Provided by: zfsutils-linux_2.1.5-1ubuntu6~22.04.4_amd64 
NAME
zfs-program — execute ZFS channel programs
SYNOPSIS
zfs program [-jn] [-t instruction-limit] [-m memory-limit] pool script [script arguments]
DESCRIPTION
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 http://www.lua.org/manual/5.2/
The channel program given by script will be run on pool, and any attempts to access or
modify other pools will cause an error.
OPTIONS
-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
Limit the 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 MB.
All remaining argument strings will be passed directly to the Lua script as described in the
LUA INTERFACE section below.
LUA INTERFACE
A channel program can be invoked either from the command line, or via a library call to
lzc_channel_program().
Arguments
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:
return:
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 equivalents:
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.
LUA STANDARD LIBRARY
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:
dofile loadfile load pcall print xpcall
ZFS API
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 used:
zfs.sync.destroy("rpool@snap")
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
end
The following global aliases for API function error return codes are defined for use in
channel programs:
EPERM ECHILD ENODEV ENOSPC ENOENT EAGAIN ENOTDIR
ESPIPE ESRCH ENOMEM EISDIR EROFS EINTR EACCES
EINVAL EMLINK EIO EFAULT ENFILE EPIPE ENXIO
ENOTBLK EMFILE EDOM E2BIG EBUSY ENOTTY ERANGE
ENOEXEC EEXIST ETXTBSY EDQUOT EBADF EXDEV EFBIG
API Functions
For detailed descriptions of the exact behavior of any ZFS administrative operations, see
the main zfs(8) manual page.
zfs.debug(msg)
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.
zfs.exists(dataset)
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 GUIDs) may wrap around and appear negative.
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.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.
[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.
zfs.sync.promote(dataset)
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 filesystem.
dataset (string) Clone to be promoted.
zfs.sync.rollback(filesystem)
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.
value (string) The value of the property to be set.
zfs.sync.snapshot(dataset)
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.sync.bookmark(source, newbookmark)
Create a bookmark of an existing source snapshot or bookmark. Returns 0 if the new
bookmark was successfully created, and a nonzero error code otherwise.
Note: Bookmarking requires the corresponding pool feature to be enabled.
source (string) Full name of the existing snapshot or bookmark.
newbookmark (string) Full name of the new bookmark.
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 dataset.
The available zfs.check functions are:
zfs.check.destroy(dataset, [defer=true|false])
zfs.check.promote(dataset)
zfs.check.rollback(filesystem)
zfs.check.set_property(dataset, property, value)
zfs.check.snapshot(dataset)
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
...
end
The available zfs.list functions are:
zfs.list.clones(snapshot)
Iterate through all clones of the given snapshot.
snapshot (string) Must be a valid snapshot path in the current pool.
zfs.list.snapshots(dataset)
Iterate through all snapshots of the given dataset. Each snapshot is returned as a
string containing the full dataset name, e.g. "pool/fs@snap".
dataset (string) Must be a valid filesystem or volume.
zfs.list.children(dataset)
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.
zfs.list.bookmarks(dataset)
Iterate through all bookmarks of the given dataset. Each bookmark is returned as a
string containing the full dataset name, e.g. "pool/fs#bookmark".
dataset (string) Must be a valid filesystem or volume.
zfs.list.holds(snapshot)
Iterate through all user holds on the given snapshot. Each hold is returned as a
pair of the hold's tag and the timestamp (in seconds since the epoch) at which it
was created.
snapshot (string) Must be a valid snapshot.
zfs.list.properties(dataset)
An alias for zfs.list.user_properties (see relevant entry).
dataset (string) Must be a valid filesystem, snapshot, or volume.
zfs.list.user_properties(dataset)
Iterate through all user properties for the given dataset. For each step of the
iteration, output the property name, its value, and its source. Throws a Lua error
if the dataset is invalid.
dataset (string) Must be a valid filesystem, snapshot, or volume.
zfs.list.system_properties(dataset)
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.
EXAMPLES
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
destroy_recursive(child)
end
for snap in zfs.list.snapshots(root) do
zfs.sync.destroy(snap)
end
zfs.sync.destroy(root)
end
destroy_recursive("pool/somefs")
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
destroy_recursive(child)
end
for snap in zfs.list.snapshots(root) do
err = zfs.sync.destroy(snap)
if (err ~= 0) then
failed[snap] = err
else
succeeded[snap] = err
end
end
err = zfs.sync.destroy(root)
if (err ~= 0) then
failed[root] = err
else
succeeded[root] = err
end
end
args = ...
argv = args["argv"]
destroy_recursive(argv[1])
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)
end
elseif (errno ~= 0) then
return errno
end
return zfs.sync.promote(ds)
end