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.
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 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 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 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 # <- 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