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Documentation/dax: Update Usage section

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Update the Usage section to reflect the new individual dax selection
functionality.

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
This commit is contained in:
Ira Weiny 2020-04-30 07:41:34 -07:00 committed by Darrick J. Wong
parent 712b2698e4
commit 83d9088659
1 changed files with 139 additions and 3 deletions
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142
Documentation/filesystems/dax.txt
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@ -20,8 +20,144 @@ Usage
If you have a block device which supports DAX, you can make a filesystem
on it as usual. The DAX code currently only supports files with a block
size equal to your kernel's PAGE_SIZE, so you may need to specify a block
size when creating the filesystem. When mounting it, use the "-o dax"
option on the command line or add 'dax' to the options in /etc/fstab.
size when creating the filesystem.
Currently 3 filesystems support DAX: ext2, ext4 and xfs. Enabling DAX on them
is different.
Enabling DAX on ext4 and ext2
-----------------------------
When mounting the filesystem, use the "-o dax" option on the command line or
add 'dax' to the options in /etc/fstab. This works to enable DAX on all files
within the filesystem. It is equivalent to the '-o dax=always' behavior below.
Enabling DAX on xfs
-------------------
Summary
-------
1. There exists an in-kernel file access mode flag S_DAX that corresponds to
the statx flag STATX_ATTR_DAX. See the manpage for statx(2) for details
about this access mode.
2. There exists a persistent flag FS_XFLAG_DAX that can be applied to regular
files and directories. This advisory flag can be set or cleared at any
time, but doing so does not immediately affect the S_DAX state.
3. If the persistent FS_XFLAG_DAX flag is set on a directory, this flag will
be inherited by all regular files and subdirectories that are subsequently
created in this directory. Files and subdirectories that exist at the time
this flag is set or cleared on the parent directory are not modified by
this modification of the parent directory.
4. There exist dax mount options which can override FS_XFLAG_DAX in the
setting of the S_DAX flag. Given underlying storage which supports DAX the
following hold:
"-o dax=inode" means "follow FS_XFLAG_DAX" and is the default.
"-o dax=never" means "never set S_DAX, ignore FS_XFLAG_DAX."
"-o dax=always" means "always set S_DAX ignore FS_XFLAG_DAX."
"-o dax" is a legacy option which is an alias for "dax=always".
This may be removed in the future so "-o dax=always" is
the preferred method for specifying this behavior.
NOTE: Modifications to and the inheritance behavior of FS_XFLAG_DAX remain
the same even when the filesystem is mounted with a dax option. However,
in-core inode state (S_DAX) will be overridden until the filesystem is
remounted with dax=inode and the inode is evicted from kernel memory.
5. The S_DAX policy can be changed via:
a) Setting the parent directory FS_XFLAG_DAX as needed before files are
created
b) Setting the appropriate dax="foo" mount option
c) Changing the FS_XFLAG_DAX flag on existing regular files and
directories. This has runtime constraints and limitations that are
described in 6) below.
6. When changing the S_DAX policy via toggling the persistent FS_XFLAG_DAX flag,
the change in behaviour for existing regular files may not occur
immediately. If the change must take effect immediately, the administrator
needs to:
a) stop the application so there are no active references to the data set
the policy change will affect
b) evict the data set from kernel caches so it will be re-instantiated when
the application is restarted. This can be achieved by:
i. drop-caches
ii. a filesystem unmount and mount cycle
iii. a system reboot
Details
-------
There are 2 per-file dax flags. One is a persistent inode setting (FS_XFLAG_DAX)
and the other is a volatile flag indicating the active state of the feature
(S_DAX).
FS_XFLAG_DAX is preserved within the filesystem. This persistent config
setting can be set, cleared and/or queried using the FS_IOC_FS[GS]ETXATTR ioctl
(see ioctl_xfs_fsgetxattr(2)) or an utility such as 'xfs_io'.
New files and directories automatically inherit FS_XFLAG_DAX from
their parent directory _when_ _created_. Therefore, setting FS_XFLAG_DAX at
directory creation time can be used to set a default behavior for an entire
sub-tree.
To clarify inheritance, here are 3 examples:
Example A:
mkdir -p a/b/c
xfs_io -c 'chattr +x' a
mkdir a/b/c/d
mkdir a/e
dax: a,e
no dax: b,c,d
Example B:
mkdir a
xfs_io -c 'chattr +x' a
mkdir -p a/b/c/d
dax: a,b,c,d
no dax:
Example C:
mkdir -p a/b/c
xfs_io -c 'chattr +x' c
mkdir a/b/c/d
dax: c,d
no dax: a,b
The current enabled state (S_DAX) is set when a file inode is instantiated in
memory by the kernel. It is set based on the underlying media support, the
value of FS_XFLAG_DAX and the filesystem's dax mount option.
statx can be used to query S_DAX. NOTE that only regular files will ever have
S_DAX set and therefore statx will never indicate that S_DAX is set on
directories.
Setting the FS_XFLAG_DAX flag (specifically or through inheritance) occurs even
if the underlying media does not support dax and/or the filesystem is
overridden with a mount option.
Implementation Tips for Block Driver Writers
@ -94,7 +230,7 @@ sysadmins have an option to restore the lost data from a prior backup/inbuilt
redundancy in the following ways:
1. Delete the affected file, and restore from a backup (sysadmin route):
This will free the file system blocks that were being used by the file,
This will free the filesystem blocks that were being used by the file,
and the next time they're allocated, they will be zeroed first, which
happens through the driver, and will clear bad sectors.