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doc/cephfs: Clarify CephFS pool guidance

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Signed-off-by: Anthony D'Atri <anthony.datri@gmail.com>
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Anthony D'Atri 2022-01-21 12:26:52 -08:00
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doc/cephfs/createfs.rst
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@ -6,19 +6,22 @@ Creating pools
==============
A Ceph file system requires at least two RADOS pools, one for data and one for metadata.
When configuring these pools, you might consider:
There are important considerations when planning these pools:
- Using a higher replication level for the metadata pool, as any data loss in
this pool can render the whole file system inaccessible.
- Using lower-latency storage such as SSDs for the metadata pool, as this will
directly affect the observed latency of file system operations on clients.
- We recommend configuring *at least* 3 replicas for the metadata pool,
as data loss in this pool can render the entire file system inaccessible.
Configuring 4 would not be extreme, especially since the metadata pool's
capacity requirements are quite modest.
- We recommend the fastest feasible low-latency storage devices (NVMe, Optane,
or at the very least SAS/SATA SSD) for the metadata pool, as this will
directly affect the latency of client file system operations.
- The data pool used to create the file system is the "default" data pool and
the location for storing all inode backtrace information, used for hard link
management and disaster recovery. For this reason, all inodes created in
CephFS have at least one object in the default data pool. If erasure-coded
pools are planned for the file system, it is usually better to use a
replicated pool for the default data pool to improve small-object write and
read performance for updating backtraces. Separately, another erasure-coded
the location for storing all inode backtrace information, which is used for hard link
management and disaster recovery. For this reason, all CephFS inodes
have at least one object in the default data pool. If erasure-coded
pools are planned for file system data, it is best to configure the default as
a replicated pool to improve small-object write and
read performance when updating backtraces. Separately, another erasure-coded
data pool can be added (see also :ref:`ecpool`) that can be used on an entire
hierarchy of directories and files (see also :ref:`file-layouts`).
@ -31,7 +34,7 @@ might run the following commands:
$ ceph osd pool create cephfs_data
$ ceph osd pool create cephfs_metadata
Generally, the metadata pool will have at most a few gigabytes of data. For
The metadata pool will typically hold at most a few gigabytes of data. For
this reason, a smaller PG count is usually recommended. 64 or 128 is commonly
used in practice for large clusters.
@ -94,7 +97,7 @@ You may use Erasure Coded pools as CephFS data pools as long as they have overwr
ceph osd pool set my_ec_pool allow_ec_overwrites true
Note that EC overwrites are only supported when using OSDS with the BlueStore backend.
Note that EC overwrites are only supported when using OSDs with the BlueStore backend.
You may not use Erasure Coded pools as CephFS metadata pools, because CephFS metadata is stored using RADOS *OMAP* data structures, which EC pools cannot store.