CedarBackup2.writers.dvdwriter.DvdWriter

Package CedarBackup2 :: Package writers :: Module dvdwriter :: Class DvdWriter |
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# Class DvdWriter

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    object --+
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            DvdWriter

Class representing a device that knows how to write some kinds of DVD media.

# Summary

This is a class representing a device that knows how to write some kinds of DVD media. It provides common operations for the device, such as ejecting the media and writing data to the media.

This class is implemented in terms of the `eject` and `growisofs` utilities, all of which should be available on most UN*X platforms.

# Image Writer Interface

The following methods make up the "image writer" interface shared with other kinds of writers:

      __init__
      initializeImage()
      addImageEntry()
      writeImage()
      setImageNewDisc()
      retrieveCapacity()
      getEstimatedImageSize()

Only these methods will be used by other Cedar Backup functionality that expects a compatible image writer.

The media attribute is also assumed to be available.

Unlike the `CdWriter`, the `DvdWriter` can only operate in terms of filesystem devices, not SCSI devices. So, although the constructor interface accepts a SCSI device parameter for the sake of compatibility, it's not used.

# Media Types

This class knows how to write to DVD+R and DVD+RW media, represented by the following constants:

- `MEDIA_DVDPLUSR`: DVD+R media (4.4 GB capacity)
- `MEDIA_DVDPLUSRW`: DVD+RW media (4.4 GB capacity)

The difference is that DVD+RW media can be rewritten, while DVD+R media cannot be (although at present, `DvdWriter` does not really differentiate between rewritable and non-rewritable media).

The capacities are 4.4 GB because Cedar Backup deals in "true" gigabytes of 1024*1024*1024 bytes per gigabyte.

The underlying `growisofs` utility does support other kinds of media (including DVD-R, DVD-RW and BlueRay) which work somewhat differently than standard DVD+R and DVD+RW media. I don't support these other kinds of media because I haven't had any opportunity to work with them. The same goes for dual-layer media of any type.

# Device Attributes vs. Media Attributes

As with the cdwriter functionality, a given dvdwriter instance has two different kinds of attributes associated with it. I call these device attributes and media attributes.

Device attributes are things which can be determined without looking at the media. Media attributes are attributes which vary depending on the state of the media. In general, device attributes are available via instance variables and are constant over the life of an object, while media attributes can be retrieved through method calls.

Compared to cdwriters, dvdwriters have very few attributes. This is due to differences between the way `growisofs` works relative to `cdrecord`.

# Media Capacity

One major difference between the `cdrecord`/`mkisofs` utilities used by the cdwriter class and the `growisofs` utility used here is that the process of estimating remaining capacity and image size is more straightforward with `cdrecord`/`mkisofs` than with `growisofs`.

In this class, remaining capacity is calculated by asking doing a dry run of `growisofs` and grabbing some information from the output of that command. Image size is estimated by asking the `IsoImage` class for an estimate and then adding on a "fudge factor" determined through experimentation.

# Testing

It's rather difficult to test this code in an automated fashion, even if you have access to a physical DVD writer drive. It's even more difficult to test it if you are running on some build daemon (think of a Debian autobuilder) which can't be expected to have any hardware or any media that you could write to.

Because of this, some of the implementation below is in terms of static methods that are supposed to take defined actions based on their arguments. Public methods are then implemented in terms of a series of calls to simplistic static methods. This way, we can test as much as possible of the "difficult" functionality via testing the static methods, while hoping that if the static methods are called appropriately, things will work properly. It's not perfect, but it's much better than no testing at all.

Instance Methods | [hide private]
__init__(self, device, scsiId=None, driveSpeed=None, mediaType=2, noEject=False, refreshMediaDelay=0, ejectDelay=0, unittest=False) Initializes a DVD writer object. | source code
isRewritable(self) Indicates whether the media is rewritable per configuration. | source code
retrieveCapacity(self, entireDisc=False) Retrieves capacity for the current media in terms of a `MediaCapacity` object. | source code
openTray(self) Opens the device's tray and leaves it open. | source code
closeTray(self) Closes the device's tray. | source code
refreshMedia(self) Opens and then immediately closes the device's tray, to refresh the device's idea of the media. | source code
initializeImage(self, newDisc, tmpdir, mediaLabel=None) Initializes the writer's associated ISO image. | source code
addImageEntry(self, path, graftPoint) Adds a filepath entry to the writer's associated ISO image. | source code
writeImage(self, imagePath=None, newDisc=False, writeMulti=True) Writes an ISO image to the media in the device. | source code
setImageNewDisc(self, newDisc) Resets (overrides) the newDisc flag on the internal image. | source code
getEstimatedImageSize(self) Gets the estimated size of the image associated with the writer. | source code
_writeImage(self, newDisc, imagePath, entries, mediaLabel=None) Writes an image to disc using either an entries list or an ISO image on disk. | source code
_getDevice(self) Property target used to get the device value. | source code
_getScsiId(self) Property target used to get the SCSI id value. | source code
_getHardwareId(self) Property target used to get the hardware id value. | source code
_getDriveSpeed(self) Property target used to get the drive speed. | source code
_getMedia(self) Property target used to get the media description. | source code
_getDeviceHasTray(self) Property target used to get the device-has-tray flag. | source code
_getDeviceCanEject(self) Property target used to get the device-can-eject flag. | source code
_getRefreshMediaDelay(self) Property target used to get the configured refresh media delay, in seconds. | source code
_getEjectDelay(self) Property target used to get the configured eject delay, in seconds. | source code
unlockTray(self) Unlocks the device's tray via 'eject -i off'. | source code
_retrieveSectorsUsed(self) Retrieves the number of sectors used on the current media. | source code
Inherited from `object`: `__delattr__`, `__format__`, `__getattribute__`, `__hash__`, `__new__`, `__reduce__`, `__reduce_ex__`, `__repr__`, `__setattr__`, `__sizeof__`, `__str__`, `__subclasshook__`

Static Methods | [hide private]
_getEstimatedImageSize(entries) Gets the estimated size of a set of image entries. | source code
_searchForOverburn(output) Search for an "overburn" error message in `growisofs` output. | source code
_buildWriteArgs(newDisc, hardwareId, driveSpeed, imagePath, entries, mediaLabel=None, dryRun=False) Builds a list of arguments to be passed to a `growisofs` command. | source code
_parseSectorsUsed(output) Parse sectors used information out of `growisofs` output. | source code

Properties | [hide private]
| device Filesystem device name for this writer.
| scsiId SCSI id for the device (saved for reference only).
| hardwareId Hardware id for this writer (always the device path).
| driveSpeed Speed at which the drive writes.
| media Definition of media that is expected to be in the device.
| deviceHasTray Indicates whether the device has a media tray.
| deviceCanEject Indicates whether the device supports ejecting its media.
| refreshMediaDelay Refresh media delay, in seconds.
| ejectDelay Eject delay, in seconds.
Inherited from `object`: `__class__`

Method Details | [hide private]

### __init__(self, device, scsiId=None, driveSpeed=None, mediaType=2, noEject=False, refreshMediaDelay=0, ejectDelay=0, unittest=False) (Constructor) | source code
Initializes a DVD writer object. Since `growisofs` can only address devices using the device path (i.e. `/dev/dvd`), the hardware id will always be set based on the device. If passed in, it will be saved for reference purposes only. We have no way to query the device to ask whether it has a tray or can be safely opened and closed. So, the `noEject` flag is used to set these values. If `noEject=False`, then we assume a tray exists and open/close is safe. If `noEject=True`, then we assume that there is no tray and open/close is not safe. Parameters: `device` (Absolute path to a filesystem device, i.e. `/dev/dvd`) - Filesystem device associated with this writer. `scsiId` (If provided, SCSI id in the form `[<method>:]scsibus,target,lun`) - SCSI id for the device (optional, for reference only). `driveSpeed` (Use `2` for 2x device, etc. or `None` to use device default.) - Speed at which the drive writes. `mediaType` (One of the valid media type as discussed above.) - Type of the media that is assumed to be in the drive. `noEject` (Boolean true/false) - Tells Cedar Backup that the device cannot safely be ejected `refreshMediaDelay` (Number of seconds, an integer >= 0) - Refresh media delay to use, if any `ejectDelay` (Number of seconds, an integer >= 0) - Eject delay to use, if any `unittest` (Boolean true/false) - Turns off certain validations, for use in unit testing. Raises: `ValueError` - If the device is not valid for some reason. `ValueError` - If the SCSI id is not in a valid form. `ValueError` - If the drive speed is not an integer >= 1. Overrides: object.__init__ Note: The `unittest` parameter should never be set to `True` outside of Cedar Backup code. It is intended for use in unit testing Cedar Backup internals and has no other sensible purpose.

### retrieveCapacity(self, entireDisc=False) | source code
Retrieves capacity for the current media in terms of a `MediaCapacity` object. If `entireDisc` is passed in as `True`, the capacity will be for the entire disc, as if it were to be rewritten from scratch. The same will happen if the disc can't be read for some reason. Otherwise, the capacity will be calculated by subtracting the sectors currently used on the disc, as reported by `growisofs` itself. Parameters: `entireDisc` (Boolean true/false) - Indicates whether to return capacity for entire disc. Returns: `MediaCapacity` object describing the capacity of the media. Raises: `ValueError` - If there is a problem parsing the `growisofs` output `IOError` - If the media could not be read for some reason.

### openTray(self) | source code
Opens the device's tray and leaves it open. This only works if the device has a tray and supports ejecting its media. We have no way to know if the tray is currently open or closed, so we just send the appropriate command and hope for the best. If the device does not have a tray or does not support ejecting its media, then we do nothing. Starting with Debian wheezy on my backup hardware, I started seeing consistent problems with the eject command. I couldn't tell whether these problems were due to the device management system or to the new kernel (3.2.0). Initially, I saw simple eject failures, possibly because I was opening and closing the tray too quickly. I worked around that behavior with the new ejectDelay flag. Later, I sometimes ran into issues after writing an image to a disc: eject would give errors like "unable to eject, last error: Inappropriate ioctl for device". Various sources online (like Ubuntu bug #875543) suggested that the drive was being locked somehow, and that the workaround was to run 'eject -i off' to unlock it. Sure enough, that fixed the problem for me, so now it's a normal error-handling strategy. Raises: `IOError` - If there is an error talking to the device.

### closeTray(self) | source code
Closes the device's tray. This only works if the device has a tray and supports ejecting its media. We have no way to know if the tray is currently open or closed, so we just send the appropriate command and hope for the best. If the device does not have a tray or does not support ejecting its media, then we do nothing. Raises: `IOError` - If there is an error talking to the device.

### refreshMedia(self) | source code
Opens and then immediately closes the device's tray, to refresh the device's idea of the media. Sometimes, a device gets confused about the state of its media. Often, all it takes to solve the problem is to eject the media and then immediately reload it. (There are also configurable eject and refresh media delays which can be applied, for situations where this makes a difference.) This only works if the device has a tray and supports ejecting its media. We have no way to know if the tray is currently open or closed, so we just send the appropriate command and hope for the best. If the device does not have a tray or does not support ejecting its media, then we do nothing. The configured delays still apply, though. Raises: `IOError` - If there is an error talking to the device.

### initializeImage(self, newDisc, tmpdir, mediaLabel=None) | source code
Initializes the writer's associated ISO image. This method initializes the `image` instance variable so that the caller can use the `addImageEntry` method. Once entries have been added, the `writeImage` method can be called with no arguments. Parameters: `newDisc` (Boolean true/false) - Indicates whether the disc should be re-initialized `tmpdir` (String representing a directory path on disk) - Temporary directory to use if needed `mediaLabel` (String, no more than 25 characters long) - Media label to be applied to the image, if any

### addImageEntry(self, path, graftPoint) | source code
Adds a filepath entry to the writer's associated ISO image. The contents of the filepath -- but not the path itself -- will be added to the image at the indicated graft point. If you don't want to use a graft point, just pass `None`. Parameters: `path` (String representing a path on disk) - File or directory to be added to the image `graftPoint` (String representing a graft point path, as described above) - Graft point to be used when adding this entry Raises: `ValueError` - If initializeImage() was not previously called `ValueError` - If the path is not a valid file or directory Note: Before calling this method, you must call initializeImage.

### writeImage(self, imagePath=None, newDisc=False, writeMulti=True) | source code
Writes an ISO image to the media in the device. If `newDisc` is passed in as `True`, we assume that the entire disc will be re-created from scratch. Note that unlike `CdWriter`, `DvdWriter` does not blank rewritable media before reusing it; however, `growisofs` is called such that the media will be re-initialized as needed. If `imagePath` is passed in as `None`, then the existing image configured with `initializeImage()` will be used. Under these circumstances, the passed-in `newDisc` flag will be ignored and the value passed in to `initializeImage()` will apply instead. The `writeMulti` argument is ignored. It exists for compatibility with the Cedar Backup image writer interface. Parameters: `imagePath` (String representing a path on disk) - Path to an ISO image on disk, or `None` to use writer's image `newDisc` (Boolean true/false.) - Indicates whether the disc should be re-initialized `writeMulti` (Boolean true/false) - Unused Raises: `ValueError` - If the image path is not absolute. `ValueError` - If some path cannot be encoded properly. `IOError` - If the media could not be written to for some reason. `ValueError` - If no image is passed in and initializeImage() was not previously called Note: The image size indicated in the log ("Image size will be...") is an estimate. The estimate is conservative and is probably larger than the actual space that `dvdwriter` will use.

### setImageNewDisc(self, newDisc) | source code
Resets (overrides) the newDisc flag on the internal image. Parameters: `newDisc` - New disc flag to set Raises: `ValueError` - If initializeImage() was not previously called

### getEstimatedImageSize(self) | source code
Gets the estimated size of the image associated with the writer. This is an estimate and is conservative. The actual image could be as much as 450 blocks (sectors) smaller under some circmstances. Returns: Estimated size of the image, in bytes. Raises: `IOError` - If there is a problem calling `mkisofs`. `ValueError` - If initializeImage() was not previously called

### _writeImage(self, newDisc, imagePath, entries, mediaLabel=None) | source code
Writes an image to disc using either an entries list or an ISO image on disk. Callers are assumed to have done validation on paths, etc. before calling this method. Parameters: `newDisc` - Indicates whether the disc should be re-initialized `imagePath` - Path to an ISO image on disk, or c{None} to use `entries` `entries` - Mapping from path to graft point, or `None` to use `imagePath` Raises: `IOError` - If the media could not be written to for some reason.

### _getEstimatedImageSize(entries) Static Method | source code
Gets the estimated size of a set of image entries. This is implemented in terms of the `IsoImage` class. The returned value is calculated by adding a "fudge factor" to the value from `IsoImage`. This fudge factor was determined by experimentation and is conservative -- the actual image could be as much as 450 blocks smaller under some circumstances. Parameters: `entries` - Dictionary mapping path to graft point. Returns: Total estimated size of image, in bytes. Raises: `ValueError` - If there are no entries in the dictionary `ValueError` - If any path in the dictionary does not exist `IOError` - If there is a problem calling `mkisofs`.

### _searchForOverburn(output) Static Method | source code
Search for an "overburn" error message in `growisofs` output. The `growisofs` command returns a non-zero exit code and puts a message into the output -- even on a dry run -- if there is not enough space on the media. This is called an "overburn" condition. The error message looks like this:
      :-( /dev/cdrom: 894048 blocks are free, 2033746 to be written!
This method looks for the overburn error message anywhere in the output. If a matching error message is found, an `IOError` exception is raised containing relevant information about the problem. Otherwise, the method call returns normally. Parameters: `output` - List of output lines to search, as from `executeCommand` Raises: `IOError` - If an overburn condition is found.

### _buildWriteArgs(newDisc, hardwareId, driveSpeed, imagePath, entries, mediaLabel=None, dryRun=False) Static Method | source code
Builds a list of arguments to be passed to a `growisofs` command. The arguments will either cause `growisofs` to write the indicated image file to disc, or will pass `growisofs` a list of directories or files that should be written to disc. If a new image is created, it will always be created with Rock Ridge extensions (-r). A volume name will be applied (-V) if `mediaLabel` is not `None`. Parameters: `newDisc` - Indicates whether the disc should be re-initialized `hardwareId` - Hardware id for the device `driveSpeed` - Speed at which the drive writes. `imagePath` - Path to an ISO image on disk, or c{None} to use `entries` `entries` - Mapping from path to graft point, or `None` to use `imagePath` `mediaLabel` - Media label to set on the image, if any `dryRun` - Says whether to make this a dry run (for checking capacity) Returns: List suitable for passing to util.executeCommand as `args`. Raises: `ValueError` - If caller does not pass one or the other of imagePath or entries. Notes: If we write an existing image to disc, then the mediaLabel is ignored. The media label is an attribute of the image, and should be set on the image when it is created. We always pass the undocumented option `-use-the-force-like=tty` to growisofs. Without this option, growisofs will refuse to execute certain actions when running from cron. A good example is -Z, which happily overwrites an existing DVD from the command-line, but fails when run from cron. It took a while to figure that out, since it worked every time I tested it by hand. :(

### unlockTray(self) | source code
Unlocks the device's tray via 'eject -i off'. Raises: `IOError` - If there is an error talking to the device.

### _retrieveSectorsUsed(self) | source code
Retrieves the number of sectors used on the current media. This is a little ugly. We need to call growisofs in "dry-run" mode and parse some information from its output. However, to do that, we need to create a dummy file that we can pass to the command -- and we have to make sure to remove it later. Once growisofs has been run, then we call `_parseSectorsUsed` to parse the output and calculate the number of sectors used on the media. Returns: Number of sectors used on the media

### _parseSectorsUsed(output) Static Method | source code
Parse sectors used information out of `growisofs` output. The first line of a growisofs run looks something like this:
      Executing 'mkisofs -C 973744,1401056 -M /dev/fd/3 -r -graft-points music4/=music | builtin_dd of=/dev/cdrom obs=32k seek=87566'
Dmitry has determined that the seek value in this line gives us information about how much data has previously been written to the media. That value multiplied by 16 yields the number of sectors used. If the seek line cannot be found in the output, then sectors used of zero is assumed. Returns: Sectors used on the media, as a floating point number. Raises: `ValueError` - If the output cannot be parsed properly.

Property Details | [hide private]

### device Filesystem device name for this writer. Get Method: _getDevice(self) - Property target used to get the device value.

### scsiId SCSI id for the device (saved for reference only). Get Method: _getScsiId(self) - Property target used to get the SCSI id value.

### hardwareId Hardware id for this writer (always the device path). Get Method: _getHardwareId(self) - Property target used to get the hardware id value.

### driveSpeed Speed at which the drive writes. Get Method: _getDriveSpeed(self) - Property target used to get the drive speed.

### media Definition of media that is expected to be in the device. Get Method: _getMedia(self) - Property target used to get the media description.

### deviceHasTray Indicates whether the device has a media tray. Get Method: _getDeviceHasTray(self) - Property target used to get the device-has-tray flag.

### deviceCanEject Indicates whether the device supports ejecting its media. Get Method: _getDeviceCanEject(self) - Property target used to get the device-can-eject flag.

### refreshMediaDelay Refresh media delay, in seconds. Get Method: _getRefreshMediaDelay(self) - Property target used to get the configured refresh media delay, in seconds.

### ejectDelay Eject delay, in seconds. Get Method: _getEjectDelay(self) - Property target used to get the configured eject delay, in seconds.