Docker on CentOS 7 machine with XFS filesystem can cause trouble when d_type is not supported

Written by Pim on Monday August 27, 2018 - Comment - Permalink
Categories: docker, devops, technology - Tags: docker, centos7, centos, xfs, storage

Docker, CentOS 7 and the XFS filesystem could be a trouble giving combination if you don't meet all the specifications of the overlay/overlay2 storage driver. The overlay storage driver relies on a technology called "directory entry type" (d_type) and is used to describe information of a directory on the filesystem.

The issue with CentOS 7, XFS and d_type support

Older CentOS 7 installations aren't configured with d_type support. This can cause some issues with chowning and chmoding, deleting and creating files and directories because the overlay storage driver can't find directory type (d_type) entries in the Linux kernel.

You can check if your existing XFS filesystem has d_type enabled by running the xfs_info command. An example output:

$ xfs_info /mount-point

meta-data=/mount-point isize=512    agcount=4, agsize=2620928 blks
         =                       sectsz=512   attr=2, projid32bit=1
         =                       crc=1        finobt=0 spinodes=0
data     =                       bsize=4096   blocks=10483712, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=1
log      =internal               bsize=4096   blocks=5119, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

The 3rd column of the 6th line of the xfs_info output is the most interesting because it contains the parameter ftype which should be 1. When ftype is 0, d_type support is disabled. When it is 1, d_type support is enabled and you're safe to use the overlay(2) storage driver with Docker on an XFS filesystem.

My XFS filesystem has no d_type support, how to resolve this?

Short answer

You've to create a new XFS filesystem with d_type support enabled.

Less short answer

You've two options: either you add a new disk and you create a new XFS partition on it, otherwise, you've to backup your existing data and recreate the XFS filesystem with d_type support enabled. Creating a new XFS filesystem with d_type enabled is as easy at the following command:

$ mkfs.xfs -n ftype=1 /mount-point

Unfortunately, it isn't possible to enable d_type support on an existing filesystem.

Docker 1.13.x and higher

The newer Docker version (1.13.x and higher) will show you if d_type is enabled by running docker info:

$ docker info

Server Version: 1.13.1
Storage Driver: overlay2
 Backing Filesystem: xfs
 Supports d_type: true
 Native Overlay Diff: true

Action required!

If d_type isn't supported, you've to change this as soon as possible to prevent strange filesystem issues with Docker containers.

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