From The Open Source Backup Wiki (Amanda, MySQL Backup, BackupPC)
The Device API is a clean interface between Amanda and data-storage systems. It provides a tape-like model -- a sequence of bytestreams on each volume, identified only by their on-volume file number -- even of non-tape devices.
Device API Features
The Device API also adds a number of new features not previously available in Amanda. These include:
- Device properties allow drivers to describe themselves (and the devices and media they control), as well as allow arbitrary user settings to propagate down to the driver.
- Smart locking means unrelated accesses can be performed without issue, while conflicting accesses wait for one another.
- Appending to volumes is supported for capable devices.
- A device can describe its supported blocksize range to the Amanda core, instead of the other way around.
- Deleting parts of volumes (without erasing the whole thing) is supported for capable devices.
- The amount of free space on a device can be reported to the Amanda core, where the device supports it (e.g., VFS Device)
The Device API is designed to replace the ancient tapeio subsystem (sometimes called the vtape API or Virtual Tape API), originally introduced to support virtual tapes. The original design of the tapeio system (located in tape-src/) was to abstract tape-related functionality into a separate library. Thus, different devices (tape, vtape and RAIT) were all phrased in terms of tape operations: Rewind, fast-forward, read a block, etc. Operations were assumed to have the same semantics as a UNIX tape device. Furthermore, the API revealed a file descriptor, with the assumption that other parts of Amanda could perform operations such as stat() or dup2() on it.
The Device API clears up a large number of limitations and device assumptions that the tapeio system made, allowing native support for new devices (such as CDs and DVDs) as well as new device-related functionality (such as parallel access, partial recycling, and appending).
The Device API does not change (or even address) media formats.
What the Device API is not
The Device API does not distinguish between random-access and linear-access media: The seek operation may take a long time for some devices, or it may be instantaneous for others. It does, however, distinguish between concurrent devices and exclusive devices: Concurrent devices may be accessed by multiple readers (and sometimes multiple writers) simultaneously, while exclusive devices cannot.
Although the Device API deals in on-medium headers and blocks, it is otherwise agnostic to media format issues. Changes to the Amanda media format, including split vs traditional formats, can be made without change to the Device API.
Using the Device API
The Device API is primarily intended for use from Perl code. See Amanda::Device for a detailed description of the interface.
Implementing a Device Driver
To add a new device to the Device API, do the following:
- Subscribe to the amanda-hackers mailing list and/or connect to #amanda and solicit the help of existing developers.
- Figure out how the data model and operational model described above will map onto your device.
- Implement a subclass of one of the existing Device classes named above. In particular, implement the various virtual functions provided for in device.h.
- Consult the existing device implementations for hints -- in particular, the VFS Device and S3 Device make good examples.
- Note that in addition to the (mostly virtual) functions discussed above, there are some additional protected functions that you may find useful, described in device.h.
- Make sure your device calls device_add_property() to register its properties.
- Arrange to call the register_device() function to register the device, so it will recieve relevant calls to device_open(). The easiest way to do this is to write a initialization function, and add a call to your function from device_api_init() in device.c. Dynamically loadable device drivers are an open project.
- add documentation of your device to the amanda-devices(7) manpage.
The Device API relies heavily on GLib's type system, so developers unfamiliar with the GObject system are encouraged to consult the relevant documentation.
Most code related to device-independent properties is in property.h and property.c. Since property values are passed around as a GValue, all value types must be registered in the GLib type system.
Every abstract property has a DevicePropertyBase, which refers to a DevicePropertyId. The Base structure is the same for all properties of all devices; it holds information about the property outside other context. Each property has a unique DevicePropertyId, though the ID of a property is only guaranteed within a single run of a program: Between runs, IDs might change, so it's best to refer to properties by name outside of a particular program.
When a device starts up, it creates a DeviceProperty structure for each supported property. This structure refers to the DevicePropertyBase, but also holds information about when the property may be accessed: Not all properties may be set or gotten at any time. The PropertyAccessFlags field access holds this information: it is the bitwise OR of any of the various PROPERTY_ACCESS_GET_ and PROPERTY_ACCESS_SET_ values defined. Note that there are five distinct "time periods" as far as access is concerned:
- BEFORE_START: Before device_start() has been called; i.e., before any permanant action may have been taken.
- BETWEEN_FILE_WRITE: When in write mode, but not actually writing a file. Specifically, after device_start() has been called with ACCESS_WRITE or ACCESS_APPEND, but outside of a pair of calls to device_start_file() and device_finish_file().
- INSIDE_FILE_WRITE: In the middle of writing a file. Specifically, after device_start_file() has been called, but before the coresponding call to device_finish_file().
- BETWEEN_FILE_READ: When in read mode, but not actually reading a file. This means after device_start() has been called with ACCESS_READ, but either before a call to device_seek_file, or after a call to device_read_block returns EOF (provided no intervening call to device_seek_file() or device_seek_block() since the EOF).
- INSIDE_FILE_READ: When in read mode, and while actually reading a file. This means after a call to device_seek_block(), but before a call to device_read_block() returns EOF.
Note that these access flags are not automatically enforced, and are currently never consulted. They exist for future expansion of the properties interface.
Device-specific properties should be defined in your own device, not in property.h. See the S3 device's properties for an example.
Future directions for the Device API include:
- new devices
- runtime loading of device modules as shared objects
- more flexible device configuration