volume.config

The volume.config file enables you to manage your cache space more efficiently and restrict disk usage by creating cache volumes of different sizes. By distributing the cache across multiple volumes, you can help decrease single-lock pressure when there are not many hard drives present. You can further configure these volumes to store data from certain origin servers and/or domains in the hosting.config file.

Format

For each volume you want to create, enter a line with the following format:

volume=volume_number  scheme=protocol_type  size=volume_size

where volume_number is a number between 1 and 255 (the maximum number of volumes is 255) and protocol_type is http. Traffic Server supports http for HTTP volume types; volume_size is the amount of cache space allocated to the volume. This value can be either a percentage of the total cache space or an absolute value. The absolute value must be a multiple of 128 MB, where 128 MB is the smallest value. If you specify a percentage, then the size is rounded down to the closest multiple of 128 MB.

Each volume is striped across several disks to achieve parallel I/O. For example: if there are four disks, then a 1-GB volume will have 256 MB on each disk (assuming each disk has enough free space available). If you do not allocate all the disk space in the cache, then the extra disk space is not used. You can use the extra space later to create new volumes without deleting and clearing the existing volumes.

Exclusive spans and volume sizes

In the following sample configuration 2 spans /dev/disk1 and /dev/disk2 are defined in storage.config, where span /dev/disk2 is assigned to volume 3 exclusively (volume 3 is forced to an “exclusive” span /dev/disk2). In volume.config there are 3 volumes defined, where volume 1 and volume 2 occupy span /dev/disk1 taking each 50% of its space and volume 3 takes 100% of span /dev/disk2 exclusively.

storage.config:

/dev/disk1
/dev/disk2 volume=3 # <- exclusinve span

volume.config:

volume=1 scheme=http size=50%
volume=2 scheme=http size=50%
volume=3 scheme=http size=512 # <- volume forced to a specific exclusive span

It is important to note that when percentages are used to specify volume sizes and “exclusive” spans are assigned (forced) to a particular volume (in this case volume 3), the “exclusive” spans (in this case /dev/disk2) are excluded from the total cache space when the “non-forced” volumes sizes are calculated (in this case volume 1 and volume 2).

Examples

The following example partitions the cache across 5 volumes to decreasing single-lock pressure for a machine with few drives.:

volume=1 scheme=http size=20%
volume=2 scheme=http size=20%
volume=3 scheme=http size=20%
volume=4 scheme=http size=20%
volume=5 scheme=http size=20%