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fs_mount_options
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fs_mount_options [2020/06/12 16:05] Ira Weiny [Details] |
fs_mount_options [2020/10/27 17:01] Ira Weiny |
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| - | **NOTE: kernel 5.8 is yet to be released. This document is therefore preliminary until it's release and subject to change.** | ||
| - | |||
| ====== Prior to kernel 5.8 ====== | ====== Prior to kernel 5.8 ====== | ||
| Line 14: | Line 12: | ||
| 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 47: | ||
| 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 68: | ||
| </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 85: | ||
| 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 100: | ||
| 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 116: | ||
| - | 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
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