13 Advanced Disk Setup #

Tip: IBM IBM Z: Hard Disks

On IBM IBM Z platforms, SUSE Linux Enterprise Server supports SCSI hard disks and DASDs (direct access storage devices). While SCSI disks can be partitioned as described below, DASDs can have no more than three partition entries in their partition tables.

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 may 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 occupy the entire remaining free cylinder range. Then create multiple logical partitions within the extended partition. The maximum number of logical partitions is 63, independent of the disk type. It does not matter which types of partitions are used for Linux. Primary and logical partitions both function normally.

Tip: GPT Partition Table

If you need to create more than 4 primary partitions on one hard disk, you need to use the GPT partition type. This type removes the primary partitions number restriction, and supports partitions bigger than 2 TB as well.

To use GPT, run the YaST Partitioner, click the relevant disk name in the System View and choose Expert › Create New Partition Table › GPT.

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:

13.1.2.1 Btrfs Partitioning #

  

The default file system for the root partition is Btrfs (see Chapter 7, System Recovery and Snapshot Management with Snapper and Chapter 1, Overview of File Systems in Linux for more information on Btrfs). The root file system is the default subvolume and it is not listed in the list of created subvolumes. As a default Btrfs subvolume, it can be mounted as a normal file system.

Important: Btrfs on an Encrypted Root Partition

The default partitioning setup suggests the root partition as Btrfs with /boot being a directory. To encrypt the root partition, make sure to use the GPT partition table type instead of the default MSDOS type. Otherwise the GRUB2 boot loader may not have enough space for the second stage loader.

It is possible to create snapshots of Btrfs subvolumes—either manually, or automatically based on system events. For example when making changes to the file system, zypper invokes the snapper command to create snapshots before and after the change. This is useful if you are not satisfied with the change zypper made and want to restore the previous state. As snapper invoked by zypper creates snapshots of the root file system by default, it makes sense to exclude specific directories from snapshots. This is the reason why YaST suggests creating the following separate subvolumes:

/boot/grub2/i386-pc, /boot/grub2/x86_64-efi, /boot/grub2/powerpc-ieee1275, /boot/grub2/s390x-emu

A rollback of the boot loader configuration is not supported. The directories listed above are architecture-specific. The first two directories are present on AMD64/Intel 64 machines, the latter two on IBM POWER and on IBM IBM Z, respectively.

/home

If /home does not reside on a separate partition, it is excluded to avoid data loss on rollbacks.

/opt, /var/opt

Third-party products usually get installed to /opt. It is excluded to avoid uninstalling these applications on rollbacks.

/srv

Contains data for Web and FTP servers. It is excluded to avoid data loss on rollbacks.

/tmp, /var/tmp, /var/cache, /var/crash

All directories containing temporary files and caches are excluded from snapshots.

/usr/local

This directory is used when manually installing software. It is excluded to avoid uninstalling these installations on rollbacks.

/var/lib/libvirt/images

The default location for virtual machine images managed with libvirt. Excluded to ensure virtual machine images are not replaced with older versions during a rollback. By default, this subvolume is created with the option no copy on write.

/var/lib/mailman, /var/spool

Directories containing mails or mail queues are excluded to avoid a loss of mails after a rollback.

/var/lib/named

Contains zone data for the DNS server. Excluded from snapshots to ensure a name server can operate after a rollback.

/var/lib/mariadb, /var/lib/mysql, /var/lib/pgqsl

These directories contain database data. By default, these subvolumes are created with the option no copy on write.

/var/log

Log file location. Excluded from snapshots to allow log file analysis after the rollback of a broken system. By default, /var/log has the NoCOW attribute set, disabling copy-on-write, which improves performance and reduces the number of duplicate blocks. Verify with lsattr:

tux > lsattr -l /var/
/var/log      No_COW

Tip: Size of Btrfs Partition

Since saved snapshots require more disk space, it is recommended to reserve enough space for Btrfs. While the minimum size for a root Btrfs partition with snapshots and default subvolumes is 16 GB, SUSE recommends at least 32 GB, or more if /home does not reside on a separate partition.

13.1.2.1.1 Managing Btrfs Subvolumes using YaST #

  

Subvolumes of a Btrfs partition can be now managed with the YaST Expert partitioner module. You can add new or remove existing subvolumes.

Procedure 13.1: Btrfs Subvolumes with YaST #

  

1. Start the YaST Expert Partitioner with System › Partitioner.

2. Choose Btrfs in the left System View pane.

3. Select the Btrfs partition whose subvolumes you need to manage and click Edit.

4. Click Subvolume Handling. You can see a list off all existing subvolumes of the selected Btrfs partition. You can notice several @/.snapshots/xyz/snapshot entries—each of these subvolumes belongs to one existing snapshot.

5. Depending on whether you want to add or remove subvolumes, do the following:

   1. To remove a subvolume, select it from the list of Exisitng Subvolumes and click Remove.

   2. To add a new subvolume, enter its name to the New Subvolume text box and click Add new.

      

      Figure 13.2: Btrfs Subvolumes in YaST Partitioner #

        

6. Confirm with OK and Finish.

7. Leave the partitioner with Finish.

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

Note: Resize File Systems

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 relevant partition before running the partitioner.

After you select a hard disk device (like sda) in the System View pane, you can access the Expert menu in the lower right part of the Expert Partitioner window. The menu contains the following commands:

After you select the host name of the computer (the top-level of the tree in the System View pane), you can access the Configure menu in the lower right part of the Expert Partitioner window. The menu contains the following commands:

The following section includes a few hints and tips on partitioning that should help you make the right decisions when setting up your system.

Tip: 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.

From the Expert partitioner, access the LVM configuration by clicking the Volume Management item in the System View pane. 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. If you already have a working 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.

Warning: 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 Storage Administration Guide.

The YaST LVM configuration can be reached from the YaST Expert Partitioner (see Section 13.1, “Using the YaST Partitioner”) within the Volume Management item in the System View pane. The Expert Partitioner allows you to edit and delete existing partitions and create new ones that need to be used with LVM. The first task is to create PVs that provide space to a volume group:

13.2.1.1 Creating Volume Groups #

  

If no volume group exists on your system, you must add one (see Figure 13.3, “Creating a Volume Group”). It is possible to create additional groups by clicking Volume Management in the System View pane, and then on Add Volume Group. One single volume group is usually sufficient.

1. Enter a name for the VG, for example, system.

2. Select the desired Physical Extend Size. This value defines the size of a physical block in the volume group. All the disk space in a volume group is handled in blocks of this size.

3. Add the prepared PVs to the VG by selecting the device and clicking Add. Selecting several devices is possible by holding Ctrl while selecting the devices.

4. Select Finish to make the VG available to further configuration steps.

Figure 13.3: Creating a Volume Group #

  

If you have multiple volume groups defined and want to add or remove PVs, select the volume group in the Volume Management list and click Resize. In the following window, you can add or remove PVs to the selected volume group.

13.2.1.2 Configuring Logical Volumes #

  

After the volume group has been filled with PVs, define the LVs which the operating system should use in the next dialog. Choose the current volume group and change to the Logical Volumes tab. Add, Edit, Resize, and Delete LVs as needed until all space in the volume group has been occupied. Assign at least one LV to each volume group.

Figure 13.4: Logical Volume Management #

  

Click Add and go through the wizard-like pop-up that opens:

1. Enter the name of the LV. For a partition that should be mounted to /home, a name like HOME could be used.

2. Select the type of the LV. It can be either Normal Volume, Thin Pool, or Thin Volume. Note that you need to create a thin pool first, which can store individual thin volumes. The big advantage of thin provisioning is that the total sum of all thin volumes stored in a thin pool can exceed the size of the pool itself.

3. Select the size and the number of stripes of the LV. If you have only one PV, selecting more than one stripe is not useful.

4. Choose the file system to use on the LV and the mount point.

By using stripes it is possible to distribute the data stream in the LV among several PVs (striping). However, striping a volume can only be done over different PVs, each providing at least the amount of space of the volume. The maximum number of stripes equals to the number of PVs, where Stripe "1" means "no striping". Striping only makes sense with PVs on different hard disks, otherwise performance will decrease.

Warning: Striping

YaST cannot, at this point, verify the correctness of your entries concerning striping. Any mistake made here is apparent only later when the LVM is implemented on disk.

If you have already configured LVM on your system, the existing logical volumes can also be used. Before continuing, assign appropriate mount points to these LVs. With Finish, return to the YaST Expert Partitioner and finish your work there.

The YaST RAID configuration can be reached from the YaST Expert Partitioner, described in Section 13.1, “Using the YaST Partitioner”. This partitioning tool enables you to edit and delete existing partitions and create new ones to be used with soft RAID:

For RAID 0 and RAID 1, at least two partitions are needed—for RAID 1, usually exactly two and no more. If RAID 5 is used, at least three partitions are required, RAID 6 and RAID 10 require at least four partitions. It is recommended to use partitions of the same size only. The RAID partitions should be located on different hard disks to decrease the risk of losing data if one is defective (RAID 1 and 5) and to optimize the performance of RAID 0. After creating all the partitions to use with RAID, click RAID › Add RAID to start the RAID configuration.

In the next dialog, choose between RAID levels 0, 1, 5, 6 and 10. Then, select all partitions with either the “Linux RAID” or “Linux native” type that should be used by the RAID system. No swap or DOS partitions are shown.

Tip: Classify Disks

For RAID types where the order of added disks matters, you can mark individual disks with one of the letters A to E. Click the Classify button, select the disk and click of the Class X buttons, where X is the letter you want to assign to the disk. Assign all available RAID disks this way, and confirm with OK. You can easily sort the classified disks with the Sorted or Interleaved buttons, or add a sort pattern from a text file with Pattern File.

Figure 13.5: RAID Partitions #

  

To add a previously unassigned partition to the selected RAID volume, first click the partition then Add. Assign all partitions reserved for RAID. Otherwise, the space on the partition remains unused. After assigning all partitions, click Next to select the available RAID Options.

In this last step, set the file system to use, encryption and the mount point for the RAID volume. After completing the configuration with Finish, see the /dev/md0 device and others indicated with RAID in the expert partitioner.

Check the file /proc/mdstat to find out whether a RAID partition has been damaged. If Th system fails, shut down your Linux system and replace the defective hard disk with a new one partitioned the same way. Then restart your system and enter the command mdadm /dev/mdX --add /dev/sdX. Replace 'X' with your particular device identifiers. This integrates the hard disk automatically into the RAID system and fully reconstructs it.

Note that although you can access all data during the rebuild, you may encounter some performance issues until the RAID has been fully rebuilt.

Configuration instructions and more details for soft RAID can be found in the Howtos at:

Linux RAID mailing lists are available, such as http://marc.info/?l=linux-raid.