Hard Disk and File System Recommendations

Hard Drive Prep

Ceph aims for data safety, which means that when the Ceph Client receives notice that data was written to a storage drive, that data was actually written to the storage drive. For old kernels (<2.6.33), disable the write cache if the journal is on a raw drive. 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 a Ceph OSD Daemon with separate drives for the operating system and the data. If you run data and an operating system on a single disk, we recommend creating a separate partition for your data.

Filesystems

Ceph OSD Daemons rely heavily upon the stability and performance of the underlying filesystem.

Recommended

We currently recommend XFS for production deployments.

We used to recommend btrfs for testing, development, and any non-critical deployments becuase it has the most promising set of features. However, we now plan to avoid using a kernel file system entirely with the new BlueStore backend. btrfs is still supported and has a comparatively compelling set of features, but be mindful of its stability and support status in your Linux distribution.

Not recommended

We recommend against using ext4 due to limitations in the size of xattrs it can store, and the problems this causes with the way Ceph handles long RADOS object names. Although these issues will generally not surface with Ceph clusters using only short object names (e.g., an RBD workload that does not include long RBD image names), other users like RGW make extensive use of long object names and can break.

Starting with the Jewel release, the ceph-osd daemon will refuse to start if the configured max object name cannot be safely stored on ext4. If the cluster is only being used with short object names (e.g., RBD only), you can continue using ext4 by setting the following configuration option:

osd max object name len = 256
osd max object namespace len = 64

Note

This may result in difficult-to-diagnose errors if you try to use RGW or other librados clients that do not properly handle or politely surface any resulting ENAMETOOLONG errors.

Filesystem Background Info

The XFS, btrfs and ext4 file systems provide numerous advantages in highly scaled data storage environments when compared to ext3.

XFS, btrfs and ext4 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.