk with free space. The actual modification can be done in the Partitions tab:

Select Add partition. If several hard disks are connected, a selection dialog appears in which to select a hard disk for the new partition.
Specify the partition type (primary or extended). Create up to four primary partitions or up to three primary partitions and one extended partition. Within the extended partition, create several logical partitions (see Section 2.1.1, “Partition Types”).
Select the file system to use, and a mount point. YaST suggests a mount point for each partition created. To use a different mount method, like mount by label, select Fstab Options.
Specify additional file system options if your setup requires them. This is necessary, for example, if you need persistent device names. For details on the available options, refer to Section 2.1.3, “Editing a Partition”.
Click OK+Apply to apply your partitioning setup and leave the partitioning module.
If you created the partition during installation, you are returned to the installation overview screen.

Editing a Partition

When you create a new partition or modify an existing partition, set various parameters. For new partitions, suitable parameters are set by YaST and usually do not require any modification. To edit your partition setup manually, proceed as follows:

Select the partition.
Click Edit to edit the partition and set the parameters:
File System ID
Even if you do not want to format the partition at this stage, assign it a file system ID to ensure that the partition is registered correctly. Possible values include Linux, Linux swap, Linux LVM, and Linux RAID.
File System
Change the file system or format the partition here. Changing the file system or reformatting partitions irreversibly deletes all data from the partition.
Swap is a special format that allows the partition to be used as virtual memory. Create a swap partition of at least 256 MB. However, if you use up your swap space, consider adding more memory to your system instead of adding more swap space.
Ext4 is now the default file system for the Linux partitions. ReiserFS, JFS, XFS, Ext4 and Ext3 are journaling file systems. These file systems are able to restore the system very quickly after a system crash, utilizing write processes logged during the operation. Furthermore, ReiserFS is very fast in handling multiple small files. Ext2 is not a journaling file system. However, it is adequate for smaller partitions, because it does not require much disk space for management.
Encrypt File System
If you activate the encryption, all data is written to the hard disk in encrypted form. This increases the security of sensitive data, but reduces the system speed, as the encryption takes some time to process. More information about the encryption of file systems is provided in Chapter Encrypting Partitions and Files (↑Security Guide).
Fstab Options
Specify various parameters contained in the global file system administration file (/etc/fstab). The default settings should suffice for most setups. You can, for example, change the file system identification from the device name to a volume label. In the volume label, use all characters except / and space.
To get persistent devices names, use the mount option Device ID, UUID or LABEL. In openSUSE, persistent device names are enabled by default.
When using the mount option LABEL to mount a partition, define an appropriate label for the selected partition. For example, you could use the partition label HOME for a partition intended to mount to /home.
If you intend to use quotas on the file system, use the mount option Enable Quota Support. This must be done before you can define quotas for users in the YaST User Management module. For further information on how to configure user quota, refer to Section 8.3.5, “Managing Quotas”.
Mount Point
Specify the directory where the partition should be mounted in the file system tree. Select from the offered YaST proposals or enter any other name.
Select OK+Apply to activate the partition.

	Resize Filesystems
To resize an existing file system, select the partition and use Resize. Note that it is not possible to resize partitions while mounted. To resize partitions, unmount the related partition before running the partitioner.

More Partitioning Tips

The following section comprises a few hints and tips on partitioning that should help you in taking the right decisions while setting up your system.

	Cylinder Numbers
Note, that different partitioning tools may start counting the cylinders of a partition with `0` or with `1`. When calculating the number of cylinders, you should always use the difference between the last and the first cylinder number and add one.

Using `swap`

Swap is used to extend the available physical memory. This makes it possible to use more memory than physical ram available. The memory management system of kernels before 2.4.10 needed swap as a safety measure. Then, if you did not have twice the size of your ram in swap, the performance of the system suffered. These limitations no longer exist.

Linux uses a page called “Least Recently Used” (LRU) to select pages that might be moved from memory to disk. Therefore, running applications have more memory available and caching works more smoothly.

If an application tries to allocate the maximum allowed memory, problems with swap can arise. There are three major cases to look at:

System with no swap: The application gets the maximum allowed memory. All caches are freed, and thus all other running applications are slowed. After a few minutes, the kernel's out-of-memory kill mechanism activates and kills the process.
System with medіum sized swap (128 MB–512 MB): At first, the system slows like a system without swap. After all physical RAM has been allocated, swap space is used as well. At this point, the system becomes very slow and it becomes impossible to run commands from remote. Depending on the speed of the hard disks that run the swap space, the system stays in this condition for about 10 to 15 minutes until the out-of-memory kill mechanism resolves the issue. Note that you will need a certain amount of swap if the computer needs to perform a “suspend to disk”. In that case, the swap size should be large enough to contain the necessary data from memory (512 MB–1GB).
System with lots of swap (several GB): It is better to not have an application that swaps in an uncontrolled manner. If you do have this problem, the system will need many hours to recover. In the process, it is likely that other processes get timeouts and faults, leaving the system in an undefined state, even if the faulty process is killed. In this case, reboot the machine hard and try to get it running again. Lots of swap is only useful if you have an application that relies on this feature. Such applications (like databases or graphics manipulation programs) often have an option to directly use hard disk space for their needs. It is advisable to use this option instead of using lots of swap space.

If your system is not out of control, but needs more swap after some time, it is possible to extend the swap space online. If you prepared a partition for swap space, just add this partition with YaST. If you do not have a partition available, you may also just use a swap file to extend the swap. Swap files are generally slower than partitions, but compared to physical RAM, the difference is negligible.

Procedure 2.1. Adding a Swap File Manually

To add a swap file in the running system, proceed as follows:

Create an empty file in your system. For example, if you want to add a swap file with 128 MB swap at /var/lib/swap/swapfile, use the commands:
```
mkdir -p /var/lib/swap
dd if=/dev/zero of=/var/lib/swap/swapfile bs=1M count=128
```
Initialize this swap file with the command
```
mkswap /var/lib/swap/swapfile
```
Activate the swap with the command
```
swapon /var/lib/swap/swapfile
```
To disable this swap file, use the command
```
swapoff /var/lib/swap/swapfile
```
Check the current available swap spaces with the command
```
cat /proc/swaps
```
Note, that at this point this is only temporary swap space. After the next reboot, it is no longer utilized.
To enable this swap file permanently, add the following line to /etc/fstab:
```
/var/lib/swap/swapfile swap swap defaults 0 0
```

Partitioning and LVM

From the expert partitioner, access the LVM configuration with Volume Management. However, if a working LVM configuration already exists on your system, it is automatically activated upon entering the initial LVM configuration of a session. In this case, all disks containing a partition (belonging to an activated volume group) cannot be repartitioned. The Linux kernel cannot reread the modified partition table of a hard disk when any partition on this disk is in use. However, if you already have a functioning LVM configuration on your system, physical repartitioning should not be necessary. Instead, change the configuration of the logical volumes.

At the beginning of the physical volumes (PVs), information about the volume is written to the partition. To reuse such a partition for other non-LVM purposes, it is advisable to delete the beginning of this volume. For example, in the VG system and PV /dev/sda2, do this with the command dd if=/dev/zero of=/dev/sda2 bs=512 count=1.

	File System for Booting
The file system used for booting (the root file system or `/boot`) must not be stored on an LVM logical volume. Instead, store it on a normal physical partition.

For more details about LVM, see the Storage Administration Guide.