Claudio Borges

Hi folks, this article shows how to create a dynamic disk partitioning scheme using Debian preseed.

TL;DR: I’ve added the partitioning file (and other preseed files) on my GitHub account. feel free to download it if you don’t want to read the entire article.

Overview

Debian preseed provides a way to set answers to questions asked during the installation process, without having to manually enter the answers while the installation is running. This makes it possible to fully automate most types of installations and it even offers some features not available during normal installations. Most of these questions are asked by Debian installer and can be preseeded by setting the answers in the debconf database.

The Debian installer has a directive called early_command. Using it, for example, you can run a Bourne Shell script (it isn’t Bash) or create a sh script inside of it. It’s powerful, but it has a problem. It doesn’t understand multiple lines, I mean, you need to create your rules in just one (logical) line.

PS: To organize and create a beautiful script, I’ve separated it in multiple lines (adding a backslash “\” at the end of each line).

Partitioning rules

The early_command will run immediately before the partitioner starts. It will count the number of disks, define the RAID level and the partitioning scheme. To do that, it will use the following rule:

1 disk = LVM Physical device
2 disks = RAID 1 + LVM Physical device
3 disks = RAID 5 + LVM Physical device
4 disks or more = RAID 10 + LVM Physical device

Partitioning methods

PartMan uses 4 different methods to partition the disks:

• regular – use the usual partition types for your architecture
• lvm – use LVM to partition the disk
• crypto – use LVM within an encrypted partition
• raid – use RAID partition the disk (with or without LVM)

To define the method, use the option “partman-auto/method”, for example:

partman-auto/method "raid"

When the method is defined, the PartMan needs to know what disks will be used in the partitioning scheme. If the system has only one disk the installer will default to using that, but otherwise, the device name must be given in traditional, non-devfs format (so e.g. /dev/hda or /dev/sda, and not e.g. dev/discs/disc0/disc). If the system has two disks (or more), you need to specify all of them:

partman-auto/disk "/dev/sda /dev/sdb /dev/sdc /dev/sdd"

In addition, it’s necessary to specify how the disks will be used in the RAID setup. Remember to use the correct partition numbers for logical partitions and that RAID levels 0, 1, 5, 6 and 10 are supported and the devices are separated using “#”. The parameters are:

<raidtype> <devcount> <sparecount> <fstype> <mountpoint> <devices> <sparedevices>

Using four disks (RAID 10 + LVM), for example, the values are defined as follow:

partman-auto-raid/recipe "10 4 0 lvm - /dev/sda1#/dev/sdb1#/dev/sdc1#/dev/sdd1 ."

Why Sofware RAID?

Perhaps are you thinking why someone would use software instead of using Hardware RAID? The answer is simple: Everything depends on your point of view. Hardware RAID (integrated into the motherboard or a separate controller) is similar in performance to the software RAID. But, it has certain features not available in software RAID and that are never implemented in low-cost controllers, such as caching, hot-swapping, and battery backup. On the other hand, software RAID is implemented by the operating system, it is cheap, easy and a fairly versatile option. However, it has certain limitations, for example, it has no hotswap. In my case, I’m using Software RAID (sometimes) because, I have a few servers with low-cost controllers.

Both of them (Hardware and Software RAID) aren’t perfect. You can still lose the array to a controller failure or operator error.

Partitioning Recipe

After choosing RAID or LVM (or RAID + LVM), it is necessary to define the recipe. You can choose predefined recipes, create your own recipe file and point at it (if you have a way to get it into the environment) or you can create your own recipe in a preseed file. In my case, I’m using just the last option (I won’t describe the others, you can read the PartMan’s documentation to understand all of them).

The recipe is defined by the option “partman-auto/expert_recipe”. Using RAID, you need to specify what devices will be included in the RAID and/or what partitions will be logical volumes. Like I said, the early_command understands just one (logical) line. But, I’ve separated it in multiple lines. Take a look in the code below:

partman-auto/expert_recipe string "multiraid :: \
    0 0 0 raid \
        $lvmignore{ } \
        $primary { } \
        method{ raid } . \

The “multiraid” in the first line is the name of the recipe (it is used by another PartMan option).

Note that second line has the values “0 0 0 raid”, they are respectively the minimum size, priority and maximum size (I’ll talk about that in the next paragraph). The other options are:

• $lvmignore{ } – The RAID partitions are tagged as “lvmignore”, while the LVM logical volumes as “defaultignore” and “lvmok”.
• $primary{ } – used to define the partition as primary.
• method { } – used to define the type of formatting. You can use swap, raid, or format.

Now, take a look in the lines below:

    512 40960 15360 ext4 \
        $defaultignore{ } \
        $lvmok{ } \
        lv_name{ lv_root } \
        method{ format } \
        format{ } \
        use_filesystem{ } \
        filesystem{ ext4 } \
        mountpoint{ / } . \

PS: The “.” in the last line is a separator between hard drives.

Like I said, in Debian preseed files, each partition has a minimum size a priority value and maximum size. As you can see, the first line has the values “512 40960 15360 ext4”. These values are used by PartMan to create a smart partitioning scheme, they mean:

• 512 – the minimum size of partition in Mb.
• 40960 – the priority if it and other listed partitions are vying for space on the disk (this is compared with the priorities of the other partitions).
• 15360 – the maximum size of partition in Mb.
• ext4 – the filesystem type.

The other options used in the previous example are:

• $defaultignore{ } – used to avoid a partition definition so that it is ignored in the default case. That is to say it will be valid in the LVM case.
• $lvmok{ } – used to define a LVM logical volume.
• lv_name{ lv_root } – set the LVM logical volume name.
• method{ format } – set to format the partition. (use “keep” to not format or “swap” for swap partitions).
• format{ } – also needed so the partition will be formatted.
• use_filesystem{ } – this partition will have a filesystem on it (it won’t be swap, LVM, etc)
• filesystem{ ext4 } – what filesystem it gets
• mountpoint{ / } – where it will be mounted

Alternatively, you can define the volume group, if you don’t specify it, the PartMan will use a default name:

partman-auto-lvm/new_vg_name string VolGroup00

Swap size and Partitioning scheme

The SWAP size will be calculated based on the amount of RAM and I’ve used the following rule:

if RAM < 2GB then SWAP = 2x physical RAM
if RAM > 2GB or MEM < 8GB then SWAP = Equal to the amount of RAM
if RAM > 8GB then SWAP = At least 4 GB

And the partitioning scheme will be as follows:

/dev/mapper/VolGroup00/lv_root   - 512Mb to 15Gb - /    ext4
/dev/mapper/VolGroup00/lv_tmp    - 256Mb to 4Gb  - /tmp ext4
/dev/mapper/VolGroup00/lv_var    - 512Mb to 15Gb - /var ext4
/dev/mapper/VolGroup00/lv_delete - all remaining space

Did you understand why I have created a logical volume called lv_delete? Of course not right? So, the Debian installer has a bug. I don’t know why it allocates all remaining space to the last logical volume (just when you’re using preseed files). Because of that, I’ve defined the lv_delete and before the install finishes, it will be deleted, giving back this space to the volume group.

Preseed file

The following preseed file is used in my production environment. Feel free to use it or change it if you want:

### Partitioning
#
# Use RAID+LVM or just LVM to partition the disk

# Partitioning scheme:
#
# /dev/md0 - RAID device / LVM Physical volume (if you have 2 or more disks)
# /dev/mapper/VolGroup00/lv_swap - the swap is calculated over the amount
# of RAM in the system, ex:
# if RAM < 2GB then SWAP = 2x physical RAM
# if RAM > 2GB or MEM < 8GB then SWAP = Equal to the amount of RAM
# if RAM > 8GB then SWAP = At least 4 GB
# /dev/mapper/VolGroup00/lv_root   - 512Mb to 15Gb   - /    ext4
# /dev/mapper/VolGroup00/lv_tmp    - 256Mb to 4Gb    - /tmp ext4
# /dev/mapper/VolGroup00/lv_var    - 512Mb to 15Gb   - /var ext4
# /dev/mapper/VolGroup00/lv_delete - all remaining space. This logical
# volume was created because the Debian installer has a bug. It allocates
# all remaining space to the last logical volume. The late_command will
# execute a lvremove VolGroup00/lv_delete and give back this space to the
# volume group.

# The options used in the partitioning can be found in the following url:
# https://wikitech.wikimedia.org/wiki/PartMan

# These commands will run immediately before the partitioner starts.
d-i partman/early_command string \
    COUNT=0; \
    SPARE=0; \
    for DISK in $(list-devices disk); do \
        DISKS="${DISKS} ${DISK}"; \
        if [ "$(echo ${DISK} | cut -d'/' -f3)" = "cciss" ]; then \
            DEVS="${DEVS}${DISK}p1#"; \
            SPARE_DEV="${DISK}p1"; \
        else \
            DEVS="${DEVS}${DISK}1#"; \
            SPARE_DEV="${DISK}1"; \
        fi; \
        COUNT=$((COUNT + 1)); \
    done; \
    DISKS=$(echo ${DISKS} | sed "s/^ //g"); \
    DEVS=$(echo ${DEVS} | sed "s/#$//g"); \
    if [ "${COUNT}" -eq "1" ]; then \
        RAID="-1"; \
    elif [ "${COUNT}" -eq "2" ]; then \
        RAID="1"; \
    elif [ "${COUNT}" -eq "3" ]; then \
        RAID="5"; \
    elif [ "${COUNT}" -ge "4" ]; then \
        RAID="10"; \
        SPARE=$((COUNT % 2));\
    fi; \
    if [ ${SPARE} -eq "1" ]; then \
        COUNT=$((COUNT - 1)); \
        DEVS=$(echo ${DEVS} | sed -e "s|${SPARE_DEV}||g;s/#$//g"); \
    else \
        SPARE_DEV=""; \
    fi; \
    MEM=$(($(sed -n 's/^MemTotal: \+\([0-9]*\) kB/\1/p' /proc/meminfo) / 1024)); \
    if [ "${MEM}" -lt "2048" ]; then \
        SWAP=$((MEM * 2)); \
    elif [ "${MEM}" -gt "2048" ] || [ "${MEM}" -le "8192" ]; then \
        SWAP=${MEM}; \
    elif [ "${MEM}" -ge "8192" ]; then \
        SWAP=4096; \
    fi; \
    debconf-set partman-auto/disk "$DISKS"; \
    if [ "${RAID}" -ge "1" ]; then \
        debconf-set partman-auto/method "raid"; \
        debconf-set partman-auto-raid/recipe "${RAID} ${COUNT} ${SPARE} lvm - ${DEVS} ${SPARE_DEV} ."; \
        debconf-set partman-auto/expert_recipe "multiraid :: \
            0 0 0 raid \
                \$lvmignore{ } \
                \$primary { } \
                method{ raid } . \
            256 256 ${SWAP} linux-swap \
                \$defaultignore{ } \
                \$lvmok{ } \
                lv_name{ lv_swap } \
                method{ swap } \
                format{ } . \
            512 40960 15360 ext4 \
                \$defaultignore{ } \
                \$lvmok{ } \
                lv_name{ lv_root } \
                method{ format } \
                format{ } \
                use_filesystem{ } \
                filesystem{ ext4 } \
                mountpoint{ / } . \
            256 5120 4096 ext4 \
                \$defaultignore{ } \
                \$lvmok{ } \
                lv_name{ lv_tmp } \
                method{ format } \
                format{ } \
                use_filesystem{ } \
                filesystem{ ext4 } \
                mountpoint{ /tmp } . \
            512 40960 15360 ext4 \
                \$defaultignore{ } \
                \$lvmok{ } \
                lv_name{ lv_var } \
                method{ format } \
                format{ } \
                use_filesystem{ } \
                filesystem{ ext4 } \
                mountpoint{ /var } . \
            1024 1024 1024 ext4 \
                \$defaultignore{ } \
                \$lvmok{ } \
                lv_name{ lv_delete } ."; \
    else \
        debconf-set partman-auto/method "lvm"; \
        debconf-set partman-auto/expert_recipe "root :: \
            256 256 ${SWAP} linux-swap \
                \$defaultignore{ } \
                \$lvmok{ } \
                lv_name{ lv_swap } \
                method{ swap } \
                format{ } . \
            512 40960 15360 ext4 \
                \$defaultignore{ } \
                \$lvmok{ } \
                lv_name{ lv_root } \
                method{ format } \
                format{ } \
                use_filesystem{ } \
                filesystem{ ext4 } \
                mountpoint{ / } . \
            256 5120 4096 ext4 \
                \$defaultignore{ } \
                \$lvmok{ } \
                lv_name{ lv_tmp } \
                method{ format } \
                format{ } \
                use_filesystem{ } \
                filesystem{ ext4 } \
                mountpoint{ /tmp } . \
            512 40960 15360 ext4 \
                \$defaultignore{ } \
                \$lvmok{ } \
                lv_name{ lv_var } \
                method{ format } \
                format{ } \
                use_filesystem{ } \
                filesystem{ ext4 } \
                mountpoint{ /var } . \
            1024 1024 1024 ext4 \
                \$defaultignore{ } \
                \$lvmok{ } \
                lv_name{ lv_delete } ."; \
    fi

# Install grub in the first device (assuming it is not a USB stick)
d-i grub-installer/bootdev string default

# Continue installation without /boot partition?
d-i partman-auto-lvm/no_boot boolean true

# Name of the volume group for the new system
d-i partman-auto-lvm/new_vg_name string VolGroup00

# Remove existing software RAID partitions?
d-i partman-md/device_remove_md boolean true

# Remove existing logical volume data?
d-i partman-lvm/device_remove_lvm boolean true

# Unable to automatically remove LVM data
d-i partman-lvm/device_remove_lvm_span boolean true

# Dummy template for preseeding unavailable questions
d-i partman-auto/purge_lvm_from_device boolean true

# Write the changes to the storage devices and configure RAID?
d-i partman-md/confirm boolean true
d-i partman-md/confirm_nooverwrite boolean true

# Write the changes to disks and configure LVM?
d-i partman-lvm/confirm boolean true
d-i partman-lvm/confirm_nooverwrite boolean true

# Write the changes to disks?
d-i partman/confirm boolean true
d-i partman/confirm_nooverwrite boolean true

# Finish partitioning and write changes to disk
d-i partman/choose_partition select finish

# This command is run just before the install finishes, and,
# it will remove the lv_delete
d-i preseed/late_command string lvremove -f /dev/VolGroup00/lv_delete > /dev/null 2>&1

The last line in the preseed file is the late_command. It works similarily to early_command, the only difference between them is the late_command runs just before the install finishes. In my case, it will remove the lv_delete, but you can use it to do other tasks, such as, for example, install packages or customize the environment before the first boot.

Final Thoughts

Probably you know that nothing is perfect, right? Debian preseed isn’t different. Its syntax isn’t easy (annoying is the right word), it has a weird bug (like I described previously) and its documentation is horrible (Debian users, I’m so sorry about that, but the Kickstart’s documentation is easier and more helpful). Besides, you need to be careful with it (especially with the early_command and the late_command), because, if you forget a minimal detail, the Debian installer will generate a generic error.

So, that is all for now folks. If you have any questions about this article, feel free to ask me.