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.Dd February 26, 2019
.Dt ZFS-PROGRAM 8
.Os
.Sh NAME
.Nm zfs program
.Nd executes ZFS channel programs
.Sh SYNOPSIS
.Cm "zfs program"
.Op Fl jn
.Op Fl t Ar instruction-limit
.Op Fl m Ar memory-limit
.Ar pool
.Ar script
.\".Op Ar optional arguments to channel program
.Sh 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.
.Pp
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:
.Bd -centered -offset indent
.Lk http://www.lua.org/manual/5.2/
.Ed
.Pp
The channel program given by
.Ar script
will be run on
.Ar pool ,
and any attempts to access or modify other pools will cause an error.
.Sh OPTIONS
.Bl -tag -width "-t"
.It Fl 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.
.It Fl 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.
.It Fl t Ar 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.
.It Fl m Ar 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.
.El
.Pp
All remaining argument strings will be passed directly to the Lua script as
described in the
.Sx LUA INTERFACE
section below.
.Sh LUA INTERFACE
A channel program can be invoked either from the command line, or via a library
call to
.Fn lzc_channel_program .
.Ss 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':
.Bd -literal -offset indent
args = ...
argv = args["argv"]
-- argv == {1="arg1", 2="arg2", ...}
.Ed
.Pp
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:
.Bd -literal -offset indent
args = ...
-- args == {"foo"="bar", "baz"={...}, ...}
.Ed
.Pp
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
.Va arr[0]
in a C array passed to a channel program will be stored in
.Va arr[1]
when accessed from Lua.
.Ss Return Values
Lua return statements take the form:
.Bd -literal -offset indent
return ret0, ret1, ret2, ...
.Ed
.Pp
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:
.Bd -literal -offset indent
a = {foo="bar", baz=2}
return a
.Ed
.Pp
Will be output formatted as:
.Bd -literal -offset indent
Channel program fully executed with return value:
    return:
        baz: 2
        foo: 'bar'
.Ed
.Ss 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:
.Bd -literal -offset indent
error: "error string, including Lua stack trace"
.Ed
.Pp
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.
.Pp
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.
.Pp
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.
.Pp
.Em 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
.Sx ZFS API
section below for function-specific details on error return codes.
.Ss 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.
.Pp
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:
.Bd -literal -offset indent
string -> string
number -> int64
boolean -> boolean_value
nil -> boolean (no value)
table -> nvlist
.Ed
.Pp
Likewise, table keys are replaced by string equivalents as follows:
.Bd -literal -offset indent
string -> no change
number -> signed decimal string ("%lld")
boolean -> "true" | "false"
.Ed
.Pp
Any collision of table key strings (for example, the string "true" and a
true boolean value) will cause a fatal error.
.Pp
Lua numbers are represented internally as signed 64-bit integers.
.Sh LUA STANDARD LIBRARY
The following Lua built-in base library functions are available:
.Bd -literal -offset indent
assert                  rawlen
collectgarbage          rawget
error                   rawset
getmetatable            select
ipairs                  setmetatable
next                    tonumber
pairs                   tostring
rawequal                type
.Ed
.Pp
All functions in the
.Em coroutine ,
.Em string ,
and
.Em table
built-in submodules are also available.
A complete list and documentation of these modules is available in the Lua
manual.
.Pp
The following functions base library functions have been disabled and are
not available for use in channel programs:
.Bd -literal -offset indent
dofile
loadfile
load
pcall
print
xpcall
.Ed
.Sh ZFS API
.Ss 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:
.Bd -literal -offset indent
zfs.sync.destroy("rpool@snap")
.Ed
.Pp
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:
.Bd -literal -offset indent
zfs.sync.destroy({1="rpool@snap", defer=true})
.Ed
.Pp
The Lua language allows curly braces to be used in place of parenthesis as
syntactic sugar for this calling convention:
.Bd -literal -offset indent
zfs.sync.snapshot{"rpool@snap", defer=true}
.Ed
.Ss 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.
.Pp
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:
.Bd -literal -offset indent
errno = zfs.sync.promote(dataset)
.Ed
.Pp
Or, the error details can be retrieved:
.Bd -literal -offset indent
errno, details = zfs.sync.promote(dataset)
if (errno == EEXIST) then
    assert(details ~= Nil)
    list_of_conflicting_snapshots = details
end
.Ed
.Pp
The following global aliases for API function error return codes are defined
for use in channel programs:
.Bd -literal -offset indent
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
.Ed
.Ss API Functions
For detailed descriptions of the exact behavior of any zfs administrative
operations, see the main
.Xr zfs 1
manual page.
.Bl -tag -width "xx"
.It Em 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:
.Bd -literal -offset indent
  dtrace -n 'zfs-dbgmsg{trace(stringof(arg0))}'
.Ed
.Pp
msg (string)
.Bd -ragged -compact -offset "xxxx"
Debug message to be printed.
.Ed
.It Em 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.
.Pp
dataset (string)
.Bd -ragged -compact -offset "xxxx"
Dataset to check for existence.
Must be in the target pool.
.Ed
.It Em 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 negative.
.Pp
dataset (string)
.Bd -ragged -compact -offset "xxxx"
Filesystem or snapshot path to retrieve properties from.
.Ed
.Pp
property (string)
.Bd -ragged -compact -offset "xxxx"
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.
.Ed
.El
.Bl -tag -width "xx"
.It Sy zfs.sync submodule
The sync submodule contains functions that modify the on-disk state.
They are executed in "syncing context".
.Pp
The available sync submodule functions are as follows:
.Bl -tag -width "xx"
.It Em 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).
.Pp
dataset (string)
.Bd -ragged -compact -offset "xxxx"
Filesystem or snapshot to be destroyed.
.Ed
.Pp
[optional] defer (boolean)
.Bd -ragged -compact -offset "xxxx"
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.
.Ed
.It Em 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.
.Pp
dataset (string)
.Bd -ragged -compact -offset "xxxx"
Clone to be promoted.
.Ed
.It Em 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.
.Pp
filesystem (string)
.Bd -ragged -compact -offset "xxxx"
Filesystem to rollback.
.Ed
.It Em zfs.sync.snapshot(dataset)
Create a snapshot of a filesystem.
Returns 0 if the snapshot was successfully created,
and a nonzero error code otherwise.
.Pp
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.
.Pp
dataset (string)
.Bd -ragged -compact -offset "xxxx"
Name of snapshot to create.
.Ed
.El
.It Sy 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,
.Em zfs.check.destroy("fs")
returns 0 if
.Em zfs.sync.destroy("fs")
would successfully destroy the dataset.
.Pp
The available zfs.check functions are:
.Bl -tag -width "xx"
.It Em zfs.check.destroy(dataset, [defer=true|false])
.It Em zfs.check.promote(dataset)
.It Em zfs.check.rollback(filesystem)
.It Em zfs.check.snapshot(dataset)
.El
.It Sy 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:
.Bd -literal -offset indent
for child in zfs.list.children("rpool") do
    ...
end
.Ed
.Pp
The available zfs.list functions are:
.Bl -tag -width "xx"
.It Em zfs.list.clones(snapshot)
Iterate through all clones of the given snapshot.
.Pp
snapshot (string)
.Bd -ragged -compact -offset "xxxx"
Must be a valid snapshot path in the current pool.
.Ed
.It Em 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".
.Pp
dataset (string)
.Bd -ragged -compact -offset "xxxx"
Must be a valid filesystem or volume.
.Ed
.It Em 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".
.Pp
dataset (string)
.Bd -ragged -compact -offset "xxxx"
Must be a valid filesystem or volume.
.Ed
.It Em zfs.list.properties(dataset)
Iterate through all user properties for the given dataset.
.Pp
dataset (string)
.Bd -ragged -compact -offset "xxxx"
Must be a valid filesystem, snapshot, or volume.
.Ed
.It Em 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.
.Pp
dataset (string)
.Bd -ragged -compact -offset "xxxx"
Must be a valid filesystem, snapshot or volume.
.Ed
.El
.El
.Sh EXAMPLES
.Ss 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.
.Bd -literal -offset indent
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")
.Ed
.Ss 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:
.Bd -literal -offset indent
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
.Ed
.Ss Example 3
The following function performs a forced promote operation by attempting to
promote the given clone and destroying any conflicting snapshots.
.Bd -literal -offset indent
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
.Ed