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doc/cephfs/createfs: Recommend default data pool on SSDs for non-EC

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Signed-off-by: Niklas Hambüchen <mail@nh2.me>
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Niklas Hambüchen 2025-10-14 03:58:08 +02:00
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doc/cephfs/createfs.rst
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@ -22,12 +22,28 @@ There are important considerations when planning these pools:
- The data pool used to create the file system is the "default" data pool and
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
have at least one RADOS 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`).
- For the same reason, even if you are not using erasure coding
and plan to store all or most of your files on HDDs,
it is recommended to set the default data pool to an SSD pool
and set a file layout for top-level directories to an HDD pool.
This gives you the option to move small files and inode objects completely off HDDs
in the future using file layouts, without having to re-create the pool from scratch.
That reduces scrub and recovery times when you have many small files,
as those operations cause at least one HDD seek per RADOS object.
This optimization cannot be retrofitted in place when deploying a CephFS
file system with an HDD default data pool,
and the default data pool cannot be subsequently removed without creating
an entirely new CephFS filesystem and migrating all files.
This strategy requires only modest capacity in the SSD default data pool
when subdirectories are aligned with an HDD data pool,
but accelerates various operations and sets your file system up for
future flexibility.
Refer to :doc:`/rados/operations/pools` to learn more about managing pools. For
example, to create two pools with default settings for use with a file system, you