ZFS-PROGRAM(8) Maintenance Commands and Procedures ZFS-PROGRAM(8)


zfs program - executes ZFS channel programs


zfs program [-jn] [-t instruction-limit] [-m memory-limit] pool script


The ZFS channel program interface allows ZFS administrative operations to
be run programmatically as a Lua script. The entire script is executed
atomically, with no other administrative operations taking effect
concurrently. A library of ZFS calls is made available to channel program
scripts. Channel programs may only be run with root privileges.

A modified version of the Lua 5.2 interpreter is used to run channel
program scripts. The Lua 5.2 manual can be found at:


The channel program given by script will be run on pool, and any attempts
to access or modify other pools will cause an error.


-j Display channel program output in JSON format. When this flag is
specified and standard output is empty - channel program encountered an
error. The details of such an error will be printed to standard error
in plain text.

-n Executes a read-only channel program, which runs faster. The program
cannot change on-disk state by calling functions from the zfs.sync
submodule. The program can be used to gather information such as
properties and determining if changes would succeed (zfs.check.*).
Without this flag, all pending changes must be synced to disk before a
channel program can complete.

-t instruction-limit
Execution time limit, in number of Lua instructions to execute. If a
channel program executes more than the specified number of
instructions, it will be stopped and an error will be returned. The
default limit is 10 million instructions, and it can be set to a
maximum of 100 million instructions.

-m memory-limit
Memory limit, in bytes. If a channel program attempts to allocate more
memory than the given limit, it will be stopped and an error returned.
The default memory limit is 10 MB, and can be set to a maximum of 100

All remaining argument strings will be passed directly to the Lua script as
described in the LUA INTERFACE section below.


A channel program can be invoked either from the command line, or via a
library call to lzc_channel_program().


Arguments passed to the channel program are converted to a Lua table. If
invoked from the command line, extra arguments to the Lua script will be
accessible as an array stored in the argument table with the key 'argv':

args = ...
argv = args["argv"]
-- argv == {1="arg1", 2="arg2", ...}

If invoked from the libZFS interface, an arbitrary argument list can be
passed to the channel program, which is accessible via the same "..."
syntax in Lua:

args = ...
-- args == {"foo"="bar", "baz"={...}, ...}

Note that because Lua arrays are 1-indexed, arrays passed to Lua from the
libZFS interface will have their indices incremented by 1. That is, the
element in arr[0] in a C array passed to a channel program will be stored
in arr[1] when accessed from Lua.

Return Values

Lua return statements take the form:

return ret0, ret1, ret2, ...

Return statements returning multiple values are permitted internally in a
channel program script, but attempting to return more than one value from
the top level of the channel program is not permitted and will throw an
error. However, tables containing multiple values can still be returned.
If invoked from the command line, a return statement:

a = {foo="bar", baz=2}
return a

Will be output formatted as:

Channel program fully executed with return value:
baz: 2
foo: 'bar'

Fatal Errors

If the channel program encounters a fatal error while running, a non-zero
exit status will be returned. If more information about the error is
available, a singleton list will be returned detailing the error:

error: "error string, including Lua stack trace"

If a fatal error is returned, the channel program may have not executed at
all, may have partially executed, or may have fully executed but failed to
pass a return value back to userland.

If the channel program exhausts an instruction or memory limit, a fatal
error will be generated and the program will be stopped, leaving the
program partially executed. No attempt is made to reverse or undo any
operations already performed. Note that because both the instruction count
and amount of memory used by a channel program are deterministic when run
against the same inputs and filesystem state, as long as a channel program
has run successfully once, you can guarantee that it will finish
successfully against a similar size system.

If a channel program attempts to return too large a value, the program will
fully execute but exit with a nonzero status code and no return value.

Note: ZFS API functions do not generate Fatal Errors when correctly
invoked, they return an error code and the channel program continues
executing. See the ZFS API section below for function-specific details on
error return codes.

Lua to C Value Conversion

When invoking a channel program via the libZFS interface, it is necessary
to translate arguments and return values from Lua values to their C
equivalents, and vice-versa.

There is a correspondence between nvlist values in C and Lua tables. A Lua
table which is returned from the channel program will be recursively
converted to an nvlist, with table values converted to their natural

string -> string
number -> int64
boolean -> boolean_value
nil -> boolean (no value)
table -> nvlist

Likewise, table keys are replaced by string equivalents as follows:

string -> no change
number -> signed decimal string ("%lld")
boolean -> "true" | "false"

Any collision of table key strings (for example, the string "true" and a
true boolean value) will cause a fatal error.

Lua numbers are represented internally as signed 64-bit integers.


The following Lua built-in base library functions are available:

assert rawlen
collectgarbage rawget
error rawset
getmetatable select
ipairs setmetatable
next tonumber
pairs tostring
rawequal type

All functions in the coroutine, string, and table built-in submodules are
also available. A complete list and documentation of these modules is
available in the Lua manual.

The following functions base library functions have been disabled and are
not available for use in channel programs:



Function Arguments

Each API function takes a fixed set of required positional arguments and
optional keyword arguments. For example, the destroy function takes a
single positional string argument (the name of the dataset to destroy) and
an optional "defer" keyword boolean argument. When using parentheses to
specify the arguments to a Lua function, only positional arguments can be


To use keyword arguments, functions must be called with a single argument
that is a Lua table containing entries mapping integers to positional
arguments and strings to keyword arguments:

zfs.sync.destroy({1="rpool@snap", defer=true})

The Lua language allows curly braces to be used in place of parenthesis as
syntactic sugar for this calling convention:

zfs.sync.snapshot{"rpool@snap", defer=true}

Function Return Values

If an API function succeeds, it returns 0. If it fails, it returns an
error code and the channel program continues executing. API functions do
not generate Fatal Errors except in the case of an unrecoverable internal
file system error.

In addition to returning an error code, some functions also return extra
details describing what caused the error. This extra description is given
as a second return value, and will always be a Lua table, or Nil if no
error details were returned. Different keys will exist in the error
details table depending on the function and error case. Any such function
may be called expecting a single return value:

errno = zfs.sync.promote(dataset)

Or, the error details can be retrieved:

errno, details = zfs.sync.promote(dataset)
if (errno == EEXIST) then
assert(details ~= Nil)
list_of_conflicting_snapshots = details

The following global aliases for API function error return codes are
defined for use in channel programs:


API Functions

For detailed descriptions of the exact behavior of any zfs administrative
operations, see the main zfs(8) manual page.

Record a debug message in the zfs_dbgmsg log. A log of these messages
can be printed via mdb's "::zfs_dbgmsg" command, or can be monitored
live by running:

dtrace -n 'zfs-dbgmsg{trace(stringof(arg0))}'

msg (string)
Debug message to be printed.

Returns true if the given dataset exists, or false if it doesn't. A
fatal error will be thrown if the dataset is not in the target pool.
That is, in a channel program running on rpool,
zfs.exists("rpool/nonexistent_fs") returns false, but
zfs.exists("somepool/fs_that_may_exist") will error.

dataset (string)
Dataset to check for existence. Must be in the target pool.

zfs.get_prop(dataset, property)
Returns two values. First, a string, number or table containing the
property value for the given dataset. Second, a string containing the
source of the property (i.e. the name of the dataset in which it was
set or nil if it is readonly). Throws a Lua error if the dataset is
invalid or the property doesn't exist. Note that Lua only supports
int64 number types whereas ZFS number properties are uint64. This
means very large values (like guid) may wrap around and appear

dataset (string)
Filesystem or snapshot path to retrieve properties from.

property (string)
Name of property to retrieve. All filesystem, snapshot and volume
properties are supported except for 'mounted' and 'iscsioptions.'
Also supports the 'written@snap' and 'written#bookmark' properties
and the '<user|group><quota|used>@id' properties, though the id
must be in numeric form.

zfs.sync submodule
The sync submodule contains functions that modify the on-disk state.
They are executed in "syncing context".

The available sync submodule functions are as follows:

zfs.sync.change_key(dataset, key)
Change the dataset encryption key. key must be in the format (raw
or hex) specified by the dataset keyformat property.

zfs.sync.destroy(dataset, [defer=true|false])
Destroy the given dataset. Returns 0 on successful destroy, or a
nonzero error code if the dataset could not be destroyed (for
example, if the dataset has any active children or clones).

dataset (string)
Filesystem or snapshot to be destroyed.

[optional] defer (boolean)
Valid only for destroying snapshots. If set to true, and the
snapshot has holds or clones, allows the snapshot to be marked
for deferred deletion rather than failing.

zfs.sync.inherit(dataset, property)
Clears the specified property in the given dataset, causing it to
be inherited from an ancestor, or restored to the default if no
ancestor property is set. The `zfs inherit -S' option has not been
implemented. Returns 0 on success, or a nonzero error code if the
property could not be cleared.

dataset (string)
Filesystem or snapshot containing the property to clear.

property (string)
The property to clear. Allowed properties are the same as
those for the zfs inherit command.

Promote the given clone to a filesystem. Returns 0 on successful
promotion, or a nonzero error code otherwise. If EEXIST is
returned, the second return value will be an array of the clone's
snapshots whose names collide with snapshots of the parent

dataset (string)
Clone to be promoted.

Rollback to the previous snapshot for a dataset. Returns 0 on
successful rollback, or a nonzero error code otherwise. Rollbacks
can be performed on filesystems or zvols, but not on snapshots or
mounted datasets. EBUSY is returned in the case where the
filesystem is mounted.

filesystem (string)
Filesystem to rollback.

zfs.sync.set_prop(dataset, property, value)
Sets the given property on a dataset. Currently only user
properties are supported. Returns 0 if the property was set, or a
nonzero error code otherwise.

dataset (string)
The dataset where the property will be set.

property (string)
The property to set. Only user properties are supported.

value (string)
The value of the property to be set.

Create a snapshot of a filesystem. Returns 0 if the snapshot was
successfully created, and a nonzero error code otherwise.

Note: Taking a snapshot will fail on any pool older than legacy
version 27. To enable taking snapshots from ZCP scripts, the pool
must be upgraded.

dataset (string)
Name of snapshot to create.

zfs.check submodule
For each function in the zfs.sync submodule, there is a corresponding
zfs.check function which performs a "dry run" of the same operation.
Each takes the same arguments as its zfs.sync counterpart and returns 0
if the operation would succeed, or a non-zero error code if it would
fail, along with any other error details. That is, each has the same
behavior as the corresponding sync function except for actually
executing the requested change. For example, zfs.check.destroy("fs")
returns 0 if zfs.sync.destroy("fs") would successfully destroy the

The available zfs.check functions are:

zfs.check.change_key(dataset, key)

zfs.check.destroy(dataset, [defer=true|false])



zfs.check.set_property(dataset, property, value)


zfs.list submodule
The zfs.list submodule provides functions for iterating over datasets
and properties. Rather than returning tables, these functions act as
Lua iterators, and are generally used as follows:

for child in zfs.list.children("rpool") do

The available zfs.list functions are:

Iterate through all clones of the given snapshot.

snapshot (string)
Must be a valid snapshot path in the current pool.

Iterate through all snapshots of the given dataset. Each snapshot
is returned as a string containing the full dataset name, e.g.

dataset (string)
Must be a valid filesystem or volume.

Iterate through all direct children of the given dataset. Each
child is returned as a string containing the full dataset name,
e.g. "pool/fs/child".

dataset (string)
Must be a valid filesystem or volume.

Iterate through all user properties for the given dataset.

dataset (string)
Must be a valid filesystem, snapshot, or volume.

Returns an array of strings, the names of the valid system (non-
user defined) properties for the given dataset. Throws a Lua error
if the dataset is invalid.

dataset (string)
Must be a valid filesystem, snapshot or volume.


Example 1
The following channel program recursively destroys a filesystem and all its
snapshots and children in a naive manner. Note that this does not involve
any error handling or reporting.

function destroy_recursive(root)
for child in zfs.list.children(root) do
for snap in zfs.list.snapshots(root) do

Example 2
A more verbose and robust version of the same channel program, which
properly detects and reports errors, and also takes the dataset to destroy
as a command line argument, would be as follows:

succeeded = {}
failed = {}

function destroy_recursive(root)
for child in zfs.list.children(root) do
for snap in zfs.list.snapshots(root) do
err = zfs.sync.destroy(snap)
if (err ~= 0) then
failed[snap] = err
succeeded[snap] = err
err = zfs.sync.destroy(root)
if (err ~= 0) then
failed[root] = err
succeeded[root] = err

args = ...
argv = args["argv"]


results = {}
results["succeeded"] = succeeded
results["failed"] = failed
return results

Example 3
The following function performs a forced promote operation by attempting to
promote the given clone and destroying any conflicting snapshots.

function force_promote(ds)
errno, details = zfs.check.promote(ds)
if (errno == EEXIST) then
assert(details ~= Nil)
for i, snap in ipairs(details) do
zfs.sync.destroy(ds .. "@" .. snap)
elseif (errno ~= 0) then
return errno
return zfs.sync.promote(ds)

illumos November 8, 2021 illumos