mirror of
https://github.com/ceph/ceph
synced 2024-12-23 03:44:23 +00:00
462 lines
17 KiB
ReStructuredText
462 lines
17 KiB
ReStructuredText
===============
|
|
Cache Tiering
|
|
===============
|
|
|
|
A cache tier provides Ceph Clients with better I/O performance for a subset of
|
|
the data stored in a backing storage tier. Cache tiering involves creating a
|
|
pool of relatively fast/expensive storage devices (e.g., solid state drives)
|
|
configured to act as a cache tier, and a backing pool of either erasure-coded
|
|
or relatively slower/cheaper devices configured to act as an economical storage
|
|
tier. The Ceph objecter handles where to place the objects and the tiering
|
|
agent determines when to flush objects from the cache to the backing storage
|
|
tier. So the cache tier and the backing storage tier are completely transparent
|
|
to Ceph clients.
|
|
|
|
|
|
.. ditaa::
|
|
+-------------+
|
|
| Ceph Client |
|
|
+------+------+
|
|
^
|
|
Tiering is |
|
|
Transparent | Faster I/O
|
|
to Ceph | +---------------+
|
|
Client Ops | | |
|
|
| +----->+ Cache Tier |
|
|
| | | |
|
|
| | +-----+---+-----+
|
|
| | | ^
|
|
v v | | Active Data in Cache Tier
|
|
+------+----+--+ | |
|
|
| Objecter | | |
|
|
+-----------+--+ | |
|
|
^ | | Inactive Data in Storage Tier
|
|
| v |
|
|
| +-----+---+-----+
|
|
| | |
|
|
+----->| Storage Tier |
|
|
| |
|
|
+---------------+
|
|
Slower I/O
|
|
|
|
|
|
The cache tiering agent handles the migration of data between the cache tier
|
|
and the backing storage tier automatically. However, admins have the ability to
|
|
configure how this migration takes place. There are two main scenarios:
|
|
|
|
- **Writeback Mode:** When admins configure tiers with ``writeback`` mode, Ceph
|
|
clients write data to the cache tier and receive an ACK from the cache tier.
|
|
In time, the data written to the cache tier migrates to the storage tier
|
|
and gets flushed from the cache tier. Conceptually, the cache tier is
|
|
overlaid "in front" of the backing storage tier. When a Ceph client needs
|
|
data that resides in the storage tier, the cache tiering agent migrates the
|
|
data to the cache tier on read, then it is sent to the Ceph client.
|
|
Thereafter, the Ceph client can perform I/O using the cache tier, until the
|
|
data becomes inactive. This is ideal for mutable data (e.g., photo/video
|
|
editing, transactional data, etc.).
|
|
|
|
- **Read-proxy Mode:** This mode will use any objects that already
|
|
exist in the cache tier, but if an object is not present in the
|
|
cache the request will be proxied to the base tier. This is useful
|
|
for transitioning from ``writeback`` mode to a disabled cache as it
|
|
allows the workload to function properly while the cache is drained,
|
|
without adding any new objects to the cache.
|
|
|
|
A word of caution
|
|
=================
|
|
|
|
Cache tiering will *degrade* performance for most workloads. Users should use
|
|
extreme caution before using this feature.
|
|
|
|
* *Workload dependent*: Whether a cache will improve performance is
|
|
highly dependent on the workload. Because there is a cost
|
|
associated with moving objects into or out of the cache, it can only
|
|
be effective when there is a *large skew* in the access pattern in
|
|
the data set, such that most of the requests touch a small number of
|
|
objects. The cache pool should be large enough to capture the
|
|
working set for your workload to avoid thrashing.
|
|
|
|
* *Difficult to benchmark*: Most benchmarks that users run to measure
|
|
performance will show terrible performance with cache tiering, in
|
|
part because very few of them skew requests toward a small set of
|
|
objects, it can take a long time for the cache to "warm up," and
|
|
because the warm-up cost can be high.
|
|
|
|
* *Usually slower*: For workloads that are not cache tiering-friendly,
|
|
performance is often slower than a normal RADOS pool without cache
|
|
tiering enabled.
|
|
|
|
* *librados object enumeration*: The librados-level object enumeration
|
|
API is not meant to be coherent in the presence of the case. If
|
|
your applicatoin is using librados directly and relies on object
|
|
enumeration, cache tiering will probably not work as expected.
|
|
(This is not a problem for RGW, RBD, or CephFS.)
|
|
|
|
* *Complexity*: Enabling cache tiering means that a lot of additional
|
|
machinery and complexity within the RADOS cluster is being used.
|
|
This increases the probability that you will encounter a bug in the system
|
|
that other users have not yet encountered and will put your deployment at a
|
|
higher level of risk.
|
|
|
|
Known Good Workloads
|
|
--------------------
|
|
|
|
* *RGW time-skewed*: If the RGW workload is such that almost all read
|
|
operations are directed at recently written objects, a simple cache
|
|
tiering configuration that destages recently written objects from
|
|
the cache to the base tier after a configurable period can work
|
|
well.
|
|
|
|
Known Bad Workloads
|
|
-------------------
|
|
|
|
The following configurations are *known to work poorly* with cache
|
|
tiering.
|
|
|
|
* *RBD with replicated cache and erasure-coded base*: This is a common
|
|
request, but usually does not perform well. Even reasonably skewed
|
|
workloads still send some small writes to cold objects, and because
|
|
small writes are not yet supported by the erasure-coded pool, entire
|
|
(usually 4 MB) objects must be migrated into the cache in order to
|
|
satisfy a small (often 4 KB) write. Only a handful of users have
|
|
successfully deployed this configuration, and it only works for them
|
|
because their data is extremely cold (backups) and they are not in
|
|
any way sensitive to performance.
|
|
|
|
* *RBD with replicated cache and base*: RBD with a replicated base
|
|
tier does better than when the base is erasure coded, but it is
|
|
still highly dependent on the amount of skew in the workload, and
|
|
very difficult to validate. The user will need to have a good
|
|
understanding of their workload and will need to tune the cache
|
|
tiering parameters carefully.
|
|
|
|
|
|
Setting Up Pools
|
|
================
|
|
|
|
To set up cache tiering, you must have two pools. One will act as the
|
|
backing storage and the other will act as the cache.
|
|
|
|
|
|
Setting Up a Backing Storage Pool
|
|
---------------------------------
|
|
|
|
Setting up a backing storage pool typically involves one of two scenarios:
|
|
|
|
- **Standard Storage**: In this scenario, the pool stores multiple copies
|
|
of an object in the Ceph Storage Cluster.
|
|
|
|
- **Erasure Coding:** In this scenario, the pool uses erasure coding to
|
|
store data much more efficiently with a small performance tradeoff.
|
|
|
|
In the standard storage scenario, you can setup a CRUSH ruleset to establish
|
|
the failure domain (e.g., osd, host, chassis, rack, row, etc.). Ceph OSD
|
|
Daemons perform optimally when all storage drives in the ruleset are of the
|
|
same size, speed (both RPMs and throughput) and type. See `CRUSH Maps`_
|
|
for details on creating a ruleset. Once you have created a ruleset, create
|
|
a backing storage pool.
|
|
|
|
In the erasure coding scenario, the pool creation arguments will generate the
|
|
appropriate ruleset automatically. See `Create a Pool`_ for details.
|
|
|
|
In subsequent examples, we will refer to the backing storage pool
|
|
as ``cold-storage``.
|
|
|
|
|
|
Setting Up a Cache Pool
|
|
-----------------------
|
|
|
|
Setting up a cache pool follows the same procedure as the standard storage
|
|
scenario, but with this difference: the drives for the cache tier are typically
|
|
high performance drives that reside in their own servers and have their own
|
|
ruleset. When setting up a ruleset, it should take account of the hosts that
|
|
have the high performance drives while omitting the hosts that don't. See
|
|
`Placing Different Pools on Different OSDs`_ for details.
|
|
|
|
|
|
In subsequent examples, we will refer to the cache pool as ``hot-storage`` and
|
|
the backing pool as ``cold-storage``.
|
|
|
|
For cache tier configuration and default values, see
|
|
`Pools - Set Pool Values`_.
|
|
|
|
|
|
Creating a Cache Tier
|
|
=====================
|
|
|
|
Setting up a cache tier involves associating a backing storage pool with
|
|
a cache pool ::
|
|
|
|
ceph osd tier add {storagepool} {cachepool}
|
|
|
|
For example ::
|
|
|
|
ceph osd tier add cold-storage hot-storage
|
|
|
|
To set the cache mode, execute the following::
|
|
|
|
ceph osd tier cache-mode {cachepool} {cache-mode}
|
|
|
|
For example::
|
|
|
|
ceph osd tier cache-mode hot-storage writeback
|
|
|
|
The cache tiers overlay the backing storage tier, so they require one
|
|
additional step: you must direct all client traffic from the storage pool to
|
|
the cache pool. To direct client traffic directly to the cache pool, execute
|
|
the following::
|
|
|
|
ceph osd tier set-overlay {storagepool} {cachepool}
|
|
|
|
For example::
|
|
|
|
ceph osd tier set-overlay cold-storage hot-storage
|
|
|
|
|
|
Configuring a Cache Tier
|
|
========================
|
|
|
|
Cache tiers have several configuration options. You may set
|
|
cache tier configuration options with the following usage::
|
|
|
|
ceph osd pool set {cachepool} {key} {value}
|
|
|
|
See `Pools - Set Pool Values`_ for details.
|
|
|
|
|
|
Target Size and Type
|
|
--------------------
|
|
|
|
Ceph's production cache tiers use a `Bloom Filter`_ for the ``hit_set_type``::
|
|
|
|
ceph osd pool set {cachepool} hit_set_type bloom
|
|
|
|
For example::
|
|
|
|
ceph osd pool set hot-storage hit_set_type bloom
|
|
|
|
The ``hit_set_count`` and ``hit_set_period`` define how much time each HitSet
|
|
should cover, and how many such HitSets to store. ::
|
|
|
|
ceph osd pool set {cachepool} hit_set_count 12
|
|
ceph osd pool set {cachepool} hit_set_period 14400
|
|
ceph osd pool set {cachepool} target_max_bytes 1000000000000
|
|
|
|
.. note:: A larger ``hit_set_count`` results in more RAM consumed by
|
|
the ``ceph-osd`` process.
|
|
|
|
Binning accesses over time allows Ceph to determine whether a Ceph client
|
|
accessed an object at least once, or more than once over a time period
|
|
("age" vs "temperature").
|
|
|
|
The ``min_read_recency_for_promote`` defines how many HitSets to check for the
|
|
existence of an object when handling a read operation. The checking result is
|
|
used to decide whether to promote the object asynchronously. Its value should be
|
|
between 0 and ``hit_set_count``. If it's set to 0, the object is always promoted.
|
|
If it's set to 1, the current HitSet is checked. And if this object is in the
|
|
current HitSet, it's promoted. Otherwise not. For the other values, the exact
|
|
number of archive HitSets are checked. The object is promoted if the object is
|
|
found in any of the most recent ``min_read_recency_for_promote`` HitSets.
|
|
|
|
A similar parameter can be set for the write operation, which is
|
|
``min_write_recency_for_promote``. ::
|
|
|
|
ceph osd pool set {cachepool} min_read_recency_for_promote 2
|
|
ceph osd pool set {cachepool} min_write_recency_for_promote 2
|
|
|
|
.. note:: The longer the period and the higher the
|
|
``min_read_recency_for_promote`` and
|
|
``min_write_recency_for_promote``values, the more RAM the ``ceph-osd``
|
|
daemon consumes. In particular, when the agent is active to flush
|
|
or evict cache objects, all ``hit_set_count`` HitSets are loaded
|
|
into RAM.
|
|
|
|
|
|
Cache Sizing
|
|
------------
|
|
|
|
The cache tiering agent performs two main functions:
|
|
|
|
- **Flushing:** The agent identifies modified (or dirty) objects and forwards
|
|
them to the storage pool for long-term storage.
|
|
|
|
- **Evicting:** The agent identifies objects that haven't been modified
|
|
(or clean) and evicts the least recently used among them from the cache.
|
|
|
|
|
|
Absolute Sizing
|
|
~~~~~~~~~~~~~~~
|
|
|
|
The cache tiering agent can flush or evict objects based upon the total number
|
|
of bytes or the total number of objects. To specify a maximum number of bytes,
|
|
execute the following::
|
|
|
|
ceph osd pool set {cachepool} target_max_bytes {#bytes}
|
|
|
|
For example, to flush or evict at 1 TB, execute the following::
|
|
|
|
ceph osd pool set hot-storage target_max_bytes 1099511627776
|
|
|
|
|
|
To specify the maximum number of objects, execute the following::
|
|
|
|
ceph osd pool set {cachepool} target_max_objects {#objects}
|
|
|
|
For example, to flush or evict at 1M objects, execute the following::
|
|
|
|
ceph osd pool set hot-storage target_max_objects 1000000
|
|
|
|
.. note:: Ceph is not able to determine the size of a cache pool automatically, so
|
|
the configuration on the absolute size is required here, otherwise the
|
|
flush/evict will not work. If you specify both limits, the cache tiering
|
|
agent will begin flushing or evicting when either threshold is triggered.
|
|
|
|
.. note:: All client requests will be blocked only when ``target_max_bytes`` or
|
|
``target_max_objects`` reached
|
|
|
|
Relative Sizing
|
|
~~~~~~~~~~~~~~~
|
|
|
|
The cache tiering agent can flush or evict objects relative to the size of the
|
|
cache pool(specified by ``target_max_bytes`` / ``target_max_objects`` in
|
|
`Absolute sizing`_). When the cache pool consists of a certain percentage of
|
|
modified (or dirty) objects, the cache tiering agent will flush them to the
|
|
storage pool. To set the ``cache_target_dirty_ratio``, execute the following::
|
|
|
|
ceph osd pool set {cachepool} cache_target_dirty_ratio {0.0..1.0}
|
|
|
|
For example, setting the value to ``0.4`` will begin flushing modified
|
|
(dirty) objects when they reach 40% of the cache pool's capacity::
|
|
|
|
ceph osd pool set hot-storage cache_target_dirty_ratio 0.4
|
|
|
|
When the dirty objects reaches a certain percentage of its capacity, flush dirty
|
|
objects with a higher speed. To set the ``cache_target_dirty_high_ratio``::
|
|
|
|
ceph osd pool set {cachepool} cache_target_dirty_high_ratio {0.0..1.0}
|
|
|
|
For example, setting the value to ``0.6`` will begin aggressively flush dirty objects
|
|
when they reach 60% of the cache pool's capacity. obviously, we'd better set the value
|
|
between dirty_ratio and full_ratio::
|
|
|
|
ceph osd pool set hot-storage cache_target_dirty_high_ratio 0.6
|
|
|
|
When the cache pool reaches a certain percentage of its capacity, the cache
|
|
tiering agent will evict objects to maintain free capacity. To set the
|
|
``cache_target_full_ratio``, execute the following::
|
|
|
|
ceph osd pool set {cachepool} cache_target_full_ratio {0.0..1.0}
|
|
|
|
For example, setting the value to ``0.8`` will begin flushing unmodified
|
|
(clean) objects when they reach 80% of the cache pool's capacity::
|
|
|
|
ceph osd pool set hot-storage cache_target_full_ratio 0.8
|
|
|
|
|
|
Cache Age
|
|
---------
|
|
|
|
You can specify the minimum age of an object before the cache tiering agent
|
|
flushes a recently modified (or dirty) object to the backing storage pool::
|
|
|
|
ceph osd pool set {cachepool} cache_min_flush_age {#seconds}
|
|
|
|
For example, to flush modified (or dirty) objects after 10 minutes, execute
|
|
the following::
|
|
|
|
ceph osd pool set hot-storage cache_min_flush_age 600
|
|
|
|
You can specify the minimum age of an object before it will be evicted from
|
|
the cache tier::
|
|
|
|
ceph osd pool {cache-tier} cache_min_evict_age {#seconds}
|
|
|
|
For example, to evict objects after 30 minutes, execute the following::
|
|
|
|
ceph osd pool set hot-storage cache_min_evict_age 1800
|
|
|
|
|
|
Removing a Cache Tier
|
|
=====================
|
|
|
|
Removing a cache tier differs depending on whether it is a writeback
|
|
cache or a read-only cache.
|
|
|
|
|
|
Removing a Read-Only Cache
|
|
--------------------------
|
|
|
|
Since a read-only cache does not have modified data, you can disable
|
|
and remove it without losing any recent changes to objects in the cache.
|
|
|
|
#. Change the cache-mode to ``none`` to disable it. ::
|
|
|
|
ceph osd tier cache-mode {cachepool} none
|
|
|
|
For example::
|
|
|
|
ceph osd tier cache-mode hot-storage none
|
|
|
|
#. Remove the cache pool from the backing pool. ::
|
|
|
|
ceph osd tier remove {storagepool} {cachepool}
|
|
|
|
For example::
|
|
|
|
ceph osd tier remove cold-storage hot-storage
|
|
|
|
|
|
|
|
Removing a Writeback Cache
|
|
--------------------------
|
|
|
|
Since a writeback cache may have modified data, you must take steps to ensure
|
|
that you do not lose any recent changes to objects in the cache before you
|
|
disable and remove it.
|
|
|
|
|
|
#. Change the cache mode to ``forward`` so that new and modified objects will
|
|
flush to the backing storage pool. ::
|
|
|
|
ceph osd tier cache-mode {cachepool} forward
|
|
|
|
For example::
|
|
|
|
ceph osd tier cache-mode hot-storage forward
|
|
|
|
|
|
#. Ensure that the cache pool has been flushed. This may take a few minutes::
|
|
|
|
rados -p {cachepool} ls
|
|
|
|
If the cache pool still has objects, you can flush them manually.
|
|
For example::
|
|
|
|
rados -p {cachepool} cache-flush-evict-all
|
|
|
|
|
|
#. Remove the overlay so that clients will not direct traffic to the cache. ::
|
|
|
|
ceph osd tier remove-overlay {storagetier}
|
|
|
|
For example::
|
|
|
|
ceph osd tier remove-overlay cold-storage
|
|
|
|
|
|
#. Finally, remove the cache tier pool from the backing storage pool. ::
|
|
|
|
ceph osd tier remove {storagepool} {cachepool}
|
|
|
|
For example::
|
|
|
|
ceph osd tier remove cold-storage hot-storage
|
|
|
|
|
|
.. _Create a Pool: ../pools#create-a-pool
|
|
.. _Pools - Set Pool Values: ../pools#set-pool-values
|
|
.. _Placing Different Pools on Different OSDs: ../crush-map/#placing-different-pools-on-different-osds
|
|
.. _Bloom Filter: http://en.wikipedia.org/wiki/Bloom_filter
|
|
.. _CRUSH Maps: ../crush-map
|
|
.. _Absolute Sizing: #absolute-sizing
|