User Tools

Site Tools


fs_mount_options

Differences

This shows you the differences between two versions of the page.

Link to this comparison view

Both sides previous revision Previous revision
Next revision
Previous revision
Last revision Both sides next revision
fs_mount_options [2020/06/12 16:05]
Ira Weiny [Details]
fs_mount_options [2020/08/03 03:55]
Ira Weiny [xfs]
Line 14: Line 14:
 As of kernel 5.8 additional DAX enabling options are available on some file systems, specifically:​ As of kernel 5.8 additional DAX enabling options are available on some file systems, specifically:​
  
-===== ext4 and ext2 =====+===== 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. 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.
  
-===== xfs =====+===== ext4 and xfs =====
  
-As of kernel 5.8 xfs supports the new per-file dax configuration.+As of kernel 5.8 ext4 and xfs supports the new per-file dax configuration.
  
 ==== Summary ==== ==== Summary ====
Line 49: Line 49:
     remounted with dax=inode and the inode is evicted from kernel memory.     remounted with dax=inode and the inode is evicted from kernel memory.
  
-==== Details ​====+==== Examples and 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). 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'​.+FS_XFLAG_DAX is preserved within the file system.  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 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. sub-tree.
  
-To clarify inheritance, here are 3 examples when using the "-o dax=inode" [default] ​option:+To clarify inheritance ​mount the file system with the inode option.
  
 <​code>​ <​code>​
Line 70: Line 70:
 </​code>​ </​code>​
  
-=== Example A: ===+And here are 3 examples showing how to enable dax on individual files and/or directories. 
 + 
 +=== Inheritance ​Example A: ===
  
 <​code>​ <​code>​
 $ mkdir -p a/b/c $ mkdir -p a/b/c
 $ xfs_io -c '​chattr +x' a $ xfs_io -c '​chattr +x' a
-$ mkdir a/b/c/d +$ xfs_io -c '​lsattr'​ a 
-$ mkdir a/e+--------------x- a 
 +$ mkdir -p a/b/c/d 
 +$ mkdir -p a/e
 </​code>​ </​code>​
         Results in:         Results in:
Line 83: Line 87:
         no dax: b,c,d         no dax: b,c,d
  
-=== Example B: ===+=== Inheritance ​Example B: ===
  
 <​code>​ <​code>​
 $ mkdir a $ mkdir a
 $ xfs_io -c '​chattr +x' a $ xfs_io -c '​chattr +x' a
 +$ xfs_io -c '​lsattr'​ a
 +--------------x- a
 $ mkdir -p a/b/c/d $ mkdir -p a/b/c/d
 </​code>​ </​code>​
Line 96: Line 102:
         no dax:         no dax:
  
-=== Example C: === +=== Inheritance ​Example C: ===
  
 <​code>​ <​code>​
 $ mkdir -p a/b/c $ mkdir -p a/b/c
 $ xfs_io -c '​chattr +x' c $ xfs_io -c '​chattr +x' c
-$ mkdir a/b/c/d+$ xfs_io -c '​lsattr'​ c 
 +--------------x- c 
 +$ mkdir -p a/b/c/d
 </​code>​ </​code>​
  
Line 111: Line 118:
  
  
-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.+=== Detecting if a file is using dax === 
 + 
 +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 file system'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. 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.
 +
 +Continuing with Example C above we create a file foo in a dax enabled directory and it is enabled for dax with the FS_XFLAG_DAX set as well as S_DAX being set.
 +
 +<​code>​
 +$ touch a/b/c/foo
 +$ xfs_io -c '​lsattr'​ a/​b/​c/​foo ​                      # FS_XFLAG_DAX == true
 +--------------x- a/b/c/foo
 +$ xfs_io -c 'statx -r' a/b/c/foo | grep attributes ​  # S_DAX == true
 +stat.attributes = 0x2000
 +</​code>​
  
 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. 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.
  
fs_mount_options.txt · Last modified: 2020/10/27 17:01 by Ira Weiny