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=============
volume.config
=============

.. configfile:: volume.config

The :file:`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 :file:`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.

.. important::

   Changing this file to add, remove or modify volumes effectively invalidates
   the cache.


Optional ramcache setting
-------------------------

You can also add an option ``ramcache=true/false`` to the volume configuration
line.  True is the default setting and so not needed unless you want to explicitly
set it.  Setting ``ramcache=false`` will disable the ramcache that normally
sits in front of a volume.  This may be desirable if you are using something like
ramdisks, to avoid wasting RAM and cpu time on double caching objects.


Optional directory entry sizing
-------------------------------

You can also add an option ``avg_obj_size=<size>`` to the volume configuration
line. This overrides the global :ts:cv:`proxy.config.cache.min_average_object_size`
configuration for this volume. This is useful if you have a volume that is dedicated
for say very small objects, and you need a lot of directory entries to store them.

Optional fragment size setting
------------------------------

You can also add an option ``fragment_size=<size>`` to the volume configuration
line. This overrides the global :ts:cv:`proxy.config.cache.target_fragment_size`
configuration for this volume. This allows for a smaller, or larger, fragment size
for a particular volume. This may be useful together with ``avg_obj_size`` as well,
since a larger fragment size could reduce the number of directory entries needed
for a large object.

Note that this setting has a maximmum value of 4MB.

Exclusive spans and volume sizes
================================

In the following sample configuration 2 spans `/dev/disk1` and `/dev/disk2` are defined
in :file:`storage.config`, where span `/dev/disk2` is assigned to `volume 3` exclusively
(`volume 3` is forced to an "exclusive" span `/dev/disk2`).
In :file:`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 # <- exclusive 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. The last volume being
an example of one that might be composed of purely ramdisks so that the
ramcache has been disabled.::

    volume=1 scheme=http size=20%
    volume=2 scheme=http size=20%
    volume=3 scheme=http size=20%
    volume=4 scheme=http size=20% avg_obj_size=4096
    volume=5 scheme=http size=20% ramcache=false fragment_size=524288
