
.. _chapter-zfs:

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, and also high performance systems by leveraging
SSD caching or even SSD only setups. ZFS can replace expensive
hardware raid cards with moderate CPU and memory load, combined with easy
management.

General advantages of ZFS:

* 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

Hardware
~~~~~~~~~

ZFS depends heavily on memory, so it's recommended to have 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 (for example, Intel SSD DC S3700 Series). This can
increase the overall performance significantly.

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


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 extensive
manual pages, which can be read with:

.. code-block:: console

  # 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.

.. code-block:: console

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

Create a new pool with RAID-0
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Minimum 1 disk

.. code-block:: console

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

Create a new pool with RAID-1
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Minimum 2 disks

.. code-block:: console

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

Create a new pool with RAID-10
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Minimum 4 disks

.. code-block:: console

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

Create a new pool with RAIDZ-1
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Minimum 3 disks

.. code-block:: console

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

Create a new pool with RAIDZ-2
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Minimum 4 disks

.. code-block:: console

  # 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).

For `<device>`, you can use multiple devices, as is shown in
"Create a new pool with RAID*".

.. code-block:: console

  # 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).

For `<device>`, you can use multiple devices, as is shown in
"Create a new pool with RAID*".

.. code-block:: console

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

Add cache and log to an existing pool
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You can add cache and log devices to a pool after its creation. In this example,
we will use a single drive for both cache and log. First, you need to create
2 partitions on the SSD with `parted` or `gdisk`

.. important:: 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.

.. code-block:: console

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


Changing a failed device
^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: console

  # 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 a bootloader.

In either case, 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.

.. code-block:: console

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

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

With `systemd-boot`:

.. code-block:: console

  # proxmox-boot-tool format <new ESP>
  # proxmox-boot-tool init <new ESP>

.. NOTE:: `ESP` stands for EFI System Partition, which is setup as partition #2 on
  bootable disks setup by the `Proxmox Backup`_ installer. For details, see
  :ref:`Setting up a new partition for use as synced ESP <systembooting-proxmox-boot-setup>`.

With `grub`:

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

.. code-block:: console

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


Activate e-mail notification
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

ZFS comes with an event daemon ``ZED``, 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 ``zfs-zed``,
which should already be installed by default in `Proxmox Backup`_.

You can configure the daemon via the file ``/etc/zfs/zed.d/zed.rc`` with your
favorite editor. The required setting for email notfication is
``ZED_EMAIL_ADDR``, which is set to ``root`` by default.

.. code-block:: console

  ZED_EMAIL_ADDR="root"

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


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 degradation of the
host. Use your preferred editor to change the configuration in
`/etc/modprobe.d/zfs.conf` and insert:

.. code-block:: console

  options zfs zfs_arc_max=8589934592

The above example limits the usage to 8 GiB ('8 * 2^30^').

.. IMPORTANT:: In case your desired `zfs_arc_max` value is lower than or equal
   to `zfs_arc_min` (which defaults to 1/32 of the system memory), `zfs_arc_max`
   will be ignored. Thus, for it to work in this case, you must set
   `zfs_arc_min` to at most `zfs_arc_max - 1`. This would require updating the
   configuration in `/etc/modprobe.d/zfs.conf`, with:

.. code-block:: console

  options zfs zfs_arc_min=8589934591
  options zfs zfs_arc_max=8589934592

This example setting limits the usage to 8 GiB ('8 * 2^30^') on
systems with more than 256 GiB of total memory, where simply setting
`zfs_arc_max` alone would not work.

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

.. code-block:: console

  # update-initramfs -u


Swap on ZFS
^^^^^^^^^^^

Swap-space created on a zvol may cause some issues, such as blocking the
server or generating a high IO load.

We strongly recommend using 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 a 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:

.. code-block:: console

  # 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:

.. code-block:: console

  vm.swappiness = 10

.. table:: Linux kernel `swappiness` parameter values
  :widths:auto

  ====================  ===============================================================
   Value                Strategy
  ====================  ===============================================================
   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`, `zstd` and `gzip-N` (where `N` is an integer from `1-9`
representing the compression ratio, where 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.

.. _local_zfs_special_device:

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
small files on the `special` device, which can further improve the
performance. Use fast SSDs for the `special` device.

.. IMPORTANT:: The redundancy of the `special` device should match the one of the
  pool, since the `special` device is a point of failure for the entire pool.

.. WARNING:: Adding a `special` device to a pool cannot be undone!

To create a pool with `special` device and RAID-1:

.. code-block:: console

  # 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:

.. code-block:: console

  # 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 this property, new file
blocks smaller than `size` will be allocated on the `special` device.

.. IMPORTANT:: 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 files smaller than 4K-blocks pool-wide:

.. code-block:: console

  # zfs set special_small_blocks=4K <pool>

Opt in for small file blocks for a single dataset:

.. code-block:: console

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

Opt out from small file blocks for a single dataset:

.. code-block:: console

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

Troubleshooting
^^^^^^^^^^^^^^^

Corrupt cache file
""""""""""""""""""

`zfs-import-cache.service` imports ZFS pools using the ZFS cache file. If this
file becomes corrupted, the service won't be able to import the pools that it's
unable to read from it.

As a result, in case of a corrupted ZFS cache file, some volumes may not be
mounted during boot and must be mounted manually later.

For each pool, run:

.. code-block:: console

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

then, update the `initramfs` by running:

.. code-block:: console

  # update-initramfs -u -k all

and finally, reboot the node.

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