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Ubuntu 18.04: Expand the atime note

Richard Laager 2018-09-23 23:17:23 -05:00
parent 7571a9e96d
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Ubuntu-18.04-Root-on-ZFS.md

@ -107,7 +107,7 @@ Choose one of the following options:
rpool /dev/mapper/luks1
**Notes:**
* If you or one of your applications cares about atime updates, you probably want `relatime`. In that case, leave `atime=off`, but also set `relatime=on`. In later steps, when `noatime` is used in `/etc/fstab`, use `relatime` instead. If you only care about atime updates for particular filesystems, you can limit your `relatime` to only those filesystems.
* Setting `atime=off` completely disables atime updates. If you or one of your applications (e.g. a mail reader following an mbox file for local delivery) cares about atime updates, you almost certainly want `relatime` rather than full `atime`. In that case, leave `atime=off`, but also set `relatime=on`. In steps below, when `noatime` is used in `/etc/fstab`, use `relatime` instead. If you only care about atime updates for particular filesystems, you can limit your `relatime` to only those filesystems. See [RedHat's documentation](https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/power_management_guide/relatime) for further information.
* The use of `ashift=12` is recommended here because many drives today have 4KiB (or larger) physical sectors, even though they present 512B logical sectors. Also, a future replacement drive may have 4KiB physical sectors (in which case `ashift=12` is desirable) or 4KiB logical sectors (in which case `ashift=12` is required).
* Setting `normalization=formD` eliminates some corner cases relating to UTF-8 filename normalization. It also implies `utf8only=on`, which means that only UTF-8 filenames are allowed. If you care to support non-UTF-8 filenames, do not use this option. For a discussion of why requiring UTF-8 filenames may be a bad idea, see [The problems with enforced UTF-8 only filenames](http://utcc.utoronto.ca/~cks/space/blog/linux/ForcedUTF8Filenames).
* Setting `xattr=sa` [vastly improves the performance of extended attributes](https://github.com/zfsonlinux/zfs/commit/82a37189aac955c81a59a5ecc3400475adb56355). Inside ZFS, extended attributes are used to implement POSIX ACLs (e.g. `acltype=posixacl`) discussed later. Extended attributes can also be used by user-space applications. [They are used by some desktop GUI applications.](https://en.wikipedia.org/wiki/Extended_file_attributes#Linux) [They can be used by Samba to store Windows ACLs and DOS attributes; they are required for a Samba Active Directory domain controller.](https://wiki.samba.org/index.php/Setting_up_a_Share_Using_Windows_ACLs) Note that [`xattr=sa` is Linux-specific.](http://open-zfs.org/wiki/Platform_code_differences) If you move your `xattr=sa` pool to another OpenZFS implementation besides ZFS-on-Linux, extended attributes will not be readable. If this is important to you, omit the `-O xattr=sa` above.