IO Classification

IO Class Configuration

Open CAS Linux provides the ability to control data caching with classification granularity. Open CAS Linux can analyze every IO on the fly to determine whether the requested block is filesystem metadata or data and, if it is data, the size of the destination file. Using this information the administrator can determine the best IO class configuration settings for the typical workload and can set which IO classes to cache and not to cache, and set a priority level for each IO class. As a result, when it becomes necessary to evict a cache line, the software will evict cache lines of the lowest priority first (a major improvement compared to traditional LRU eviction).

The IO classification is configured using an IO classification file. The table below summarizes the configurable fields in this IO classification file.

IO Class Configuration File Fields

Field Description
IO class id Unique ID for the IO class. (NOTE: The ID for unclassified must always be 0)
IO class name The name of the IO class. The name can be a known class name such as metadata or unclassified or direct . It can also consist of a user-defined rule. A rule consists of conditions separated by logical operators and/or arithmetic operators.
Eviction priority Sets the priority number for the IO class. Priority range is 0-255 with zero having the highest priority. The IO classes with the lowest priority will be evicted first.
Allocation Boolean value that allows the user to decide whether data of this IO class will be cached or not. 0=do not cache, 1=cache.

IO class names can consist of user-specified rules to satisfy. Multiple conditions can be combined using logical operators and can be fine-tuned using arithmetic operators. The available operators and names are specified below.

IO Class Configuration File Logical Operators

Operator Description
& Logical and
| Logical or

IO Class Configuration File Arithmetic Operators

Operator Description
eq Equals
ne Not equal
lt Less than
gt Greater than
le Less than or equal to
ge Greater than or equal to

Available IO Class Names and Conditions

IO Class Name Description
unclassified The IO cannot be classified by any other class ID.
metadata The IO contains filesystem metadata.
direct The IO is flagged to be performed directly on the device without buffering. This may be set by the application using the O_DIRECT flag.
core_id The IO belongs to a particular core. For example to classify the IO to a core with id 1: core_id:eq:1
file_size The IO belongs to a file with a specific size in bytes. For example if the file size is less or equal to 4096 bytes: file_size:le:4096
directory The IO belongs to a specific directory path. For example: directory:/data/datab
io_class The IO can be classified by a previously defined io_class ID. For example, to reference IO class with ID 3: io_class:3
extension The IO belongs to a file with a specific extension. For example: extension:txt
file_name_prefix The IO belongs to a file with a specific name prefix. For example, all files with name starting with foo: file_name_prefix:foo
lba The IO belongs to a specific range of lba’s on core device. For example: lba:ge:2000&lba:le:5000
pid The IO was triggered by a process with particular pid. For example: pid:eq:7890
process_name The IO was triggered by a process with particular name. For example: process_name:fio
file_offset The IO belongs to an offset within a file. For example: file_offset:gt:2000 &file_offset:lt:5000
request_size The IO belongs to a request with particular size. For example: request_size:ge:8192&request_size:le:16384&done
wlth The IO uses application/user-space write hints. For example: wlth:eq:0

The table below shows the structure of the IO classification file. This table shows IO class 0 is unclassified and has a high eviction priority of 22 to ensure unclassified data is evicted last. The allocation of 1 specifies this data type is cachable.

IO Class Configuration File Structure

IO Class IO Class Name Eviction Priority Allocation
0 unclassified 22 1

The IO classification file entries are comma-separated and should follow this format:

IO class id,IO class name,Eviction priority,Allocation

Open CAS Linux iterates over all the IO classes specified in the classification file and if an I/O request satisfies the given rule, the I/O will be assigned to this IO class and move on to the next IO class in the file. Open CAS Linux can optionally be configured to terminate upon IO class assignment and not move on to the next IO class with the special keyword “&done”.

Enabling IO Classification

  • To enable IO classification and selective allocation, first look at the provided example IO class configuration file and edit it to suit your needs:

# vi utils/ioclass-config.csv

  • After you have completed your changes to the example IO class configuration and saved the file, you must load the configuration for your cache device with ID number represented by <ID> from file <FILE>:

# casadm --io-class --load-config --cache-id <ID> -f <FILE>

  • Verify that the configuration file was successfully loaded:

# casadm --io-class --list --cache-id <ID>

File Size-Based Caching

The default IO class configuration file (which has been revised from previous versions) allows the user to specify the file size range of interest using keyword “file_size”. It is shown below.

IO class id,IO class name,Eviction priority,Allocation


Directory-Based Caching

IO classification by directory is also configurable using the keyword “directory”. Directory-based caching adds new options to user-specified caching, and works in conjunction with file size-based caching.

The IO classification example below shows a directory caching entry:

IO class id,IO class name,Eviction priority,Allocation


In the above example, to specify the directories to be cached, IO Class 1 shows an IO Class Name of:


This example specifies that all data in the /data/dataa or /data/datab directories will be cached; unclassified data will not be cached; metadata will be cached (due to allocation 1 or 0).

If a directory referenced in classification rule “directory” condition is modified (created / removed / moved), it might take a few seconds until the CAS classification procedure picks up the change and starts classifying IO according to the changed directory contents.

Combined IO Classification Rules

The IO classification example below shows a combination of file size and directory based caching. Class ID 4 demonstrates that ID’s may be combined to form a new classification rule:

IO class id,IO class name,Eviction priority,Allocation


IO class name length limit is 1024 characters. If exceeded, IO class configuration will fail. The valid IO class id range is 0-32. Please note that multiple classification rules must be combined using the same logical operator. For example, metadata|file_size:lt:4096|io_class:3 only uses the “|” operator. Or in another example, file_size:gt:4096&file_size:lt:40960&io_class:3 only uses the “&” operator.

Rules with more than one logical operator will lead to an unmatched evaluation (for example, metadata|file_size:lt:4096&io_class:3 will lead to an undefined condition evaluation due to the use of both “&” and “|” operators).

The keyword &done can always be added at the end of any valid combined rule. Note that the classifier is unable to further categorize IO when it is in direct mode, therefore, IO classified as direct cannot jointly classify other conditions such as file_size or directory.