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Documentation/filesystem/dax: DAX on virtiofs

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Record DAX on virtiofs and the semantic difference with that on ext4
and xfs.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
This commit is contained in:
Jeffle Xu 2021-11-25 15:05:30 +08:00 committed by Miklos Szeredi
parent c3cb6f935e
commit 073c3ab6ae
1 changed files with 18 additions and 2 deletions
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20
Documentation/filesystems/dax.rst
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@ -23,8 +23,8 @@ 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.
Currently 3 filesystems support `DAX`: ext2, ext4 and xfs. Enabling `DAX` on them
is different.
Currently 4 filesystems support `DAX`: ext2, ext4, xfs and virtiofs.
Enabling `DAX` on them is different.
Enabling DAX on ext2
--------------------
@ -168,6 +168,22 @@ if the underlying media does not support dax and/or the filesystem is
overridden with a mount option.
Enabling DAX on virtiofs
----------------------------
The semantic of DAX on virtiofs is basically equal to that on ext4 and xfs,
except that when '-o dax=inode' is specified, virtiofs client derives the hint
whether DAX shall be enabled or not from virtiofs server through FUSE protocol,
rather than the persistent `FS_XFLAG_DAX` flag. That is, whether DAX shall be
enabled or not is completely determined by virtiofs server, while virtiofs
server itself may deploy various algorithm making this decision, e.g. depending
on the persistent `FS_XFLAG_DAX` flag on the host.
It is still supported to set or clear persistent `FS_XFLAG_DAX` flag inside
guest, but it is not guaranteed that DAX will be enabled or disabled for
corresponding file then. Users inside guest still need to call statx(2) and
check the statx flag `STATX_ATTR_DAX` to see if DAX is enabled for this file.
Implementation Tips for Block Driver Writers
--------------------------------------------