add `xattr=sa` (woinder why I didn't do that earlier)

Debian-Stretch-Root-on-ZFS.md

@@ -71,13 +71,15 @@ Always use the long `/dev/disk/by-id/*` aliases with ZFS.  Using the `/dev/sd*`
 
 2.3  Create the root pool:
 
-    # zpool create -o ashift=12 -o feature@large_dnode=disabled \
-          -O atime=off -O canmount=off -O compression=lz4 -O normalization=formD \
+    # zpool create -o ashift=12 \
+          -o feature@large_dnode=disabled \
+          -O atime=off -O canmount=off -O compression=lz4 -O normalization=formD -O xattr=sa \
           -O mountpoint=/ -R /mnt \
           rpool /dev/disk/by-id/scsi-SATA_disk1-part1
 
 **Notes:**
 * 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 `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.
 * 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).
 * Make sure to include the `-part1` portion of the drive path. If you forget that, you are specifying the whole disk, which ZFS will then re-partition, and you will lose the bootloader partition(s).
 * **[GRUB doesn't support all pool features](http://savannah.gnu.org/bugs/?func=detailitem&item_id=48885) of 0.7 release**, if you install ZFS from backports or use Proxmox, disable them: `-o feature@large_dnode=disabled`. If you use 0.6 release, remove this text from command.