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109 lines
4.1 KiB
ReStructuredText
109 lines
4.1 KiB
ReStructuredText
================================
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RBD Persistent Write-back Cache
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================================
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.. index:: Ceph Block Device; Persistent Write-back Cache
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Persistent Write-back Cache
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===========================
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The persistent write-back cache provides a persistent, fault-tolerant write-back
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cache for librbd-based RBD clients.
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This cache uses a log-ordered write-back design which maintains checkpoints
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internally so that writes that get flushed back to the cluster are always
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crash consistent. Even if the client cache is lost entirely, the disk image is
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still consistent but the data will appear to be stale.
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This cache can be used with PMEM or SSD as a cache device.
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Usage
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=====
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The persistent write-back cache manages the cache data in a persistent device.
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It looks for and creates cache files in a configured directory, and then caches
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data in the file.
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The persistent write-back cache can't be enabled without the exclusive-lock
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feature. It tries to enable the write-back cache only when the exclusive lock
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is acquired.
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The cache provides two different persistence modes. In persistent-on-write mode,
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the writes are completed only when they are persisted to the cache device and
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will be readable after a crash. In persistent-on-flush mode, the writes are
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completed as soon as it no longer needs the caller's data buffer to complete
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the writes, but does not guarantee that writes will be readable after a crash.
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The data is persisted to the cache device when a flush request is received.
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Initially it defaults to the persistent-on-write mode and it switches to
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persistent-on-flush mode after the first flush request is received.
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Enable Persistent Write-back Cache
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========================================
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To enable the persistent write-back cache, the following Ceph settings
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need to be enabled.::
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rbd persistent cache mode = {cache-mode}
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rbd plugins = pwl_cache
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Value of {cache-mode} can be ``rwl``, ``ssd`` or ``disabled``. By default the
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cache is disabled.
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Here are some cache configuration settings:
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- ``rbd_persistent_cache_path`` A file folder to cache data. This folder must
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have DAX enabled (see `DAX`_) when using ``rwl`` mode to avoid performance
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degradation.
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- ``rbd_persistent_cache_size`` The cache size per image. The minimum cache
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size is 1 GB.
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- ``rbd_persistent_cache_log_periodic_stats`` This is a debug option. It is
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used to emit periodic perf stats to the debug log if ``debug rbd pwl`` is
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set to ``1`` or higher.
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The above configurations can be set per-host, per-pool, per-image etc. Eg, to
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set per-host, add the overrides to the appropriate `section`_ in the host's
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``ceph.conf`` file. To set per-pool, per-image, etc, please refer to the
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``rbd config`` `commands`_.
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Cache Status
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------------
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The persistent write-back cache is enabled when the exclusive lock is acquired,
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and it is closed when the exclusive lock is released. To check the cache status,
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users may use the command ``rbd status``. ::
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rbd status {pool-name}/{image-name}
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The status of the cache is shown, including present, clean, cache size and the
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location. Currently the status is updated only at the time the cache is opened
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and closed and therefore may appear to be out of date (e.g. show that the cache
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is clean when it is actually dirty).
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For example::
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$ rbd status rbd/foo
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Watchers: none
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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}
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Discard Cache
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-------------
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To discard a cache file with ``rbd``, specify the ``image-cache invalidate``
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option, the pool name and the image name. ::
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rbd image-cache invalidate {pool-name}/{image-name}
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The command removes the cache metadata of the corresponding image, disable
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the cache feature and deletes the local cache file if it exists.
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For example::
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$ rbd image-cache invalidate rbd/foo
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.. _section: ../../rados/configuration/ceph-conf/#configuration-sections
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.. _commands: ../../man/8/rbd#commands
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.. _DAX: https://www.kernel.org/doc/Documentation/filesystems/dax.txt
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