As part of the ongoing conversion to YAML for configuration files, Masaori and
I have been collaborating a configuration file format which combines
"volume.config" and "storage.config" for the cache, because we think it's
always been a bit silly for these to be separate files. Documentation for the
new format
==============
cache.yaml
==============
.. configfile:: cache.yaml
The :file:`cache.yaml` file (by default, located in
``/usr/local/etc/trafficserver/``) lists all the files, directories, and/or
hard disk partitions that make up the Traffic Server cache. After you
modify the :file:`cache.yaml` file the new settings will not be effective until
Traffic Server is restarted.
Format
======
The format of the :file:`cache.yaml` file is a series of lines of the form
.. code-block:: yaml
cache: # file level key
spans:
- id: # identifier / name of the span.
path: # path to storage.
size: # size in bytes, required for file system storage, optional
for raw device.
hash_seed: # optional, used to isolate lookup from path changes.
volumes:
- id: # identifier / name of the volume.
size: # optional, size in percentage
scheme: # optional, default to "http"
ram-cache: # optional, default to "true"
spans: # optional
- use: # Span identifier
size: # size allocated to this volume.
:code:`spans` lists the raw storage used for the cache. :code:`volumes`
organizes the storage into locations for
storing cached objects. This is very similar to operating system partitions and
file systems.
For :code:`spans` the keys are
+---------------+-------------+-------------------------------------------------------------+
| Key | Type | Meaning
|
+===============+=============+=============================================================+
| id | string | Name of the span.
|
+---------------+-------------+-------------------------------------------------------------+
| path | string | File system of the storage. This must be a
block device or |
| | | directory.
|
+---------------+-------------+-------------------------------------------------------------+
| size | bytes | Size in bytes. This is optional for devices but
required |
| | _or_ | for directories.
|
| | percentage |
|
+---------------+-------------+-------------------------------------------------------------+
| hash_seed | string | Hashing for object location uses a seed to
randomize the |
| | | hash. By default this is the path for the span.
|
+---------------+-------------+-------------------------------------------------------------+
For :code:`volumes` the keys are
+---------------+-------------+-------------------------------------------------------------+
| Key | Type | Meaning
|
+===============+=============+=============================================================+
| id | string | Name of the volume.
|
+---------------+-------------+-------------------------------------------------------------+
| size | bytes | Target size of the entire volume. This can be
an absolute |
| | _or_ | number of bytes or a percentage.
|
| | percentage |
+---------------+-------------+-------------------------------------------------------------+
| scheme | enumeration | Protocol scheme, defaults to "http". Preserved
for future |
| | string | use.
|
+---------------+-------------+-------------------------------------------------------------+
| ram-cache | boolean | Control of ram caching for this volume. Default
is ``true`` |
+---------------+-------------+-------------------------------------------------------------+
| spans | list | Spans that provide storage for this volume.
Defaults to |
| | | all spans.
|
+---------------+-------------+-------------------------------------------------------------+
For :code:`volumes:spans` the keys are
+---------------+-------------+-------------------------------------------------------------+
| Key | Type | Meaning
|
+===============+=============+=============================================================+
| use | string | Name of the span to use.
|
+---------------+-------------+-------------------------------------------------------------+
| size | bytes | Amount of the span to use. The total across all
uses of |
| | _or_ | this specific span must be less than 100% and
less than the |
| | percentage | total size of the span.
|
+---------------+-------------+-------------------------------------------------------------+
.. important::
Any change to this files can (and almost always will) invalidate the
existing cache in its entirety.
You can use any partition of any size. For best performance:
- Use raw disk partitions.
- For each disk, make all partitions the same size.
- Group similar kinds of storage into different volumes. For example
split out SSD's or RAM drives into their own volume.
Specify pathnames according to your operating system requirements. See
the following examples. In the :file:`cache.yaml` file, a formatted or
raw disk must be at least 128 MB.
When using raw disk or partitions, you should make sure the :ts:cv:`Traffic
Server user <proxy.config.admin.user_id>` used by the Traffic Server process
has read and write privileges on the raw disk device or partition. One good
practice is to make sure the device file is set with 'g+rw' and the Traffic
Server user is in the group which owns the device file. However, some
operating systems have stronger requirements - see the following examples for
more information.
As with standard ``records.yaml`` integers, human readable prefixes are also
supported. They include
- ``K`` Kilobytes (1024 bytes)
- ``M`` Megabytes (1024^2 or 1,048,576 bytes)
- ``G`` Gigabytes (1024^3 or 1,073,741,824 bytes)
- ``T`` Terabytes (1024^4 or 1,099,511,627,776 bytes)
.. _assignment-table:
Storage Allocation
------------------
Allocation of span storage to volumes is done in stages.
* Convert span use percentages to absolute bytes based on the span size. The
values are rounded down to a multiple
of 128 megabytes.
* Explicit span storage is allocated to volumes. It is an error if the total
allocated is larger than the span size.
* Span storage is allocated to volumes by the :code:`volumes::size` values.
Percentages are converted to absolute
sizes, rounded down to 128 megabytes. These percentages are applied against
remaining span storage.
Assignment Table
----------------
Each storage element defined in :file:`cache.yaml` is divided in to
:term:`stripes <cache stripe>`. The
assignment table maps from an object URL to a specific stripe. The table is
initialized based on a
pseudo-random process which is seeded by hashing a string for each stripe. This
string is composed
of a base string, an offset (the start of the stripe on the storage element),
and the length of the
stripe. By default the path for the storage is used as the base string. This
ensures that each
stripe has a unique string for the assignment hash. This does make the
assignment table very
sensitive to the path for the storage elements and changing even one can have a
cascading effect
which will effectively clear most of the cache. This can be problem when drives
fail and a system
reboot causes the path names to change.
The :arg:`id` option can be used to create a fixed string that an administrator
can use to keep the
assignment table consistent by maintaining the mapping from physical device to
base string even in the presence of hardware changes and failures.
Backwards Compatibility
-----------------------
In previous versions of |TS| it was possible to have "exclusive" spans which
were used by only one volume. This is
now down by specifying the span in the volume and using a size of "100%". E.g.
old configuration like ::
/dev/disk2 volume=3 # storage.config
volume=3 scheme=http size=512 # volume.config
The corresponding configuration would be
.. code-block:: yaml
cache:
spans:
- id: disk.2
path: /dev/disk2
volumes:
- id: linear
spans:
- use: disk.2
size: 100%
Because volume sizes that are percentages are computed on span storage not
already explicitly allocated, this will
leave none of "disk.2" for such allocation and therefore "disk.2" will be used
only by volume "linear". Note this
configuration is more flexible. If it was useful to have two linear volumes,
each using exclusively half of the
span, this would be
.. code-block:: yaml
cache:
spans:
- id: disk.2
path: /dev/disk2
volumes:
- id: linear.1
spans:
- use: disk.2
size: 50%
- id: linear.2
spans:
- use: disk.2
size: 50%
Examples
========
The following basic example shows 128 MB of cache storage in the
"/big_dir" directory
.. code-block: yaml
cache:
spans:
- id: store
path: /big_dir
size: 134217728
volumes:
- id: primary
By default a volume uses all spans, therefore volume "primary" uses all of span
"store" because there are no other
volumes. It would be equivalent is using the spans explicitly, e.g.
.. code-block: yaml
cache:
spans:
- id: store
path: /big_dir
size: 134217728
volumes:
- id: primary
size: 100%
spans:
- id: store
You can use the ``.`` symbol for the current directory. Here is an
example for 128 MB of cache storage in the current directory
.. code-block: yaml
cache:
spans:
- id: store
path: "."
size: 134217728
volumes:
- id: primary
size: 100%
.. note::
When using on-filesystem cache disk storage, you can only have one such
directory specified. This will be addressed in a future version.
Linux Example
-------------
.. note::
Rather than refer to disk devices like ``/dev/sda``, ``/dev/sdb``, etc.,
modern Linux supports `alternative symlinked names for disk devices
https://wiki.archlinux.org/index.php/persistent_block_device_naming#by-id_and_by-path`_
in the ``/dev/disk``
directory structure. As noted for the :ref:`assignment-table` the path used
for the disk can effect
the cache if it changes. This can be ameliorated in some cases by using one
of the alternate paths
in via ``/dev/disk``. Note that if the ``by-id`` or ``by-path`` style is
used, replacing a failed drive will cause
that path to change because the new drive will have a different physical ID
or path.
If this is not sufficient then the :arg:`hash_seed` key should be used to
create a more permanent
assignment table. An example would be
.. code-block:: yaml
- id: "span.0"
path: "/dev/sde"
hash-seed: "cache.disk.0"
- id: "span.1"
path: "/dev/sdg"
hash-seed: "cache.disk.1"
The following example will use an entire raw disk in the Linux operating
system
.. code-block: yaml
cache:
spans:
- id: a
path: "/dev/disk/by-id/disk-A-id"
- id: b
path: "/dev/disk/by-id/disk-B-id"
volumes:
- id: one
spans:
- use: a
size: 100%
- id: two
spans:
- use: b
size: 100%
In order to make sure :program:`traffic_server` will have access to this disk
you can use :manpage:`udev(7)` to persistently set the right permissions. The
following rules are targeted for an Ubuntu system, and stored in
``/etc/udev/rules.d/51-cache-disk.rules``::
# Assign DiskA and DiskB to the tserver group
# make the assignment final, no later changes allowed to the group!
SUBSYSTEM=="block", KERNEL=="sd[ef]", GROUP:="tserver"
In order to apply these settings, trigger a reload with :manpage:`udevadm(8)`:::
udevadm trigger --subsystem-match=block
FreeBSD Example
---------------
Starting with 5.1 FreeBSD dropped support for explicit raw devices. All
devices on FreeBSD can be accessed raw now.
The following example will use an entire raw disk in the FreeBSD
operating system
.. code-block: yaml
cache:
spans:
- id: ada.1
path: "/dev/ada1"
- id: ada.2
path: "/dev/ada2"
volumes:
- id: primary
size: 100%
In order to make sure :program:`traffic_server` will have access to this disk
you can use :manpage:`devfs(8)` to persistently set the right permissions. The
following rules are stored in :manpage:`devfs.conf(5)`::
# Assign /dev/ada1 and /dev/ada2 to the tserver user
own ada[12] tserver:tserver
Advanced
--------
Because relative paths in :file:`cache.yaml` are relative to the base prefix,
when using customized runroot
it may be necessary to adjust such paths in :file:`cache.yaml` or adjust
``runroot.yaml`` itself.
Despite the name, the cachedir value is not used for this file.
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
ram cache has been disabled.
.. code-block:: yaml
cache:
spans:
- id: disk
path: "/dev/sdb"
volumes:
- id: vol.1
size: 20%
- id: vol.2
size: 20%
- id: vol.3
size: 20%
- id: vol.4
size: 20%
- id: vol.5
size: 20%
ram-cache: false
