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ReStructuredText
462 lines
17 KiB
ReStructuredText
===============
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Cache Tiering
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===============
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A cache tier provides Ceph Clients with better I/O performance for a subset of
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the data stored in a backing storage tier. Cache tiering involves creating a
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pool of relatively fast/expensive storage devices (e.g., solid state drives)
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configured to act as a cache tier, and a backing pool of either erasure-coded
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or relatively slower/cheaper devices configured to act as an economical storage
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tier. The Ceph objecter handles where to place the objects and the tiering
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agent determines when to flush objects from the cache to the backing storage
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tier. So the cache tier and the backing storage tier are completely transparent
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to Ceph clients.
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.. ditaa::
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+-------------+
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| Ceph Client |
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+------+------+
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^
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Tiering is |
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Transparent | Faster I/O
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to Ceph | +---------------+
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Client Ops | | |
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| +----->+ Cache Tier |
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| | | |
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| | +-----+---+-----+
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| | | ^
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v v | | Active Data in Cache Tier
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+------+----+--+ | |
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| Objecter | | |
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+-----------+--+ | |
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^ | | Inactive Data in Storage Tier
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| v |
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| +-----+---+-----+
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+----->| Storage Tier |
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+---------------+
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Slower I/O
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The cache tiering agent handles the migration of data between the cache tier
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and the backing storage tier automatically. However, admins have the ability to
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configure how this migration takes place. There are two main scenarios:
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- **Writeback Mode:** When admins configure tiers with ``writeback`` mode, Ceph
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clients write data to the cache tier and receive an ACK from the cache tier.
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In time, the data written to the cache tier migrates to the storage tier
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and gets flushed from the cache tier. Conceptually, the cache tier is
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overlaid "in front" of the backing storage tier. When a Ceph client needs
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data that resides in the storage tier, the cache tiering agent migrates the
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data to the cache tier on read, then it is sent to the Ceph client.
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Thereafter, the Ceph client can perform I/O using the cache tier, until the
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data becomes inactive. This is ideal for mutable data (e.g., photo/video
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editing, transactional data, etc.).
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- **Read-proxy Mode:** This mode will use any objects that already
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exist in the cache tier, but if an object is not present in the
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cache the request will be proxied to the base tier. This is useful
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for transitioning from ``writeback`` mode to a disabled cache as it
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allows the workload to function properly while the cache is drained,
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without adding any new objects to the cache.
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A word of caution
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=================
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Cache tiering will *degrade* performance for most workloads. Users should use
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extreme caution before using this feature.
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* *Workload dependent*: Whether a cache will improve performance is
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highly dependent on the workload. Because there is a cost
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associated with moving objects into or out of the cache, it can only
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be effective when there is a *large skew* in the access pattern in
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the data set, such that most of the requests touch a small number of
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objects. The cache pool should be large enough to capture the
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working set for your workload to avoid thrashing.
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* *Difficult to benchmark*: Most benchmarks that users run to measure
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performance will show terrible performance with cache tiering, in
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part because very few of them skew requests toward a small set of
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objects, it can take a long time for the cache to "warm up," and
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because the warm-up cost can be high.
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* *Usually slower*: For workloads that are not cache tiering-friendly,
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performance is often slower than a normal RADOS pool without cache
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tiering enabled.
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* *librados object enumeration*: The librados-level object enumeration
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API is not meant to be coherent in the presence of the case. If
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your applicatoin is using librados directly and relies on object
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enumeration, cache tiering will probably not work as expected.
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(This is not a problem for RGW, RBD, or CephFS.)
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* *Complexity*: Enabling cache tiering means that a lot of additional
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machinery and complexity within the RADOS cluster is being used.
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This increases the probability that you will encounter a bug in the system
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that other users have not yet encountered and will put your deployment at a
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higher level of risk.
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Known Good Workloads
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--------------------
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* *RGW time-skewed*: If the RGW workload is such that almost all read
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operations are directed at recently written objects, a simple cache
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tiering configuration that destages recently written objects from
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the cache to the base tier after a configurable period can work
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well.
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Known Bad Workloads
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-------------------
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The following configurations are *known to work poorly* with cache
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tiering.
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* *RBD with replicated cache and erasure-coded base*: This is a common
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request, but usually does not perform well. Even reasonably skewed
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workloads still send some small writes to cold objects, and because
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small writes are not yet supported by the erasure-coded pool, entire
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(usually 4 MB) objects must be migrated into the cache in order to
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satisfy a small (often 4 KB) write. Only a handful of users have
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successfully deployed this configuration, and it only works for them
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because their data is extremely cold (backups) and they are not in
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any way sensitive to performance.
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* *RBD with replicated cache and base*: RBD with a replicated base
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tier does better than when the base is erasure coded, but it is
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still highly dependent on the amount of skew in the workload, and
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very difficult to validate. The user will need to have a good
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understanding of their workload and will need to tune the cache
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tiering parameters carefully.
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Setting Up Pools
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================
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To set up cache tiering, you must have two pools. One will act as the
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backing storage and the other will act as the cache.
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Setting Up a Backing Storage Pool
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---------------------------------
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Setting up a backing storage pool typically involves one of two scenarios:
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- **Standard Storage**: In this scenario, the pool stores multiple copies
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of an object in the Ceph Storage Cluster.
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- **Erasure Coding:** In this scenario, the pool uses erasure coding to
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store data much more efficiently with a small performance tradeoff.
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In the standard storage scenario, you can setup a CRUSH ruleset to establish
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the failure domain (e.g., osd, host, chassis, rack, row, etc.). Ceph OSD
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Daemons perform optimally when all storage drives in the ruleset are of the
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same size, speed (both RPMs and throughput) and type. See `CRUSH Maps`_
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for details on creating a ruleset. Once you have created a ruleset, create
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a backing storage pool.
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In the erasure coding scenario, the pool creation arguments will generate the
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appropriate ruleset automatically. See `Create a Pool`_ for details.
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In subsequent examples, we will refer to the backing storage pool
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as ``cold-storage``.
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Setting Up a Cache Pool
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-----------------------
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Setting up a cache pool follows the same procedure as the standard storage
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scenario, but with this difference: the drives for the cache tier are typically
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high performance drives that reside in their own servers and have their own
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ruleset. When setting up a ruleset, it should take account of the hosts that
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have the high performance drives while omitting the hosts that don't. See
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`Placing Different Pools on Different OSDs`_ for details.
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In subsequent examples, we will refer to the cache pool as ``hot-storage`` and
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the backing pool as ``cold-storage``.
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For cache tier configuration and default values, see
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`Pools - Set Pool Values`_.
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Creating a Cache Tier
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=====================
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Setting up a cache tier involves associating a backing storage pool with
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a cache pool ::
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ceph osd tier add {storagepool} {cachepool}
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For example ::
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ceph osd tier add cold-storage hot-storage
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To set the cache mode, execute the following::
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ceph osd tier cache-mode {cachepool} {cache-mode}
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For example::
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ceph osd tier cache-mode hot-storage writeback
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The cache tiers overlay the backing storage tier, so they require one
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additional step: you must direct all client traffic from the storage pool to
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the cache pool. To direct client traffic directly to the cache pool, execute
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the following::
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ceph osd tier set-overlay {storagepool} {cachepool}
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For example::
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ceph osd tier set-overlay cold-storage hot-storage
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Configuring a Cache Tier
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========================
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Cache tiers have several configuration options. You may set
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cache tier configuration options with the following usage::
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ceph osd pool set {cachepool} {key} {value}
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See `Pools - Set Pool Values`_ for details.
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Target Size and Type
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--------------------
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Ceph's production cache tiers use a `Bloom Filter`_ for the ``hit_set_type``::
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ceph osd pool set {cachepool} hit_set_type bloom
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For example::
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ceph osd pool set hot-storage hit_set_type bloom
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The ``hit_set_count`` and ``hit_set_period`` define how much time each HitSet
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should cover, and how many such HitSets to store. ::
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ceph osd pool set {cachepool} hit_set_count 12
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ceph osd pool set {cachepool} hit_set_period 14400
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ceph osd pool set {cachepool} target_max_bytes 1000000000000
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.. note:: A larger ``hit_set_count`` results in more RAM consumed by
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the ``ceph-osd`` process.
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Binning accesses over time allows Ceph to determine whether a Ceph client
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accessed an object at least once, or more than once over a time period
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("age" vs "temperature").
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The ``min_read_recency_for_promote`` defines how many HitSets to check for the
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existence of an object when handling a read operation. The checking result is
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used to decide whether to promote the object asynchronously. Its value should be
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between 0 and ``hit_set_count``. If it's set to 0, the object is always promoted.
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If it's set to 1, the current HitSet is checked. And if this object is in the
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current HitSet, it's promoted. Otherwise not. For the other values, the exact
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number of archive HitSets are checked. The object is promoted if the object is
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found in any of the most recent ``min_read_recency_for_promote`` HitSets.
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A similar parameter can be set for the write operation, which is
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``min_write_recency_for_promote``. ::
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ceph osd pool set {cachepool} min_read_recency_for_promote 2
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ceph osd pool set {cachepool} min_write_recency_for_promote 2
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.. note:: The longer the period and the higher the
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``min_read_recency_for_promote`` and
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``min_write_recency_for_promote``values, the more RAM the ``ceph-osd``
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daemon consumes. In particular, when the agent is active to flush
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or evict cache objects, all ``hit_set_count`` HitSets are loaded
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into RAM.
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Cache Sizing
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------------
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The cache tiering agent performs two main functions:
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- **Flushing:** The agent identifies modified (or dirty) objects and forwards
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them to the storage pool for long-term storage.
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- **Evicting:** The agent identifies objects that haven't been modified
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(or clean) and evicts the least recently used among them from the cache.
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Absolute Sizing
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~~~~~~~~~~~~~~~
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The cache tiering agent can flush or evict objects based upon the total number
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of bytes or the total number of objects. To specify a maximum number of bytes,
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execute the following::
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ceph osd pool set {cachepool} target_max_bytes {#bytes}
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For example, to flush or evict at 1 TB, execute the following::
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ceph osd pool set hot-storage target_max_bytes 1099511627776
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To specify the maximum number of objects, execute the following::
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ceph osd pool set {cachepool} target_max_objects {#objects}
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For example, to flush or evict at 1M objects, execute the following::
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ceph osd pool set hot-storage target_max_objects 1000000
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.. note:: Ceph is not able to determine the size of a cache pool automatically, so
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the configuration on the absolute size is required here, otherwise the
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flush/evict will not work. If you specify both limits, the cache tiering
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agent will begin flushing or evicting when either threshold is triggered.
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.. note:: All client requests will be blocked only when ``target_max_bytes`` or
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``target_max_objects`` reached
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Relative Sizing
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~~~~~~~~~~~~~~~
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The cache tiering agent can flush or evict objects relative to the size of the
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cache pool(specified by ``target_max_bytes`` / ``target_max_objects`` in
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`Absolute sizing`_). When the cache pool consists of a certain percentage of
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modified (or dirty) objects, the cache tiering agent will flush them to the
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storage pool. To set the ``cache_target_dirty_ratio``, execute the following::
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ceph osd pool set {cachepool} cache_target_dirty_ratio {0.0..1.0}
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For example, setting the value to ``0.4`` will begin flushing modified
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(dirty) objects when they reach 40% of the cache pool's capacity::
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ceph osd pool set hot-storage cache_target_dirty_ratio 0.4
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When the dirty objects reaches a certain percentage of its capacity, flush dirty
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objects with a higher speed. To set the ``cache_target_dirty_high_ratio``::
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ceph osd pool set {cachepool} cache_target_dirty_high_ratio {0.0..1.0}
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For example, setting the value to ``0.6`` will begin aggressively flush dirty objects
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when they reach 60% of the cache pool's capacity. obviously, we'd better set the value
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between dirty_ratio and full_ratio::
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ceph osd pool set hot-storage cache_target_dirty_high_ratio 0.6
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When the cache pool reaches a certain percentage of its capacity, the cache
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tiering agent will evict objects to maintain free capacity. To set the
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``cache_target_full_ratio``, execute the following::
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ceph osd pool set {cachepool} cache_target_full_ratio {0.0..1.0}
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For example, setting the value to ``0.8`` will begin flushing unmodified
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(clean) objects when they reach 80% of the cache pool's capacity::
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ceph osd pool set hot-storage cache_target_full_ratio 0.8
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Cache Age
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---------
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You can specify the minimum age of an object before the cache tiering agent
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flushes a recently modified (or dirty) object to the backing storage pool::
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ceph osd pool set {cachepool} cache_min_flush_age {#seconds}
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For example, to flush modified (or dirty) objects after 10 minutes, execute
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the following::
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ceph osd pool set hot-storage cache_min_flush_age 600
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You can specify the minimum age of an object before it will be evicted from
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the cache tier::
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ceph osd pool {cache-tier} cache_min_evict_age {#seconds}
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For example, to evict objects after 30 minutes, execute the following::
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ceph osd pool set hot-storage cache_min_evict_age 1800
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Removing a Cache Tier
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=====================
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Removing a cache tier differs depending on whether it is a writeback
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cache or a read-only cache.
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Removing a Read-Only Cache
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--------------------------
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Since a read-only cache does not have modified data, you can disable
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and remove it without losing any recent changes to objects in the cache.
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#. Change the cache-mode to ``none`` to disable it. ::
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ceph osd tier cache-mode {cachepool} none
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For example::
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ceph osd tier cache-mode hot-storage none
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#. Remove the cache pool from the backing pool. ::
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ceph osd tier remove {storagepool} {cachepool}
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For example::
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ceph osd tier remove cold-storage hot-storage
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Removing a Writeback Cache
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--------------------------
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Since a writeback cache may have modified data, you must take steps to ensure
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that you do not lose any recent changes to objects in the cache before you
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disable and remove it.
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#. Change the cache mode to ``forward`` so that new and modified objects will
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flush to the backing storage pool. ::
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ceph osd tier cache-mode {cachepool} forward
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For example::
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ceph osd tier cache-mode hot-storage forward
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#. Ensure that the cache pool has been flushed. This may take a few minutes::
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rados -p {cachepool} ls
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If the cache pool still has objects, you can flush them manually.
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For example::
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rados -p {cachepool} cache-flush-evict-all
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#. Remove the overlay so that clients will not direct traffic to the cache. ::
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ceph osd tier remove-overlay {storagetier}
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For example::
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ceph osd tier remove-overlay cold-storage
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#. Finally, remove the cache tier pool from the backing storage pool. ::
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ceph osd tier remove {storagepool} {cachepool}
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For example::
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ceph osd tier remove cold-storage hot-storage
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.. _Create a Pool: ../pools#create-a-pool
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.. _Pools - Set Pool Values: ../pools#set-pool-values
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.. _Placing Different Pools on Different OSDs: ../crush-map/#placing-different-pools-on-different-osds
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.. _Bloom Filter: http://en.wikipedia.org/wiki/Bloom_filter
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.. _CRUSH Maps: ../crush-map
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.. _Absolute Sizing: #absolute-sizing
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