ExNodes contain the metadata (file size, location of data, data encoding, etc) associated with a given object. Similar to a Linux inode. An exNode contains multiple segment definitions defining the data layout. The exNode section itself is pretty minimal. It just contains a unique id arbitrary text name. The [view] section has a list of segments for use and also defines the default segment to use. Each of these points to a different section to load with the naming convention [segment-**id*]* with id the segment id whish is a random 63-bit integer.
Data blocks define allocations acquired via the resource service and is where object data is stored. Each block has it’s own unique section in the exNode using a similar naming convention as for segments – [block-**id*]* where the id is a unique identifier assigned to the block. Each block has the following minimal set of parameters.
The Segment Service consists of software drivers that control the logical layout of data on physical resources. They perform block-level I/O using the Data Service to perform sophisticated fault tolerance and caching schemes.
Simplest segment driver. Maps the segment to a local file.
Provides a hook into the caching service for storing data and handle impedance differences between the application wanting random I/O with the need for the segment driver perform fixed size block I/O.
This segment driver implements the functionality contained in Jim Plank’s Jerasure library.
Takes a collection of data blocks and turns them into a logical unit or LON. Similar to what a hardware based RAID applicance does.
Default query string for selecting resources. All available resource attributes can be used. The query is comprised of a collection of ”.” separated tuples. The tuples are individual operaions broken into the following ”:” separated format:
The OP, KEY_OP, and VAL_OP are integers representing the operations. These are given in the table below. The key and val are text string arguments that KEY_OP and VAL_OP use. For some operations these values are not used. In that case, “any” is normally used to signify this.
Value OP meaning 1 This is key/val matching operation. 2 Boolean NOT operation 3 Boolean AND operation 4 Boolean OR operation
Value KEY_OP/VAL_OP meaning 1 Exact match is required of the argument. 2 The argument must be a prefix for the resource key or val being checked. 3 Any value matches
Value Mutually exclusive options to be &’ed with the Key/VAl options above 64 Make sure that each selection is unique if multiple resources are requested. 128 Pick one for the field and use it for all resources requested.
Tuple definiing the data blocks and physical layout of the LUN row. Multiple row definitions are allowed for large files. The tuples have the format:
The first three fields define where the row fits in the file and define the file beginning and end offset along with the length of the row. The remaining fields define which data blocks and starting offsets to use. There are n_devices entries. One for each device. The block_i represents the block ID.
Implements a log-structured file. A log-structured file only supports append operaionts. Dta is never overwritten. Instead it is appended to file file along with a new extent defining what was appended. This allows one to replay the log and create different versions of the file at any point in the log. The log segment is comprised of three other segments.