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 Hard Disk and File System Recommendations
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Hard Disk Prep
==============

Ceph aims for data safety, which means that when the application receives notice
that data was written to the disk, that data was actually written to the disk.
For old kernels (<2.6.33), disable the write cache if the journal is on a raw
disk. Newer kernels should work fine.

Use ``hdparm`` to disable write caching on the hard disk::

	sudo hdparm -W 0 /dev/hda 0

In production environments, we recommend running OSDs with an operating system
disk, and a separate disk(s) for data. If you run data and an operating system
on a single disk, create a separate partition for your data before configuring
your OSD cluster.


File Systems
============

Ceph OSDs rely heavily upon the stability and performance of the
underlying file system.

.. note:: We currently recommend ``XFS`` for production deployments.
   We recommend ``btrfs`` for testing, development, and any
   non-critical deployments.  We believe that ``btrfs`` has the correct
   feature set and roadmap to serve Ceph in the long-term, but ``XFS``
   and ``ext4`` provide the necessary stability for today's deployments.
   ``btrfs`` development is proceeding rapidly: users should be
   comfortable installing the latest released upstream kernels and be
   able to track development activity for critical bug fixes.

Ceph OSDs depend on the Extended Attributes (XATTRs) of the underlying
file system for various forms of internal object state and metadata.
The underlying file system must provide sufficient capacity for
XATTRs.  ``btrfs`` does not bound the total xattr metadata stored with
a file.  ``XFS`` has a relatively large limit (64 KB) that most
deployments won't encounter, but the ``ext4`` is too small to be
usable.

You should always add the following line to the ``[osd]`` section of your
``ceph.conf`` file for ``ext4`` filesystems; you can optionally use
it for ``btrfs`` and ``XFS``.::

	filestore xattr use omap = true

FS Background Info
==================

The ``XFS`` and ``btrfs`` file systems provide numerous advantages in highly 
scaled data storage environments when `compared`_ to ``ext3`` and ``ext4``.
Both ``XFS`` and ``btrfs`` are `journaling file systems`_, which means that
they are more robust when recovering from crashes, power outages, etc. These
filesystems journal all of the changes they will make before performing writes.

``XFS`` was developed for Silicon Graphics, and is a mature and stable
filesystem. By contrast, ``btrfs`` is a relatively new file system that aims
to address the long-standing wishes of system administrators working with 
large scale data storage environments. ``btrfs`` has some unique features
and advantages compared to other Linux filesystems. 

``btrfs`` is a `copy-on-write`_ filesystem. It supports file creation
timestamps and checksums that verify metadata integrity, so it can detect
bad copies of data and fix them with the good copies. The copy-on-write 
capability means that ``btrfs`` can support snapshots that are writable.
``btrfs`` supports transparent compression and other features.

``btrfs`` also incorporates multi-device management into the file system,
which enables you to support heterogeneous disk storage infrastructure,
data allocation policies. The community also aims to provide ``fsck``, 
deduplication, and data encryption support in the future. This compelling 
list of features makes ``btrfs`` the ideal choice for Ceph clusters.

.. _copy-on-write: http://en.wikipedia.org/wiki/Copy-on-write
.. _compared: http://en.wikipedia.org/wiki/Comparison_of_file_systems
.. _journaling file systems: http://en.wikipedia.org/wiki/Journaling_file_system