mirror of
https://github.com/ceph/ceph
synced 2025-02-27 04:52:54 +00:00
1b4f4841bb
The full name of PWL is persistent write log, and the full name of RWL is replica write log. Change the title to make it consistent with the name of the feature and better reflect its design. Signed-off-by: Yin Congmin <congmin.yin@intel.com>
110 lines
4.1 KiB
ReStructuredText
110 lines
4.1 KiB
ReStructuredText
================================
|
|
RBD Persistent Write Log Cache
|
|
================================
|
|
|
|
.. index:: Ceph Block Device; Persistent Write Log Cache
|
|
|
|
Persistent Write Log Cache
|
|
===========================
|
|
|
|
The Persistent Write Log Cache (PWL) provides a persistent, fault-tolerant
|
|
write-back cache for librbd-based RBD clients.
|
|
|
|
This cache uses a log-ordered write-back design which maintains checkpoints
|
|
internally so that writes that get flushed back to the cluster are always
|
|
crash consistent. Even if the client cache is lost entirely, the disk image is
|
|
still consistent but the data will appear to be stale.
|
|
|
|
This cache can be used with PMEM or SSD as a cache device. For PMEM, the cache
|
|
mode is called ``replica write log (rwl)``. At present, only local cache is
|
|
supported, and the replica function is under development. For SSD, the cache
|
|
mode is called ``ssd``.
|
|
|
|
Usage
|
|
=====
|
|
|
|
The PWL cache manages the cache data in a persistent device. It looks for and
|
|
creates cache files in a configured directory, and then caches data in the
|
|
file.
|
|
|
|
The PWL cache depends on the exclusive-lock feature. The cache can be loaded
|
|
only after the exclusive lock is acquired.
|
|
|
|
The cache provides two different persistence modes. In persistent-on-write mode,
|
|
the writes are completed only when they are persisted to the cache device and
|
|
will be readable after a crash. In persistent-on-flush mode, the writes are
|
|
completed as soon as it no longer needs the caller's data buffer to complete
|
|
the writes, but does not guarantee that writes will be readable after a crash.
|
|
The data is persisted to the cache device when a flush request is received.
|
|
|
|
Initially it defaults to the persistent-on-write mode and it switches to
|
|
persistent-on-flush mode after the first flush request is received.
|
|
|
|
Enable Cache
|
|
========================================
|
|
|
|
To enable the PWL cache, set the following configuration settings::
|
|
|
|
rbd persistent cache mode = {cache-mode}
|
|
rbd plugins = pwl_cache
|
|
|
|
Value of {cache-mode} can be ``rwl``, ``ssd`` or ``disabled``. By default the
|
|
cache is disabled.
|
|
|
|
Here are some cache configuration settings:
|
|
|
|
- ``rbd_persistent_cache_path`` A file folder to cache data. This folder must
|
|
have DAX enabled (see `DAX`_) when using ``rwl`` mode to avoid performance
|
|
degradation.
|
|
|
|
- ``rbd_persistent_cache_size`` The cache size per image. The minimum cache
|
|
size is 1 GB.
|
|
|
|
- ``rbd_persistent_cache_log_periodic_stats`` This is a debug option. It is
|
|
used to emit periodic perf stats to the debug log if ``debug rbd pwl`` is
|
|
set to ``1`` or higher.
|
|
|
|
The above configurations can be set per-host, per-pool, per-image etc. Eg, to
|
|
set per-host, add the overrides to the appropriate `section`_ in the host's
|
|
``ceph.conf`` file. To set per-pool, per-image, etc, please refer to the
|
|
``rbd config`` `commands`_.
|
|
|
|
Cache Status
|
|
------------
|
|
|
|
The PWL cache is enabled when the exclusive lock is acquired,
|
|
and it is closed when the exclusive lock is released. To check the cache status,
|
|
users may use the command ``rbd status``. ::
|
|
|
|
rbd status {pool-name}/{image-name}
|
|
|
|
The status of the cache is shown, including present, clean, cache size and the
|
|
location. Currently the status is updated only at the time the cache is opened
|
|
and closed and therefore may appear to be out of date (e.g. show that the cache
|
|
is clean when it is actually dirty).
|
|
|
|
For example::
|
|
|
|
$ rbd status rbd/foo
|
|
Watchers: none
|
|
Image cache state: {"present":"true","empty":"false","clean":"true","cache_type":"ssd","pwl_host":"sceph9","pwl_path":"/tmp/rbd-pwl.rbd.abcdef123456.pool","pwl_size":1073741824}
|
|
|
|
Discard Cache
|
|
-------------
|
|
|
|
To discard a cache file with ``rbd``, specify the ``image-cache invalidate``
|
|
option, the pool name and the image name. ::
|
|
|
|
rbd image-cache invalidate {pool-name}/{image-name}
|
|
|
|
The command removes the cache metadata of the corresponding image, disable
|
|
the cache feature and deletes the local cache file if it exists.
|
|
|
|
For example::
|
|
|
|
$ rbd image-cache invalidate rbd/foo
|
|
|
|
.. _section: ../../rados/configuration/ceph-conf/#configuration-sections
|
|
.. _commands: ../../man/8/rbd#commands
|
|
.. _DAX: https://www.kernel.org/doc/Documentation/filesystems/dax.txt
|