Imported Upstream version 3.2.0

[debian/amanda] / perl / Amanda / Device.pod

/*
 * Copyright (c) 2009, 2010 Zmanda, Inc.  All Rights Reserved.
 *
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 * under the terms of the GNU General Public License version 2 as published
 * by the Free Software Foundation.
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 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
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 *
 * Contact information: Zmanda Inc., 465 S. Mathilda Ave., Suite 300
 * Sunnyvale, CA 94085, USA, or: http://www.zmanda.com
 */

%perlcode %{

=head1 NAME

Amanda::Device - interact with Amanda data-storage devices

=head1 SYNOPSIS

  use Amanda::Device qw( :constants );

  my $dev = Amanda::Device->new($device_name);
  if ($dev->read_label() == $DEVICE_STATUS_SUCCESS) {
      print "Label on $device_name is '$dev->volume_label'\n";
  }

=head1 DATA MODEL

A volume is a container for data which can be "loaded" into a particular
device. For tape devices, a volume is a tape, but most other devices do not
deal with such physical objects. Each volume has a volume header giving, among
other things, the label of the volume and the timestamp on which it was
written. The header may also indicate that the volume is not an Amanda volume.
Aside from the header, a volume contains a sequence of files, numbered starting
at 1. While writing, devices number files sequentially, but devices that
support partial volume recycling may have "holes" in the sequence of file
numbers where files have been deleted. The C<seek_file> method, below,
describes how the API represents this situation. Each file has a header, too,
which contains lots of information about the file. See L<Amanda::Header> for
the full list. After the header, a file is just a sequence of bytes.

Reads and writes to devices take place in blocks. Unlike a typical
operating-system file, in which any block boundaries are lost after the file is
written, devices must be read back with the block sizes that were used to read.
See C<amanda-devices(7)> for more in block sizes, and the read_block and
write_block sections, below, for more information.

=head1 USING THE DEVICE API

The Device API is object-oriented, so the first task in using the API is to
make a Device object:

    $dev = Amanda::Device->new("tape:/dev/nst0");

This function takes a device name (possibly a device alias) and returns a
device object. This function always returns a Device, although it may be a Null
device with an error condition. Any C<new> call should be followed by a check
of the device's status:

    $dev = Amanda::Device->new($device_name);
    if ($dev->status() != $Amanda::Device::DEVICE_STATUS_SUCCESS) {
      die "Could not open '$device_name': " . $dev->error();
    }

This function does not access the underlying hardware or any other external
systems in any way: that doesn't happen until C<read_label> or C<start>.  An
Amanda configuration must be loaded when this function is called, as it
searches the configuation for device definitions.  The member variable
C<device_name> is set when this function has returned.

It is unusual for higher-level code to call C<< Amanda::Device->new >>.
Intead, use L<Amanda::Changer> to load a volume and reserve access to it; the
resulting reservation will contain an already-created Device object.

While Amanda proivdes multiple implementations of the Device class, they are
not distinguishable via the usual Perl methods (C<ref> or C<< $dev->isa >>).

Device users generally call device methods in the following order for reading:

    read_label (optional)
    start
    seek_file (optional)
    read_block (repeated)

or, when writing or appending:

    read_label (optional)
    start
    start_file
    write_block (repeated)
    finish_file
    finish

=head2 Alternate Constructor

To create a new RAIT device from a collection of device objects, call
C<< Amanda::Device->new_rait_from_children($child1, $child2, ..) >>.
If one of the child objects is C<undef>, the resulting RAIT device
will operate in degraded mode.

=head2 Error Handling

Device methods return a particular value to signal the presence of an error
condition. In many cases, this is simply false (exceptions are listed below).

When a device signals an error, C<< $dev->status >> and C<< $dev->error >>
contain details of what went wrong. Status is a bitfield where each bit that is
set indicates a possible problem. Unfortunately, some devices are unable to
distinguish states due to limitations of an external system. For example, the
tape device often cannot distinguish an empty drive
(C<$DEVICE_STATUS_VOLUME_MISSING>) from a hard device error
(C<$DEVICE_STATUS_DEVICE_ERROR>), leading to the status
C<$DEVICE_STATUS_VOLUME_MISSING>|C<$DEVICE_STATUS_DEVICE_ERROR>.  To be clear:
as few as one of the status bits may represent a actual problem.  If
C<$DEVICE_STATUS_VOLUME_UNLABELED> is set along with other bits, it is I<not>
safe to assume that an unlabeled volume is available.  However, if the bit is
not set, then it is safe to assume there is no unlabeled volume present.

=over 2

=item C<$DEVICE_STATUS_SUCCESS>

All OK (no bits set)

=item C<$DEVICE_STATUS_DEVICE_ERROR>

The device is in an unresolvable error state, and further retries are unlikely
to change the status

=item C<$DEVICE_STATUS_DEVICE_BUSY>

The device is in use, and should be retried later

=item C<$DEVICE_STATUS_VOLUME_MISSING>

The device itself is OK, but has no media loaded. This may change if media is
loaded by the user or a changer

=item C<$DEVICE_STATUS_VOLUME_UNLABELED>

The device is OK and media is laoded, but there is no Amanda header or an
invalid header on the media.

=item C<$DEVICE_STATUS_VOLUME_ERROR>

The device is OK, but there was an unresolvable error loading the header from
the media, so subsequent reads or writes will probably fail.

=back

At the risk of being repetitive, never test a device's status with C<==>,
unless it is to C<$DEVICE_STATUS_SUCCESS>. Furthermore, never try to parse the
device error messages -- they are only for user consumption, and may differ
from device to device.

In addition to the status bitfield, a Device also provides a
user-comprehensible error message, available from the methods C<error>
(returning the error message), C<status_error> (returning the string form of
the status), or C<status_or_error> (returning the error message if one is set,
otherwise the string form of the status). None of these functions will ever
return C<undef>.

=head2 Properties

Device properties provide a bidirectional means of communication between
devices and their users. A device provides values for some properties, which
other parts of Amanda can use to adjust their behavior to suit the device. For
example, Amanda will only attempt to append to a volume if the device's
properties indicate that it supports this activity. Some devices have
additional properties that can be set to control its activity. For example, the
S3 Device requires that the users' keys be given via properties.

See C<amanda-devices(7)> for more information on device properties and their
meanings.

The methods C<property_get> and C<property_set> are used to get and set
properties, respectively. If the indicated property simply does not exist,
these functions return an error indication (FALSE), but the device's status
remains C<$DEVICE_STATUS_SUCCESS>. If a more serious error occurs, then the
device's status is set appropriately.

Device properties are easy to handle, as the Perl-to-C glue takes care of all
necessary type conversions:

    $success = $device->property_set("BLOCK_SIZE", $blocksize);
    $blocksize = $device->property_get("BLOCK_SIZE");

If C<property_get> is called in an array context, it returns the property
value, its surety, and its source, in that order.  If there is an error
fetching the property, C<property_get> returns C<undef>.

The C<property_list()> method returns a list of all properties:

  my @props = $device->property_list();

its return is an array of hashes:

  ( { 'access' => $access_flags,
      'name' => $property_name,
      'description' => $property_description },
    ...
  )

=head3 Surety and Source

All properties have a source - where the value came from - and surety - a level
of confidence in the value. This can be used to decide which of two potentially
contradictory properties to believe. For example, the RAIT device examines the
source and surety of child devices' block sizes, prefering properties set by
the user (C<$PROPERTY_SOURCE_USER>) over others.

Set a property's source and surety with C<property_set_ex>:
    $dev->property_set_ex("my_prop", 13, $PROPERTY_SURETY_BAD, $PROPERTY_SOURCE_DEFAULT);
The surety and source are returned after the property value in list context:
    my ($val, $sur, $sou) = $dev->property_get("my_prop");

The available sureties are:

  $PROPERTY_SURETY_GOOD
  $PROPERTY_SURETY_BAD

and the sources are:

  $PROPERTY_SOURCE_DEFAULT
  $PROPERTY_SOURCE_DETECTED
  $PROPERTY_SOURCE_USER

=head2 Concurrency

Some devices can perform more than one operation simultaneously, while others
are more limited. For example, a tape device is exclusive to a single process
while it is in use, while a VFS device can support concurrent reads and writes
on the same volume.

As of this writing, device locking is not correctly implemented in many
devices; consult the source code and check with the Amanda developers before
depending on concurrent operation of devices.

=head2 EOM and EOF

When writing to a volume, an EOM (end-of-media) condition occurs when no more
space is available on the volume.  Some devices (currently only those
supporting DirectTCP) distinguish a logical EOM (LEOM) from a physical EOM
(PEOM).  The logical EOM comes some distance before the physical EOM, with
enough space left to finish a data block and write any additional bookkeeping
data before PEOM.

In such devices, the C<is_eom> attribute is set once LEOM is detected.  Such
detection can happen in any method that writes to the volume, including
C<start>, C<start_file>, C<finish_file>, and C<finish>.  API users that
understand LEOM should take this as a signal to complete writing to the device
and move on before hitting PEOM.

Devices which do not support LEOM simply return a VOLUME_ERROR when the volume
is full.  If this occurs during a C<write_block> operation, then the volume may
or may not contain the block - the situation is indeterminate.

Devices indicate their support for LEOM with the LEOM property.

=head2 Device Resources

Some device types have a "locking" mechanism that prevents other parts of the
system from accessing the underlying resource while an operation is in
progress.  For example, a typical UNIX tape driver cannot be opened by two
processes at once.

Amanda Devices will lock the underlying resource when C<start> or C<read_label>
is called, and unlock the resource either when the Device object is
garbage-collected or in the C<finish> method.  Thus in a calling sequence such as

    read_label
    start
    seek_file
    ...
    finish

the underlying resource remains locked for the entire sequence, even between
read_label and finish.

It is unwise to rely on Perl's garbage-collection to automatically release
resources.  Instead, always explicitly release resources with a C<finish> call.
The Changer API is careful to do this in its C<release> method.

=head2 Member Variables

All member variables are implemented using accessor methods, rather than the
more common hashref technique.  Use

  print $dev->device_name, "\n";

instead of

  print $dev->{'device_name'}, "\n";

The member variables are:

=over 4

=item C<file>

the current file number, if any

=item C<block>

the current block number, if any

=item C<in_file>

true if the device is in the middle of reading or writing a file

=item C<device_name>

the name with which the device was constructed; note that this is not set until after open_device is finished -- it is an error to access this variable in an open_device implementation

=item C<access_mode>

the current access mode (C<$ACCESS_NULL>, or that supplied to start)

=item C<is_eof>

true if an EOF occurred while reading; also used by C<write_from_connection>

=item C<is_eom>

true if a write operation reached the end of the volume (end-of-medium)

=item C<volume_label>

the label of the current volume, set by start and read_label

=item C<volume_time>

the timestamp of the current volume, set by start and read_label

=item C<volume_header>

the header of the current volume, set by read_label

=item C<status>

the device's error status (bit flags) as an integer

=item C<status_error>

the device's error status (bit flags) as a string

=item C<error>

the device's error message

=item C<error_or_status>

the device's error message, if set, otherwise the same as C<status_error> --
use this to display error messages from devices

=item C<block_size>

the device's currently configured block size. This is also available via the
BLOCK_SIZE property. Writers should use block_size-byte blocks, and readers
should initially use block_size, and expand buffers as directed by
C<read_block>.

=item C<min_block_size>

minimum allowed block size for this device

=item C<max_block_size>

maximum allowed block size for this device

=back

=head2 Object Methods

=head3 configure($use_global_config)

    $dev->configure(1);

Once you have a new device, you should configure it. This sets properties on
the device based on the user's configuation. If C<$use_global_config> is true,
then any global C<device_property> parameters are processed, along with
tapetype and other relevant parameters. Otherwise, only parameters from the
device definition (if the device was opened via an alias) are processed.

This method is I<deprecated>.  All access to Devices should be via the Changer
API (see L<Amanda::Changer>), which implements its own, more advanced method of
configuring devices.  The C<configure> method may be removed in a future
release.

=head3 read_label

    $status = $dev->read_label();

This function reads the tape header of the current volume, returning the
Device's status (see "Error Handling", above). Since this is often the first
function to accses the underlying hardware, its error status is the one most
often reported to the user. In fact, C<amdevcheck(8)> is little more than a
wrapper around read_label.

The method sets the following member variables:

=over 4

=item C<volume_header>

if any header data was read from the volume, it is represented here. The
header's type may be F_WEIRD if the header was not recognized by Amanda.

=item C<volume_label>

if read_label read the header successfully, then volume_label contains the
label

=item C<volume_time>

smililarly, if read_label read the header successfully, then volume_time
contains the timestamp from the header

=back

=head3 start

    $succss = $dev->start($ACCESS_WRITE, $label, $timestamp);

Start starts the device and prepares it for the use described by its second
parameter. This function can be called regardless of whether C<read_label> has
already been called.

If the access mode is C<$ACCESS_WRITE>, then the label and timestamp must be
supplied (although leaving the timestamp undef will use the current time), and
they will be used to write a new volume header. Otherwise, these parameters
should be undef.

On completion, start leaves the device's C<access_mode>, C<volume_label> and
C<volume_time> member variables set, by reading the tape header if necessary.
Note that in mode C<$ACCESS_APPEND>, the C<file> member variable is not set
until after C<start_file> has been called.

=head3 start_file

 $success = $dev->start_file($header);

This method prepares the device to write data into a file, beginning by writing
the supplied header to the volume. On successful completion, the device's
C<file> is set to the current file number, C<block> is zero, and C<in_file> is
true.  If the volume is out of space, the C<is_eom> member is set to true and
the method returns false with status C<DEVICE_STATUS_VOLUME_ERROR>.

=head3 write_block

 # (not available from Perl)
 success = device_write_block(dev, blocksize, buf);

This method writes a single block of data to the volume.  It is only available
from C -- Perl code should not be handling raw data, as it is far too slow.
Use the transfer architecture (L<Amanda::Xfer>) for that purpose.

The C<blocksize> must be the device's block size, unless
this is a short write.  A short write must be the last block
of a file. Some devices will zero-pad a short write to a full
blocksize. This method returns false on error.  If the volume is
out of space, C<is_eom> is set and the method returns false with
status C<DEVICE_STATUS_VOLUME_ERROR>.  Note that not all devices can
differentiate an EOM condition from other errors; these devices will
set C<is_eom> whenever the situation is ambiguous.

This function ensures that C<block> is correct on exit. Even in an
error condition, it does not finish the current file for the caller.

=head3 finish_file

 $success = $dev->finish_file();

Once an entire file has been written, finish_file performs any
cleanup required on the volume, such as writing filemarks. On exit,
C<in_file> is false.  If the device runs out of space while finishing
(e.g., the filemark does not fit), then this method returns false
with status C<DEVICE_STATUS_VOLUME_ERROR> and C<is_eom> is set.

This function should not be used while reading -- instead, just seek
to the next file.

=head3 seek_file

 $header = $dev->seek_file($fileno);

In C<$ACCESS_READ>, C<seek_file> sets up the device to read from file
C<$fileno>. This function is not available in C<$ACCESS_WRITE> and
C<$ACCESS_APPEND>. It returns the header from the requested file on success, or
undef on error.

If the requested file doesn't exist, as might happen when a volume has had
files recycled, then C<seek_file> will seek to the next file that does exist. The
file this function selected is indicated by the C<file> member variable on exit.
If the requested file number is exactly one more than the last valid file, this
function returns a C<$F_TAPEEND> header.

As an example, on a volume with only files 1 and 3:

 $dev->seek_file(1) returns header for file 1, $dev->file == 1
 $dev->seek_file(2) returns header for file 3, $dev->file == 3
 $dev->seek_file(3) returns header for file 3, $dev->file == 3
 $dev->seek_file(4) returns a tapend header, $dev->file == 4
 $dev->seek_file(5) returns NULL/undef

On exit, C<is_eof> is false, C<in_file> is true unless no file was found (tapeend or NULL), C<file> is the discovered file, and C<block> is zero.

=head3 seek_block

 $success = $dev->seek_block($block);

After seeking to a file, the caller can optionally seek to a particular block
in the file. This function will set C<block> appropriately. Note that it may
not be possible to detect EOF, so this function may fail to set C<is_eof> even
though a subsequent C<read_block> will return no data.

=head3 read_block

 # (not available from Perl)
 bytes_read = device_read_block(dev, buffer, *blocksize);

This method is the complement of C<write_block>, and reads the next block from
the device, or returns -1 on error. Pass a buffer and its size. If the buffer
is not big enough, no read is performed, the parameter C<blocksize> is set to
the required blocksize, and the method returns 0. As a special case, passing a
C<NULL> buffer and C<*blocksize == 0> is treated as a request for the required block
size. It is not an error to pass a buffer that is too large (and, in fact, this
is precisely the effect of setting the C<read_block_size> configuration
parameter).

On EOF, this method returns -1, but sets C<is_eof> and leaves the device's
status set to C<$DEVICE_STATUS_SUCCESS>. Some devices may be able to detect EOF
while reading the last block, and will set C<is_eof> at that time. Others must
wait for the next read to fail. It is never an error to call C<read_block>
after an EOF, so there is no need to check C<is_eof> except when C<read_block>
returns -1.

=head3 finish

 $success = $dev->finish();

This undoes the effects of start, returning the device to a neutral state
(C<$ACCESS_NULL>).  It will also release any resources acquired by
C<read_label>, even if C<start> was not called.  After C<finish>, it is not an
error to call C<start> again, even with a different mode.

=head3 recycle_file

 $success = $dev->recycle_file(fileno);

On devices that support it, this removes the indicated file from the volume,
presumably freeing its space to be used for other files. File numbers of
existing files will not change, so this operation may leave "holes" in the
sequence of file numbers. See C<seek_file> to see how this is handled.

This method cannot be called while in a file, nor while in C<$ACCESS_READ>
mode.

=head3 erase

 $success = $dev->erase(fileno);

On devices that support it, this erases all data from the volume, presumably
freeing the space.  This method must be called before start and after finish --
that is, while the device is in a neutral state (C<$ACCESS_NULL>). You can
detect whether or not this operation is supported using the C<full_deletion>
property.

=head3 eject

 $success = $dev->eject();

On devices that support it, this eject the volume.  This method can be called
before start and after finish.

=head3 directtcp_supported

  $supp = $dev->directtcp_supported();

This method returns TRUE if the DirectTCP-related methods (C<listen>,
C<accept>, C<write_from_connection>, and C<read_to_connection>) are implemented
by this device.

=head3 listen

  $addrs = $dev->listen($for_writing);

The C<listen> method starts the device listening for an incoming DirectTCP
connection.  The method returns a set of IP:PORT pairs to which a TCP
connection can be made.  The boolean C<for_writing> is TRUE if
this connection will be used to write to the device.

This method can be called at any time, but between the time C<listen> is called
and when C<accept> returns, no other methods of the device should be called.

The return value might look like:

  $addrs = [ [ "127.0.0.1", 9382 ] ]

In C, the memory for these addresses remains the responsibility of the device,
and will remain unchanged until C<accept> returns.

=head3 accept

  $conn = $dev->accept();

This method accepts a connection to one of the addresses returned by C<listen>,
returning an established DirectTCPConnection object (see below).  It returns
C<undef> on failure.  Note that this method may block indefinitely if no
connection ever occurs.  The C implementation returns an already-referenced
connection object, so the caller should call C<g_object_unref> when the
connection is no longer needed.

=head3 connect

  $conn = $dev->connect($for_writing, $addrs);

This method initiates a connection to one of the addresses in C<$addrs>,
returning an established DirectTCPConnection object (see below).  The
C<$for_writing> parameter is TRUE if the connection will be used to write to
the device.  It returns C<undef> on failure.  Note that this method may block
indefinitely if no connection ever occurs.  The C implementation returns an
already-referenced connection object, so the caller should call
C<g_object_unref> when the connection is no longer needed.

=head3 use_connection

  my $ok = $dev->use_connection($conn);

Call this method to use a DirectTCPConnection object created with another
device.  The method must be called before the device is started (so
C<access_mode> is C<$ACCESS_NULL>), as some devices cannot support switching
connections without rewinding.  Any subsequent C<read_to_connection> or
C<write_from_connection> calls will use this connection.

=head3 write_from_connection

  ($ok, $actual_size) = $dev->write_from_connection($size);

This method reads data from the DirectTCPConnection specified with
C<use_connection> or returned from C<accept> or C<connect> and writes it to the
volume.   It writes at most C<$size> bytes, and returns the number of bytes
written in C<$actual_size>.  If C<$size> is zero, it will write until EOF, EOM,
or a device error.  On error, C<$ok> is false.

When an EOF is received over the connection, signalling the end of the data
stream, then this method returns without error (C<$ok> is true), with
C<$actual_size> indicating the number of bytes written to the device (which may
be zero).  In this case, the C<is_eof> attribute is true on return.

Similarly, when the device encounters logical EOM in this method, it returns
the total bytes transferred in C<$actual_size>, with C<$ok> true, and the
C<is_eom> attribute true.  No data is lost.  If writes continue until physical
EOM, data may be lost.

=head3 read_to_connection

  ($ok, $actual_size) = $dev->read_to_connection($size);

This method is similar to C<write_from_connection> but the data flows in the
opposite direction.  It reads at most C<$size> bytes, and returns the total
number of bytes read in C<$actual_size>.

When the method encounters an EOF, it stops early and returns successfully with
the number of bytes actually read (which may be zero).

=head3 property_get

Get a property value, where the property is specified by name.  See "Properties", above.

=head3 property_set

Set a simple property value.  See "Properties", above.

=head3 property_set_ex

Set a property value with surety and source.  See "Properties", above.

=head2 CONSTANTS

This module defines a large number of constant scalars.  These constants are
available from the package namespace (e.g., C<$Amanda::Device::ACCESS_WRITE>),
or imported with the C<:constant> import tag.

=head2 DirectTCPConnection objects

The C<accept> and C<connect> methods return an object to represent the ongoing
DirectTCP connection.  This object is mostly useful as a "token" to be passed
to C<write_from_connection> and C<read_to_connection>.  In particular, a
connection created by one device can be used with another device; this is how
DirectTCP dumps are spanned over multiple volumes.

The class does have one critical method, though:

  $conn->close();

This method closes the connection, releasing all resources allocated to it.  It
can be called at any time, whether the remote side has closed the connection
already or not.

=cut

%}