Host System Administration

Proxmox Backup is based on the famous Debian Linux distribution. That means that you have access to the whole world of Debian packages, and the base system is well documented. The Debian Administrator's Handbook is available online, and provides a comprehensive introduction to the Debian operating system.

A standard Proxmox Backup installation uses the default repositories from Debian, so you get bug fixes and security updates through that channel. In addition, we provide our own package repository to roll out all Proxmox related packages. This includes updates to some Debian packages when necessary.

We also deliver a specially optimized Linux kernel, where we enable all required virtualization and container features. That kernel includes drivers for ZFS, and several hardware drivers. For example, we ship Intel network card drivers to support their newest hardware.

The following sections will concentrate on backup related topics. They either explain things which are different on Proxmox Backup, or tasks which are commonly used on Proxmox Backup. For other topics, please refer to the standard Debian documentation.

ZFS on Linux

ZFS is a combined file system and logical volume manager designed by Sun Microsystems. There is no need to manually compile ZFS modules - all packages are included.

By using ZFS, it's possible to achieve maximum enterprise features with low budget hardware, but also high performance systems by leveraging SSD caching or even SSD only setups. ZFS can replace cost intense hardware raid cards by moderate CPU and memory load combined with easy management.

General ZFS advantages

  • Easy configuration and management with GUI and CLI.

  • Reliable

  • Protection against data corruption

  • Data compression on file system level

  • Snapshots

  • Copy-on-write clone

  • Various raid levels: RAID0, RAID1, RAID10, RAIDZ-1, RAIDZ-2 and RAIDZ-3

  • Can use SSD for cache

  • Self healing

  • Continuous integrity checking

  • Designed for high storage capacities

  • Asynchronous replication over network

  • Open Source

  • Encryption


ZFS depends heavily on memory, so you need at least 8GB to start. In practice, use as much you can get for your hardware/budget. To prevent data corruption, we recommend the use of high quality ECC RAM.

If you use a dedicated cache and/or log disk, you should use an enterprise class SSD (e.g. Intel SSD DC S3700 Series). This can increase the overall performance significantly.

IMPORTANT: Do not use ZFS on top of hardware controller which has its own cache management. ZFS needs to directly communicate with disks. An HBA adapter is the way to go, or something like LSI controller flashed in IT mode.

ZFS Administration

This section gives you some usage examples for common tasks. ZFS itself is really powerful and provides many options. The main commands to manage ZFS are zfs and zpool. Both commands come with great manual pages, which can be read with:

# man zpool
# man zfs

Create a new zpool

To create a new pool, at least one disk is needed. The ashift should have the same sector-size (2 power of ashift) or larger as the underlying disk.

# zpool create -f -o ashift=12 <pool> <device>

Create a new pool with RAID-0

Minimum 1 disk

# zpool create -f -o ashift=12 <pool> <device1> <device2>

Create a new pool with RAID-1

Minimum 2 disks

# zpool create -f -o ashift=12 <pool> mirror <device1> <device2>

Create a new pool with RAID-10

Minimum 4 disks

# zpool create -f -o ashift=12 <pool> mirror <device1> <device2> mirror <device3> <device4>

Create a new pool with RAIDZ-1

Minimum 3 disks

# zpool create -f -o ashift=12 <pool> raidz1 <device1> <device2> <device3>

Create a new pool with RAIDZ-2

Minimum 4 disks

# zpool create -f -o ashift=12 <pool> raidz2 <device1> <device2> <device3> <device4>

Create a new pool with cache (L2ARC)

It is possible to use a dedicated cache drive partition to increase the performance (use SSD).

As <device> it is possible to use more devices, like it's shown in "Create a new pool with RAID*".

# zpool create -f -o ashift=12 <pool> <device> cache <cache_device>

Create a new pool with log (ZIL)

It is possible to use a dedicated cache drive partition to increase the performance (SSD).

As <device> it is possible to use more devices, like it's shown in "Create a new pool with RAID*".

# zpool create -f -o ashift=12 <pool> <device> log <log_device>

Add cache and log to an existing pool

If you have a pool without cache and log. First partition the SSD in 2 partition with parted or gdisk


Always use GPT partition tables.

The maximum size of a log device should be about half the size of physical memory, so this is usually quite small. The rest of the SSD can be used as cache.

# zpool add -f <pool> log <device-part1> cache <device-part2>

Changing a failed device

# zpool replace -f <pool> <old device> <new device>

Changing a failed bootable device

Depending on how Proxmox Backup was installed it is either using grub or systemd-boot as bootloader.

The first steps of copying the partition table, reissuing GUIDs and replacing the ZFS partition are the same. To make the system bootable from the new disk, different steps are needed which depend on the bootloader in use.

# sgdisk <healthy bootable device> -R <new device>
# sgdisk -G <new device>
# zpool replace -f <pool> <old zfs partition> <new zfs partition>


Use the zpool status -v command to monitor how far the resilvering process of the new disk has progressed.

With systemd-boot:

# pve-efiboot-tool format <new disk's ESP>
# pve-efiboot-tool init <new disk's ESP>


ESP stands for EFI System Partition, which is setup as partition #2 on bootable disks setup by the {pve} installer since version 5.4. For details, see xref:sysboot_systemd_boot_setup[Setting up a new partition for use as synced ESP].

With grub:

Usually grub.cfg is located in /boot/grub/grub.cfg

# grub-install <new disk>
# grub-mkconfig -o /path/to/grub.cfg

Activate E-Mail Notification

ZFS comes with an event daemon, which monitors events generated by the ZFS kernel module. The daemon can also send emails on ZFS events like pool errors. Newer ZFS packages ship the daemon in a separate package, and you can install it using apt-get:

# apt-get install zfs-zed

To activate the daemon it is necessary to edit /etc/zfs/zed.d/zed.rc with your favorite editor, and uncomment the ZED_EMAIL_ADDR setting:


Please note Proxmox Backup forwards mails to root to the email address configured for the root user.

IMPORTANT: The only setting that is required is ZED_EMAIL_ADDR. All other settings are optional.

Limit ZFS Memory Usage

It is good to use at most 50 percent (which is the default) of the system memory for ZFS ARC to prevent performance shortage of the host. Use your preferred editor to change the configuration in /etc/modprobe.d/zfs.conf and insert:

options zfs zfs_arc_max=8589934592

This example setting limits the usage to 8GB.


If your root file system is ZFS you must update your initramfs every time this value changes:

# update-initramfs -u


Swap-space created on a zvol may generate some troubles, like blocking the server or generating a high IO load, often seen when starting a Backup to an external Storage.

We strongly recommend to use enough memory, so that you normally do not run into low memory situations. Should you need or want to add swap, it is preferred to create a partition on a physical disk and use it as swap device. You can leave some space free for this purpose in the advanced options of the installer. Additionally, you can lower the swappiness value. A good value for servers is 10:

# sysctl -w vm.swappiness=10

To make the swappiness persistent, open /etc/sysctl.conf with an editor of your choice and add the following line:

vm.swappiness = 10
Linux kernel swappiness parameter values :widths:auto



vm.swappiness = 0

The kernel will swap only to avoid an 'out of memory' condition

vm.swappiness = 1

Minimum amount of swapping without disabling it entirely.

vm.swappiness = 10

Sometimes recommended to improve performance when sufficient memory exists in a system.

vm.swappiness = 60

The default value.

vm.swappiness = 100

The kernel will swap aggressively.

ZFS Compression

To activate compression: .. code-block:: console

# zpool set compression=lz4 <pool>

We recommend using the lz4 algorithm, since it adds very little CPU overhead. Other algorithms such as lzjb and gzip-N (where N is an integer 1-9 representing the compression ratio, 1 is fastest and 9 is best compression) are also available. Depending on the algorithm and how compressible the data is, having compression enabled can even increase I/O performance.

You can disable compression at any time with: .. code-block:: console

# zfs set compression=off <dataset>

Only new blocks will be affected by this change.

ZFS Special Device

Since version 0.8.0 ZFS supports special devices. A special device in a pool is used to store metadata, deduplication tables, and optionally small file blocks.

A special device can improve the speed of a pool consisting of slow spinning hard disks with a lot of metadata changes. For example workloads that involve creating, updating or deleting a large number of files will benefit from the presence of a special device. ZFS datasets can also be configured to store whole small files on the special device which can further improve the performance. Use fast SSDs for the special device.


The redundancy of the special device should match the one of the pool, since the special device is a point of failure for the whole pool.


Adding a special device to a pool cannot be undone!

Create a pool with special device and RAID-1:

# zpool create -f -o ashift=12 <pool> mirror <device1> <device2> special mirror <device3> <device4>

Adding a special device to an existing pool with RAID-1:

# zpool add <pool> special mirror <device1> <device2>

ZFS datasets expose the special_small_blocks=<size> property. size can be 0 to disable storing small file blocks on the special device or a power of two in the range between 512B to 128K. After setting the property new file blocks smaller than size will be allocated on the special device.


If the value for special_small_blocks is greater than or equal to the recordsize (default 128K) of the dataset, all data will be written to the special device, so be careful!

Setting the special_small_blocks property on a pool will change the default value of that property for all child ZFS datasets (for example all containers in the pool will opt in for small file blocks).

Opt in for all file smaller than 4K-blocks pool-wide:

# zfs set special_small_blocks=4K <pool>

Opt in for small file blocks for a single dataset:

# zfs set special_small_blocks=4K <pool>/<filesystem>

Opt out from small file blocks for a single dataset:

# zfs set special_small_blocks=0 <pool>/<filesystem>


Corrupted cachefile

In case of a corrupted ZFS cachefile, some volumes may not be mounted during boot until mounted manually later.

For each pool, run:

# zpool set cachefile=/etc/zfs/zpool.cache POOLNAME

and afterwards update the initramfs by running:

# update-initramfs -u -k all

and finally reboot your node.

Sometimes the ZFS cachefile can get corrupted, and zfs-import-cache.service doesn't import the pools that aren't present in the cachefile.

Another workaround to this problem is enabling the zfs-import-scan.service, which searches and imports pools via device scanning (usually slower).

Service Daemons


This daemon exposes the whole Proxmox Backup Server API on TCP port 8007 using HTTPS. It runs as user backup and has very limited permissions. Operation requiring more permissions are forwarded to the local proxmox-backup service.


This daemon exposes the Proxmox Backup Server management API on It runs as root and has permission to do all privileged operations.

NOTE: The daemon listens to a local address only, so you cannot access it from outside. The proxmox-backup-proxy daemon exposes the API to the outside world.