mirror of
https://github.com/ceph/ceph
synced 2024-12-24 04:14:07 +00:00
430 lines
17 KiB
ReStructuredText
430 lines
17 KiB
ReStructuredText
==================
|
|
Placement Groups
|
|
==================
|
|
|
|
.. _preselection:
|
|
|
|
A preselection of pg_num
|
|
========================
|
|
|
|
When creating a new pool with::
|
|
|
|
ceph osd pool set {pool-name} pg_num
|
|
|
|
it is mandatory to choose the value of ``pg_num`` because it cannot be
|
|
calculated automatically. Here are a few values commonly used:
|
|
|
|
- Less than 5 OSDs set ``pg_num`` to 128
|
|
|
|
- Between 5 and 10 OSDs set ``pg_num`` to 512
|
|
|
|
- Between 10 and 50 OSDs set ``pg_num`` to 4096
|
|
|
|
- If you have more than 50 OSDs, you need to understand the tradeoffs
|
|
and how to calculate the ``pg_num`` value by yourself
|
|
|
|
As the number of OSDs increases, chosing the right value for pg_num
|
|
becomes more important because it has a significant influence on the
|
|
behavior of the cluster as well as the durability of the data when
|
|
something goes wrong (i.e. the probability that a catastrophic event
|
|
leads to data loss).
|
|
|
|
How are Placement Groups used ?
|
|
===============================
|
|
|
|
A placement group (PG) aggregates objects within a pool because
|
|
tracking object placement and object metadata on a per-object basis is
|
|
computationally expensive--i.e., a system with millions of objects
|
|
cannot realistically track placement on a per-object basis.
|
|
|
|
.. ditaa::
|
|
/-----\ /-----\ /-----\ /-----\ /-----\
|
|
| obj | | obj | | obj | | obj | | obj |
|
|
\-----/ \-----/ \-----/ \-----/ \-----/
|
|
| | | | |
|
|
+--------+--------+ +---+----+
|
|
| |
|
|
v v
|
|
+-----------------------+ +-----------------------+
|
|
| Placement Group #1 | | Placement Group #2 |
|
|
| | | |
|
|
+-----------------------+ +-----------------------+
|
|
| |
|
|
+------------------------------+
|
|
|
|
|
v
|
|
+-----------------------+
|
|
| Pool |
|
|
| |
|
|
+-----------------------+
|
|
|
|
Placement groups are invisible to the Ceph user: the CRUSH algorithm
|
|
determines in which placement group the object will be
|
|
placed. Although CRUSH is a deterministic function using the object
|
|
name as a parameter, there is no way to force an object into a given
|
|
placement group.
|
|
|
|
The object's contents within a placement group are stored in a set of
|
|
OSDs. For instance, in a replicated pool of size two, each placement
|
|
group will store objects on two OSDs, as shown below.
|
|
|
|
.. ditaa::
|
|
|
|
+-----------------------+ +-----------------------+
|
|
| Placement Group #1 | | Placement Group #2 |
|
|
| | | |
|
|
+-----------------------+ +-----------------------+
|
|
| | | |
|
|
v v v v
|
|
/----------\ /----------\ /----------\ /----------\
|
|
| | | | | | | |
|
|
| OSD #1 | | OSD #2 | | OSD #2 | | OSD #3 |
|
|
| | | | | | | |
|
|
\----------/ \----------/ \----------/ \----------/
|
|
|
|
|
|
Should OSD #2 fail, another will be assigned to Placement Group #1 and
|
|
will be filled with copies of all objects in OSD #1. If the pool size
|
|
is changed from two to three, an additional OSD will be assigned to
|
|
the placement group and will receive copies of all objects in the
|
|
placement group.
|
|
|
|
Placement groups do not own the OSD, they share it with other
|
|
placement groups from the same pool or even other pools. If OSD #2
|
|
fails, the Placement Group #2 will also have to restore copies of
|
|
objects, using OSD #3.
|
|
|
|
When the number of placement groups increases, the new placement
|
|
groups will be assigned OSDs. The result of the CRUSH function will
|
|
also change and some objects from the former placement groups will be
|
|
copied over to the new Placement Groups and removed from the old ones.
|
|
|
|
Placement Groups Tradeoffs
|
|
==========================
|
|
|
|
Data durability and even distribution among all OSDs call for more
|
|
placement groups but their number should be reduced to the minimum to
|
|
save CPU and memory.
|
|
|
|
.. _data durability:
|
|
|
|
Data durability
|
|
---------------
|
|
|
|
After an OSD fails, the risk of data loss increases until the data it
|
|
contained is fully recovered. Let's imagine a scenario that causes
|
|
permanent data loss in a single placement group:
|
|
|
|
- The OSD fails and all copies of the object it contains are lost.
|
|
For all objects within the placement group the number of replica
|
|
suddently drops from three to two.
|
|
|
|
- Ceph starts recovery for this placement group by chosing a new OSD
|
|
to re-create the third copy of all objects.
|
|
|
|
- Another OSD, within the same placement group, fails before the new
|
|
OSD is fully populated with the third copy. Some objects will then
|
|
only have one surviving copies.
|
|
|
|
- Ceph picks yet another OSD and keeps copying objects to restore the
|
|
desired number of copies.
|
|
|
|
- A third OSD, within the same placement group, fails before recovery
|
|
is complete. If this OSD contained the only remaining copy of an
|
|
object, it is permanently lost.
|
|
|
|
In a cluster containing 10 OSDs with 512 placement groups in a three
|
|
replica pool, CRUSH will give each placement groups three OSDs. In the
|
|
end, each OSDs will end up hosting (512 * 3) / 10 = ~150 Placement
|
|
Groups. When the first OSD fails, the above scenario will therefore
|
|
start recovery for all 150 placement groups at the same time.
|
|
|
|
The 150 placement groups being recovered are likely to be
|
|
homogeneously spread over the 9 remaining OSDs. Each remaining OSD is
|
|
therefore likely to send copies of objects to all others and also
|
|
receive some new objects to be stored because they became part of a
|
|
new placement group.
|
|
|
|
The amount of time it takes for this recovery to complete entirely
|
|
depends on the architecture of the Ceph cluster. Let say each OSD is
|
|
hosted by a 1TB SSD on a single machine and all of them are connected
|
|
to a 10Gb/s switch and the recovery for a single OSD completes within
|
|
M minutes. If there are two OSDs per machine using spinners with no
|
|
SSD journal and a 1Gb/s switch, it will at least be an order of
|
|
magnitude slower.
|
|
|
|
In a cluster of this size, the number of placement groups has almost
|
|
no influence on data durability. It could be 128 or 8192 and the
|
|
recovery would not be slower or faster.
|
|
|
|
However, growing the same Ceph cluster to 20 OSDs instead of 10 OSDs
|
|
is likely to speed up recovery and therefore improve data durability
|
|
significantly. Each OSD now participates in only ~75 placement groups
|
|
instead of ~150 when there were only 10 OSDs and it will still require
|
|
all 19 remaining OSDs to perform the same amount of object copies in
|
|
order to recover. But where 10 OSDs had to copy approximately 100GB
|
|
each, they now have to copy 50GB each instead. If the network was the
|
|
bottleneck, recovery will happen twice as fast. In other words,
|
|
recovery goes faster when the number of OSDs increases.
|
|
|
|
If this cluster grows to 40 OSDs, each of them will only host ~35
|
|
placement groups. If an OSD dies, recovery will keep going faster
|
|
unless it is blocked by another bottleneck. However, if this cluster
|
|
grows to 200 OSDs, each of them will only host ~7 placement groups. If
|
|
an OSD dies, recovery will happen between at most of ~21 (7 * 3) OSDs
|
|
in these placement groups: recovery will take longer than when there
|
|
were 40 OSDs, meaning the number of placement groups should be
|
|
increased.
|
|
|
|
No matter how short the recovery time is, there is a chance for a
|
|
second OSD to fail while it is in progress. In the 10 OSDs cluster
|
|
described above, if any of them fail, then ~17 placement groups
|
|
(i.e. ~150 / 9 placement groups being recovered) will only have one
|
|
surviving copy. And if any of the 8 remaining OSD fail, the last
|
|
objects of two placement groups are likely to be lost (i.e. ~17 / 8
|
|
placement groups with only one remaining copy being recovered).
|
|
|
|
When the size of the cluster grows to 20 OSDs, the number of Placement
|
|
Groups damaged by the loss of three OSDs drops. The second OSD lost
|
|
will degrade ~4 (i.e. ~75 / 19 placement groups being recovered)
|
|
instead of ~17 and the third OSD lost will only lose data if it is one
|
|
of the four OSDs containing the surviving copy. In other words, if the
|
|
probability of losing one OSD is 0.0001% during the recovery time
|
|
frame, it goes from 17 * 10 * 0.0001% in the cluster with 10 OSDs to 4 * 20 *
|
|
0.0001% in the cluster with 20 OSDs.
|
|
|
|
In a nutshell, more OSDs mean faster recovery and a lower risk of
|
|
cascading failures leading to the permanent loss of a Placement
|
|
Group. Having 512 or 4096 Placement Groups is roughly equivalent in a
|
|
cluster with less than 50 OSDs as far as data durability is concerned.
|
|
|
|
Note: It may take a long time for a new OSD added to the cluster to be
|
|
populated with placement groups that were assigned to it. However
|
|
there is no degradation of any object and it has no impact on the
|
|
durability of the data contained in the Cluster.
|
|
|
|
.. _object distribution:
|
|
|
|
Object distribution within a pool
|
|
---------------------------------
|
|
|
|
Ideally objects are evenly distributed in each placement group. Since
|
|
CRUSH computes the placement group for each object, but does not
|
|
actually know how much data is stored in each OSD within this
|
|
placement group, the ratio between the number of placement groups and
|
|
the number of OSDs may influence the distribution of the data
|
|
significantly.
|
|
|
|
For instance, if there was single a placement group for ten OSDs in a
|
|
three replica pool, only three OSD would be used because CRUSH would
|
|
have no other choice. When more placement groups are available,
|
|
objects are more likely to be evenly spread among them. CRUSH also
|
|
makes every effort to evenly spread OSDs among all existing Placement
|
|
Groups.
|
|
|
|
As long as there are one or two orders of magnitude more Placement
|
|
Groups than OSDs, the distribution should be even. For instance, 300
|
|
placement groups for 3 OSDs, 1000 placement groups for 10 OSDs etc.
|
|
|
|
Uneven data distribution can be caused by factors other than the ratio
|
|
between OSDs and placement groups. Since CRUSH does not take into
|
|
account the size of the objects, a few very large objects may create
|
|
an imbalance. Let say one million 4K objects totaling 4GB are evenly
|
|
spread among 1000 placement groups on 10 OSDs. They will use 4GB / 10
|
|
= 400MB on each OSD. If one 400MB object is added to the pool, the
|
|
three OSDs supporting the placement group in which the object has been
|
|
placed will be filled with 400MB + 400MB = 800MB while the seven
|
|
others will remain occupied with only 400MB.
|
|
|
|
.. _resource usage:
|
|
|
|
Memory, CPU and network usage
|
|
-----------------------------
|
|
|
|
For each placement group, OSDs and MONs need memory, network and CPU
|
|
at all times and even more during recovery. Sharing this overhead by
|
|
clustering objects within a placement group is one of the main reasons
|
|
they exist.
|
|
|
|
Minimizing the number of placement groups saves significant amounts of
|
|
resources.
|
|
|
|
Choosing the number of Placement Groups
|
|
=======================================
|
|
|
|
If you have more than 50 OSDs, we recommend approximately 50-100
|
|
placement groups per OSD to balance out resource usage, data
|
|
durability and distribution. If you have less than 50 OSDs, chosing
|
|
among the `preselection`_ above is best. For a single pool of objects,
|
|
you can use the following formula to get a baseline::
|
|
|
|
(OSDs * 100)
|
|
Total PGs = ------------
|
|
pool size
|
|
|
|
Where **pool size** is either the number of replicas for replicated
|
|
pools or the K+M sum for erasure coded pools (as returned by **ceph
|
|
osd erasure-code-profile get**).
|
|
|
|
You should then check if the result makes sense with the way you
|
|
designed your Ceph cluster to maximize `data durability`_,
|
|
`object distribution`_ and minimize `resource usage`_.
|
|
|
|
The result should be **rounded up to the nearest power of two.**
|
|
Rounding up is optional, but recommended for CRUSH to evenly balance
|
|
the number of objects among placement groups.
|
|
|
|
As an example, for a cluster with 200 OSDs and a pool size of 3
|
|
replicas, you would estimate your number of PGs as follows::
|
|
|
|
(200 * 100)
|
|
----------- = 6667. Nearest power of 2: 8192
|
|
3
|
|
|
|
When using multiple data pools for storing objects, you need to ensure
|
|
that you balance the number of placement groups per pool with the
|
|
number of placement groups per OSD so that you arrive at a reasonable
|
|
total number of placement groups that provides reasonably low variance
|
|
per OSD without taxing system resources or making the peering process
|
|
too slow.
|
|
|
|
For instance a cluster of 10 pools each with 512 placement groups on
|
|
ten OSDs is a total of 5,120 placement groups spread over ten OSDs,
|
|
that is 512 placement groups per OSD. That does not use too many
|
|
resources. However, if 1,000 pools were created with 512 placement
|
|
groups each, the OSDs will handle ~50,000 placement groups each and it
|
|
would require significantly more resources and time for peering.
|
|
|
|
.. _setting the number of placement groups:
|
|
|
|
Set the Number of Placement Groups
|
|
==================================
|
|
|
|
To set the number of placement groups in a pool, you must specify the
|
|
number of placement groups at the time you create the pool.
|
|
See `Create a Pool`_ for details. Once you've set placement groups for a
|
|
pool, you may increase the number of placement groups (but you cannot
|
|
decrease the number of placement groups). To increase the number of
|
|
placement groups, execute the following::
|
|
|
|
ceph osd pool set {pool-name} pg_num {pg_num}
|
|
|
|
Once you increase the number of placement groups, you must also
|
|
increase the number of placement groups for placement (``pgp_num``) before your
|
|
cluster will rebalance. The ``pgp_num`` should be equal to the ``pg_num``.
|
|
To increase the number of placement groups for placement, execute the
|
|
following::
|
|
|
|
ceph osd pool set {pool-name} pgp_num {pgp_num}
|
|
|
|
|
|
Get the Number of Placement Groups
|
|
==================================
|
|
|
|
To get the number of placement groups in a pool, execute the following::
|
|
|
|
ceph osd pool get {pool-name} pg_num
|
|
|
|
|
|
Get a Cluster's PG Statistics
|
|
=============================
|
|
|
|
To get the statistics for the placement groups in your cluster, execute the following::
|
|
|
|
ceph pg dump [--format {format}]
|
|
|
|
Valid formats are ``plain`` (default) and ``json``.
|
|
|
|
|
|
Get Statistics for Stuck PGs
|
|
============================
|
|
|
|
To get the statistics for all placement groups stuck in a specified state,
|
|
execute the following::
|
|
|
|
ceph pg dump_stuck inactive|unclean|stale|undersized|degraded [--format <format>] [-t|--threshold <seconds>]
|
|
|
|
**Inactive** Placement groups cannot process reads or writes because they are waiting for an OSD
|
|
with the most up-to-date data to come up and in.
|
|
|
|
**Unclean** Placement groups contain objects that are not replicated the desired number
|
|
of times. They should be recovering.
|
|
|
|
**Stale** Placement groups are in an unknown state - the OSDs that host them have not
|
|
reported to the monitor cluster in a while (configured by ``mon_osd_report_timeout``).
|
|
|
|
Valid formats are ``plain`` (default) and ``json``. The threshold defines the minimum number
|
|
of seconds the placement group is stuck before including it in the returned statistics
|
|
(default 300 seconds).
|
|
|
|
|
|
Get a PG Map
|
|
============
|
|
|
|
To get the placement group map for a particular placement group, execute the following::
|
|
|
|
ceph pg map {pg-id}
|
|
|
|
For example::
|
|
|
|
ceph pg map 1.6c
|
|
|
|
Ceph will return the placement group map, the placement group, and the OSD status::
|
|
|
|
osdmap e13 pg 1.6c (1.6c) -> up [1,0] acting [1,0]
|
|
|
|
|
|
Get a PGs Statistics
|
|
====================
|
|
|
|
To retrieve statistics for a particular placement group, execute the following::
|
|
|
|
ceph pg {pg-id} query
|
|
|
|
|
|
Scrub a Placement Group
|
|
=======================
|
|
|
|
To scrub a placement group, execute the following::
|
|
|
|
ceph pg scrub {pg-id}
|
|
|
|
Ceph checks the primary and any replica nodes, generates a catalog of all objects
|
|
in the placement group and compares them to ensure that no objects are missing
|
|
or mismatched, and their contents are consistent. Assuming the replicas all
|
|
match, a final semantic sweep ensures that all of the snapshot-related object
|
|
metadata is consistent. Errors are reported via logs.
|
|
|
|
|
|
Revert Lost
|
|
===========
|
|
|
|
If the cluster has lost one or more objects, and you have decided to
|
|
abandon the search for the lost data, you must mark the unfound objects
|
|
as ``lost``.
|
|
|
|
If all possible locations have been queried and objects are still
|
|
lost, you may have to give up on the lost objects. This is
|
|
possible given unusual combinations of failures that allow the cluster
|
|
to learn about writes that were performed before the writes themselves
|
|
are recovered.
|
|
|
|
Currently the only supported option is "revert", which will either roll back to
|
|
a previous version of the object or (if it was a new object) forget about it
|
|
entirely. To mark the "unfound" objects as "lost", execute the following::
|
|
|
|
ceph pg {pg-id} mark_unfound_lost revert|delete
|
|
|
|
.. important:: Use this feature with caution, because it may confuse
|
|
applications that expect the object(s) to exist.
|
|
|
|
|
|
.. toctree::
|
|
:hidden:
|
|
|
|
pg-states
|
|
pg-concepts
|
|
|
|
|
|
.. _Create a Pool: ../pools#createpool
|