Difference between revisions of "ZFS on Linux"
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Revision as of 18:40, 14 October 2016
ZFS is a combined file system and logical volume manager designed by Sun Microsystems. Starting with Proxmox VE 3.4, the native Linux kernel port of the ZFS file system is introduced as optional file system and also as an additional selection for the root file system. There is no need for manually compile ZFS modules - all packages are included.
By using ZFS, its 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.
Easy configuration and management with Proxmox VE GUI and CLI.
Protection against data corruption
Data compression on file system level
Various raid levels: RAID0, RAID1, RAID10, RAIDZ-1, RAIDZ-2 and RAIDZ-3
Can use SSD for cache
Continuous integrity checking
Designed for high storage capacities
Asynchronous replication over network
ZFS depends heavily on memory, so you need at least 8GB to start. In practice, use as much as 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.
|Do not use ZFS on top of a hardware RAID controller which has its own cache management. ZFS needs to communicate directly with the disks. An HBA adapter or something like an LSI controller flashed in “IT” mode is more appropriate.|
If you are experimenting with an installation of Proxmox VE inside a VM (Nested Virtualization), don’t use virtio for disks of that VM, as they are not supported by ZFS. Use IDE or SCSI instead (also works with the virtio SCSI controller type).
Installation as Root File System
When you install using the Proxmox VE installer, you can choose ZFS for the root file system. You need to select the RAID type at installation time:
Also called “striping”. The capacity of such volume is the sum of the capacities of all disks. But RAID0 does not add any redundancy, so the failure of a single drive makes the volume unusable.
Also called “mirroring”. Data is written identically to all disks. This mode requires at least 2 disks with the same size. The resulting capacity is that of a single disk.
A combination of RAID0 and RAID1. Requires at least 4 disks.
A variation on RAID-5, single parity. Requires at least 3 disks.
A variation on RAID-5, double parity. Requires at least 4 disks.
A variation on RAID-5, triple parity. Requires at least 5 disks.
The installer automatically partitions the disks, creates a ZFS pool called rpool, and installs the root file system on the ZFS subvolume rpool/ROOT/pve-1.
Another subvolume called rpool/data is created to store VM images. In order to use that with the Proxmox VE tools, the installer creates the following configuration entry in /etc/pve/storage.cfg:
zfspool: local-zfs pool rpool/data sparse content images,rootdir
After installation, you can view your ZFS pool status using the zpool command:
# zpool status pool: rpool state: ONLINE scan: none requested config: NAME STATE READ WRITE CKSUM rpool ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 sda2 ONLINE 0 0 0 sdb2 ONLINE 0 0 0 mirror-1 ONLINE 0 0 0 sdc ONLINE 0 0 0 sdd ONLINE 0 0 0 errors: No known data errors
The zfs command is used configure and manage your ZFS file systems. The following command lists all file systems after installation:
# zfs list NAME USED AVAIL REFER MOUNTPOINT rpool 4.94G 7.68T 96K /rpool rpool/ROOT 702M 7.68T 96K /rpool/ROOT rpool/ROOT/pve-1 702M 7.68T 702M / rpool/data 96K 7.68T 96K /rpool/data rpool/swap 4.25G 7.69T 64K -
ZFS RAID Level Considerations
There are a few factors to take into consideration when choosing the layout of a ZFS pool. The basic building block of a ZFS pool is the virtual device, or vdev. All vdevs in a pool are used equally and the data is striped among them (RAID0). Check the zpool(8) manpage for more details on vdevs.
Each vdev type has different performance behaviors. The two parameters of interest are the IOPS (Input/Output Operations per Second) and the bandwidth with which data can be written or read.
A mirror vdev (RAID1) will approximately behave like a single disk in regards to both parameters when writing data. When reading data if will behave like the number of disks in the mirror.
A common situation is to have 4 disks. When setting it up as 2 mirror vdevs (RAID10) the pool will have the write characteristics as two single disks in regard of IOPS and bandwidth. For read operations it will resemble 4 single disks.
A RAIDZ of any redundancy level will approximately behave like a single disk in regard of IOPS with a lot of bandwidth. How much bandwidth depends on the size of the RAIDZ vdev and the redundancy level.
For running VMs, IOPS is the more important metric in most situations.
Size, Space usage and Redundancy
While a pool made of mirror vdevs will have the best performance characteristics, the usable space will be 50% of the disks available. Less if a mirror vdev consists of more than 2 disks, for example in a 3-way mirror. At least one healthy disk per mirror is needed for the pool to stay functional.
The usable space of a RAIDZ type vdev of N disks is roughly N-P, with P being the RAIDZ-level. The RAIDZ-level indicates how many arbitrary disks can fail without losing data. A special case is a 4 disk pool with RAIDZ2. In this situation it is usually better to use 2 mirror vdevs for the better performance as the usable space will be the same.
Another important factor when using any RAIDZ level is how ZVOL datasets, which are used for VM disks, behave. For each data block the pool needs parity data which is at least the size of the minimum block size defined by the ashift value of the pool. With an ashift of 12 the block size of the pool is 4k. The default block size for a ZVOL is 8k. Therefore, in a RAIDZ2 each 8k block written will cause two additional 4k parity blocks to be written, 8k + 4k + 4k = 16k. This is of course a simplified approach and the real situation will be slightly different with metadata, compression and such not being accounted for in this example.
This behavior can be observed when checking the following properties of the ZVOL:
refreservation (if the pool is not thin provisioned)
used (if the pool is thin provisioned and without snapshots present)
# zfs get volsize,refreservation,used <pool>/vm-<vmid>-disk-X
volsize is the size of the disk as it is presented to the VM, while refreservation shows the reserved space on the pool which includes the expected space needed for the parity data. If the pool is thin provisioned, the refreservation will be set to 0. Another way to observe the behavior is to compare the used disk space within the VM and the used property. Be aware that snapshots will skew the value.
There are a few options to counter the increased use of space:
Increase the volblocksize to improve the data to parity ratio
Use mirror vdevs instead of RAIDZ
Use ashift=9 (block size of 512 bytes)
The volblocksize property can only be set when creating a ZVOL. The default value can be changed in the storage configuration. When doing this, the guest needs to be tuned accordingly and depending on the use case, the problem of write amplification if just moved from the ZFS layer up to the guest.
Using ashift=9 when creating the pool can lead to bad performance, depending on the disks underneath, and cannot be changed later on.
Mirror vdevs (RAID1, RAID10) have favorable behavior for VM workloads. Use them, unless your environment has specific needs and characteristics where RAIDZ performance characteristics are acceptable.
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>
To activate compression (see section Compression in ZFS):
# zfs set compression=lz4 <pool>
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 VE was installed it is either using proxmox-boot-tool
[Systems installed with Proxmox VE 6.4 or later, EFI systems installed with Proxmox VE 5.4 or later]
or plain grub as bootloader (see Host Bootloader). You can check by running:
# proxmox-boot-tool status
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.|
# proxmox-boot-tool format <new disk's ESP> # proxmox-boot-tool init <new disk's ESP>
|ESP stands for EFI System Partition, which is setup as partition #2 on bootable disks setup by the Proxmox VE installer since version 5.4. For details, see Setting up a new partition for use as synced ESP.|
# grub-install <new disk>
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 VE forwards mails to root to the email address configured for the root user.
|The only setting that is required is ZED_EMAIL_ADDR. All other settings are optional.|
Limit ZFS Memory Usage
ZFS uses 50 % of the host memory for the Adaptive Replacement Cache (ARC) by default. Allocating enough memory for the ARC is crucial for IO performance, so reduce it with caution. As a general rule of thumb, allocate at least 2 GiB Base + 1 GiB/TiB-Storage. For example, if you have a pool with 8 TiB of available storage space then you should use 10 GiB of memory for the ARC.
You can change the ARC usage limit for the current boot (a reboot resets this change again) by writing to the zfs_arc_max module parameter directly:
echo "$[10 * 1024*1024*1024]" >/sys/module/zfs/parameters/zfs_arc_max
To permanently change the ARC limits, add the following line to /etc/modprobe.d/zfs.conf:
options zfs zfs_arc_max=8589934592
This example setting limits the usage to 8 GiB (8 * 230).
|In case your desired zfs_arc_max value is lower than or equal to zfs_arc_min (which defaults to 1/32 of the system memory), zfs_arc_max will be ignored unless you also set zfs_arc_min to at most zfs_arc_max - 1.|
echo "$[8 * 1024*1024*1024 - 1]" >/sys/module/zfs/parameters/zfs_arc_min echo "$[8 * 1024*1024*1024]" >/sys/module/zfs/parameters/zfs_arc_max
This example setting (temporarily) limits the usage to 8 GiB (8 * 230) on systems with more than 256 GiB of total memory, where simply setting zfs_arc_max alone would not work.
If your root file system is ZFS, you must update your initramfs every time this value changes:
# update-initramfs -u
You must reboot to activate these changes.
SWAP on ZFS
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 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:
# 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
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
This value is 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.
Encrypted ZFS Datasets
ZFS on Linux version 0.8.0 introduced support for native encryption of datasets. After an upgrade from previous ZFS on Linux versions, the encryption feature can be enabled per pool:
# zpool get feature@encryption tank NAME PROPERTY VALUE SOURCE tank feature@encryption disabled local # zpool set feature@encryption=enabled # zpool get feature@encryption tank NAME PROPERTY VALUE SOURCE tank feature@encryption enabled local
|There is currently no support for booting from pools with encrypted datasets using Grub, and only limited support for automatically unlocking encrypted datasets on boot. Older versions of ZFS without encryption support will not be able to decrypt stored data.|
|It is recommended to either unlock storage datasets manually after booting, or to write a custom unit to pass the key material needed for unlocking on boot to zfs load-key.|
|Establish and test a backup procedure before enabling encryption of production data. If the associated key material/passphrase/keyfile has been lost, accessing the encrypted data is no longer possible.|
Encryption needs to be setup when creating datasets/zvols, and is inherited by default to child datasets. For example, to create an encrypted dataset tank/encrypted_data and configure it as storage in Proxmox VE, run the following commands:
# zfs create -o encryption=on -o keyformat=passphrase tank/encrypted_data Enter passphrase: Re-enter passphrase: # pvesm add zfspool encrypted_zfs -pool tank/encrypted_data
All guest volumes/disks create on this storage will be encrypted with the shared key material of the parent dataset.
To actually use the storage, the associated key material needs to be loaded and the dataset needs to be mounted. This can be done in one step with:
# zfs mount -l tank/encrypted_data Enter passphrase for 'tank/encrypted_data':
It is also possible to use a (random) keyfile instead of prompting for a passphrase by setting the keylocation and keyformat properties, either at creation time or with zfs change-key on existing datasets:
# dd if=/dev/urandom of=/path/to/keyfile bs=32 count=1 # zfs change-key -o keyformat=raw -o keylocation=file:///path/to/keyfile tank/encrypted_data
|When using a keyfile, special care needs to be taken to secure the keyfile against unauthorized access or accidental loss. Without the keyfile, it is not possible to access the plaintext data!|
A guest volume created underneath an encrypted dataset will have its encryptionroot property set accordingly. The key material only needs to be loaded once per encryptionroot to be available to all encrypted datasets underneath it.
See the encryptionroot, encryption, keylocation, keyformat and keystatus properties, the zfs load-key, zfs unload-key and zfs change-key commands and the Encryption section from man zfs for more details and advanced usage.
Compression in ZFS
When compression is enabled on a dataset, ZFS tries to compress all new blocks before writing them and decompresses them on reading. Already existing data will not be compressed retroactively.
You can enable compression with:
# zfs set compression=<algorithm> <dataset>
We recommend using the lz4 algorithm, because it adds very little CPU overhead. Other algorithms like lzjb and gzip-N, where N is an integer from 1 (fastest) to 9 (best compression ratio), 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:
# zfs set compression=off <dataset>
Again, 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!|
# zpool create -f -o ashift=12 <pool> mirror <device1> <device2> special mirror <device3> <device4>
# 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).
# zfs set special_small_blocks=4K <pool>
# zfs set special_small_blocks=4K <pool>/<filesystem>
# zfs set special_small_blocks=0 <pool>/<filesystem>
ZFS Pool Features
Changes to the on-disk format in ZFS are only made between major version changes and are specified through features. All features, as well as the general mechanism are well documented in the zpool-features(5) manpage.
Since enabling new features can render a pool not importable by an older version of ZFS, this needs to be done actively by the administrator, by running zpool upgrade on the pool (see the zpool-upgrade(8) manpage).
Unless you need to use one of the new features, there is no upside to enabling them.
In fact, there are some downsides to enabling new features:
A system with root on ZFS, that still boots using grub will become unbootable if a new feature is active on the rpool, due to the incompatible implementation of ZFS in grub.
The system will not be able to import any upgraded pool when booted with an older kernel, which still ships with the old ZFS modules.
Booting an older Proxmox VE ISO to repair a non-booting system will likewise not work.
|Do not upgrade your rpool if your system is still booted with grub, as this will render your system unbootable. This includes systems installed before Proxmox VE 5.4, and systems booting with legacy BIOS boot (see how to determine the bootloader).|
# zpool upgrade <pool>