Introduction¶

Storage Instantiation Daemon (SID) is a project that aims to help with Linux storage device state tracking that encompasses device layers, groups and whole stacks by monitoring progression of events. Based on monitored states and further recorded information, it is able to trigger associated actions for well-defined triggers, including activation and deactivation of devices and their layers in the stack.

SID positions itself on top of udev, reacting to uevents. It is closely interlinked and cooperating with udev daemon. The udev daemon is enhanced with specialized sid udev builtin command that is used to communicate with SID. SID also listens to udev uevents issued by udev daemon which in turn triggers further processing.

At its core, SID provides an infrastructure and an API for various device subsystems to create modules that handle specific device types and their abstractions. For this purpose, it organizes overall uevent processing into discrete and controlled steps and, through an API, it provides access to its own SID database for use within core parts as well as modules to store and retrieve extended information besides udev daemon’s database capabilities.

The SID database contains both internal and general-purpose records. The internal records are accessed only through dedicated API calls which are used to track and support device dependencies, grouping, triggers and associated actions. The general-purpose records can be stored and retrieved by modules directly with a possibility to define their key components, scope of visibility and access type.

Since SID keeps track of the overall storage device stack and grouping, it provides an insight into the stack, the device and group dependencies and it is able to notify when there are changes to certain parts of the device stack to which there needs to be an appropriate action executed either inside (using SID’s triggers and associated actions) or outside SID itself (using extended notifications that SID sends out).