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Delegated Administration
A comprehensive permission delegation system allows unprivileged users to perform <acronym>ZFS</acronym> administration functions. For example, if each user's home directory is a dataset, users can be given permission to create and destroy snapshots of their home directories. A backup user can be given permission to use replication features. A usage statistics script can be allowed to run with access only to the space utilization data for all users. It is even possible to delegate the ability to delegate permissions. Permission delegation is possible for each subcommand and most properties.
Delegating Dataset Creation
<command>zfs allow <replaceable>someuser</replaceable> create <replaceable>mydataset</replaceable></command> gives the specified user permission to create child datasets under the selected parent dataset. There is a caveat: creating a new dataset involves mounting it. That requires setting the FreeBSD <literal>vfs.usermount</literal> <citerefentry><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry> to <literal>1</literal> to allow non-root users to mount a file system. There is another restriction aimed at preventing abuse: non-<systemitem class="username">root</systemitem> users must own the mountpoint where the file system is to be mounted.
Delegating Permission Delegation
<command>zfs allow <replaceable>someuser</replaceable> allow <replaceable>mydataset</replaceable></command> gives the specified user the ability to assign any permission they have on the target dataset, or its children, to other users. If a user has the <literal>snapshot</literal> permission and the <literal>allow</literal> permission, that user can then grant the <literal>snapshot</literal> permission to other users.
Tuning
There are a number of tunables that can be adjusted to make <acronym>ZFS</acronym> perform best for different workloads.
<emphasis><varname>vfs.zfs.arc_max</varname></emphasis> - Maximum size of the <link linkend="zfs-term-arc"><acronym>ARC</acronym></link>. The default is all <acronym>RAM</acronym> but 1 GB, or 5/8 of all <acronym>RAM</acronym>, whichever is more. However, a lower value should be used if the system will be running any other daemons or processes that may require memory. This value can be adjusted at runtime with <citerefentry><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry> and can be set in <filename>/boot/loader.conf</filename> or <filename>/etc/sysctl.conf</filename>.
<emphasis><varname>vfs.zfs.arc_meta_limit</varname></emphasis> - Limit the portion of the <link linkend="zfs-term-arc"><acronym>ARC</acronym></link> that can be used to store metadata. The default is one fourth of <varname>vfs.zfs.arc_max</varname>. Increasing this value will improve performance if the workload involves operations on a large number of files and directories, or frequent metadata operations, at the cost of less file data fitting in the <link linkend="zfs-term-arc"><acronym>ARC</acronym></link>. This value can be adjusted at runtime with <citerefentry><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry> and can be set in <filename>/boot/loader.conf</filename> or <filename>/etc/sysctl.conf</filename>.
<emphasis><varname>vfs.zfs.arc_min</varname></emphasis> - Minimum size of the <link linkend="zfs-term-arc"><acronym>ARC</acronym></link>. The default is one half of <varname>vfs.zfs.arc_meta_limit</varname>. Adjust this value to prevent other applications from pressuring out the entire <link linkend="zfs-term-arc"><acronym>ARC</acronym></link>. This value can be adjusted at runtime with <citerefentry><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry> and can be set in <filename>/boot/loader.conf</filename> or <filename>/etc/sysctl.conf</filename>.
<emphasis><varname>vfs.zfs.vdev.cache.size</varname></emphasis> - A preallocated amount of memory reserved as a cache for each device in the pool. The total amount of memory used will be this value multiplied by the number of devices. This value can only be adjusted at boot time, and is set in <filename>/boot/loader.conf</filename>.
<emphasis><varname>vfs.zfs.min_auto_ashift</varname></emphasis> - Minimum <varname>ashift</varname> (sector size) that will be used automatically at pool creation time. The value is a power of two. The default value of <literal>9</literal> represents <literal>2^9 = 512</literal>, a sector size of 512 bytes. To avoid <emphasis>write amplification</emphasis> and get the best performance, set this value to the largest sector size used by a device in the pool.
Many drives have 4 KB sectors. Using the default <varname>ashift</varname> of <literal>9</literal> with these drives results in write amplification on these devices. Data that could be contained in a single 4 KB write must instead be written in eight 512-byte writes. <acronym>ZFS</acronym> tries to read the native sector size from all devices when creating a pool, but many drives with 4 KB sectors report that their sectors are 512 bytes for compatibility. Setting <varname>vfs.zfs.min_auto_ashift</varname> to <literal>12</literal> (<literal>2^12 = 4096</literal>) before creating a pool forces <acronym>ZFS</acronym> to use 4 KB blocks for best performance on these drives.
Forcing 4 KB blocks is also useful on pools where disk upgrades are planned. Future disks are likely to use 4 KB sectors, and <varname>ashift</varname> values cannot be changed after a pool is created.
In some specific cases, the smaller 512-byte block size might be preferable. When used with 512-byte disks for databases, or as storage for virtual machines, less data is transferred during small random reads. This can provide better performance, especially when using a smaller <acronym>ZFS</acronym> record size.
<emphasis><varname>vfs.zfs.prefetch_disable</varname></emphasis> - Disable prefetch. A value of <literal>0</literal> is enabled and <literal>1</literal> is disabled. The default is <literal>0</literal>, unless the system has less than 4 GB of <acronym>RAM</acronym>. Prefetch works by reading larger blocks than were requested into the <link linkend="zfs-term-arc"><acronym>ARC</acronym></link> in hopes that the data will be needed soon. If the workload has a large number of random reads, disabling prefetch may actually improve performance by reducing unnecessary reads. This value can be adjusted at any time with <citerefentry><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
<emphasis><varname>vfs.zfs.vdev.trim_on_init</varname></emphasis> - Control whether new devices added to the pool have the <literal>TRIM</literal> command run on them. This ensures the best performance and longevity for <acronym>SSD</acronym>s, but takes extra time. If the device has already been secure erased, disabling this setting will make the addition of the new device faster. This value can be adjusted at any time with <citerefentry><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
<emphasis><varname>vfs.zfs.vdev.max_pending</varname></emphasis> - Limit the number of pending I/O requests per device. A higher value will keep the device command queue full and may give higher throughput. A lower value will reduce latency. This value can be adjusted at any time with <citerefentry><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
<emphasis><varname>vfs.zfs.top_maxinflight</varname></emphasis> - Maxmimum number of outstanding I/Os per top-level <link linkend="zfs-term-vdev">vdev</link>. Limits the depth of the command queue to prevent high latency. The limit is per top-level vdev, meaning the limit applies to each <link linkend="zfs-term-vdev-mirror">mirror</link>, <link linkend="zfs-term-vdev-raidz">RAID-Z</link>, or other vdev independently. This value can be adjusted at any time with <citerefentry><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
<emphasis><varname>vfs.zfs.l2arc_write_max</varname></emphasis> - Limit the amount of data written to the <link linkend="zfs-term-l2arc"><acronym>L2ARC</acronym></link> per second. This tunable is designed to extend the longevity of <acronym>SSD</acronym>s by limiting the amount of data written to the device. This value can be adjusted at any time with <citerefentry><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
<emphasis><varname>vfs.zfs.l2arc_write_boost</varname></emphasis> - The value of this tunable is added to <link linkend="zfs-advanced-tuning-l2arc_write_max"><varname>vfs.zfs.l2arc_write_max</varname></link> and increases the write speed to the <acronym>SSD</acronym> until the first block is evicted from the <link linkend="zfs-term-l2arc"><acronym>L2ARC</acronym></link>. This <quote>Turbo Warmup Phase</quote> is designed to reduce the performance loss from an empty <link linkend="zfs-term-l2arc"><acronym>L2ARC</acronym></link> after a reboot. This value can be adjusted at any time with <citerefentry><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
<emphasis><varname>vfs.zfs.scrub_delay</varname></emphasis> - Number of ticks to delay between each I/O during a <link linkend="zfs-term-scrub"><command>scrub</command></link>. To ensure that a <command>scrub</command> does not interfere with the normal operation of the pool, if any other <acronym>I/O</acronym> is happening the <command>scrub</command> will delay between each command. This value controls the limit on the total <acronym>IOPS</acronym> (I/Os Per Second) generated by the <command>scrub</command>. The granularity of the setting is determined by the value of <varname>kern.hz</varname> which defaults to 1000 ticks per second. This setting may be changed, resulting in a different effective <acronym>IOPS</acronym> limit. The default value is <literal>4</literal>, resulting in a limit of: 1000 ticks/sec / 4 = 250 <acronym>IOPS</acronym>. Using a value of <replaceable>20</replaceable> would give a limit of: 1000 ticks/sec / 20 = 50 <acronym>IOPS</acronym>. The speed of <command>scrub</command> is only limited when there has been recent activity on the pool, as determined by <link linkend="zfs-advanced-tuning-scan_idle"><varname>vfs.zfs.scan_idle</varname></link>. This value can be adjusted at any time with <citerefentry><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
<emphasis><varname>vfs.zfs.resilver_delay</varname></emphasis> - Number of milliseconds of delay inserted between each I/O during a <link linkend="zfs-term-resilver">resilver</link>. To ensure that a resilver does not interfere with the normal operation of the pool, if any other I/O is happening the resilver will delay between each command. This value controls the limit of total <acronym>IOPS</acronym> (I/Os Per Second) generated by the resilver. The granularity of the setting is determined by the value of <varname>kern.hz</varname> which defaults to 1000 ticks per second. This setting may be changed, resulting in a different effective <acronym>IOPS</acronym> limit. The default value is 2, resulting in a limit of: 1000 ticks/sec / 2 = 500 <acronym>IOPS</acronym>. Returning the pool to an <link linkend="zfs-term-online">Online</link> state may be more important if another device failing could <link linkend="zfs-term-faulted">Fault</link> the pool, causing data loss. A value of 0 will give the resilver operation the same priority as other operations, speeding the healing process. The speed of resilver is only limited when there has been other recent activity on the pool, as determined by <link linkend="zfs-advanced-tuning-scan_idle"><varname>vfs.zfs.scan_idle</varname></link>. This value can be adjusted at any time with <citerefentry><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
<emphasis><varname>vfs.zfs.scan_idle</varname></emphasis> - Number of milliseconds since the last operation before the pool is considered idle. When the pool is idle the rate limiting for <link linkend="zfs-term-scrub"><command>scrub</command></link> and <link linkend="zfs-term-resilver">resilver</link> are disabled. This value can be adjusted at any time with <citerefentry><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
<emphasis><varname>vfs.zfs.txg.timeout</varname></emphasis> - Maximum number of seconds between <link linkend="zfs-term-txg">transaction group</link>s. The current transaction group will be written to the pool and a fresh transaction group started if this amount of time has elapsed since the previous transaction group. A transaction group my be triggered earlier if enough data is written. The default value is 5 seconds. A larger value may improve read performance by delaying asynchronous writes, but this may cause uneven performance when the transaction group is written. This value can be adjusted at any time with <citerefentry><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
<acronym>ZFS</acronym> on i386
Some of the features provided by <acronym>ZFS</acronym> are memory intensive, and may require tuning for maximum efficiency on systems with limited <acronym>RAM</acronym>.
Memory
As a bare minimum, the total system memory should be at least one gigabyte. The amount of recommended <acronym>RAM</acronym> depends upon the size of the pool and which <acronym>ZFS</acronym> features are used. A general rule of thumb is 1 GB of RAM for every 1 TB of storage. If the deduplication feature is used, a general rule of thumb is 5 GB of RAM per TB of storage to be deduplicated. While some users successfully use <acronym>ZFS</acronym> with less <acronym>RAM</acronym>, systems under heavy load may panic due to memory exhaustion. Further tuning may be required for systems with less than the recommended RAM requirements.
Kernel Configuration

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Source string comment
(itstool) path: listitem/para
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read-only
Source string location
book.translate.xml:42583
String age
a year ago
Source string age
a year ago
Translation file
books/handbook.pot, string 6980