more zfs docs

docs/zfs.md

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+# ZFS
+
+Swan runs its root on ext4. The ZFS drives are managed imperatively (this
+[disko configuration](../disks/zfs.nix) is an unused work-in-progress).
+
+The basic ZFS settings are managed [here](../modules/nixos/hardware/zfs.nix).
+
+## Creating a New Dataset
+
+```
+sudo zfs create tank/mydataset
+sudo zfs set compression=zstd tank/myzstddataset
+sudo zfs set mountpoint=/data/mydataset tank/mydataset
+```
+
+## Maintenance
+
+### Get Status
+
+```
+sudo zpool status
+```
+
+### Replace Disk
+
+```
+sudo zdb
+sudo zpool status -g # Show by GUID
+sudo zpool offline tank <GUID>
+sudo zpool status
+# Remove old disk, insert new disk
+sudo zdb
+sudo zpool replace tank <OLD GUID> /dev/disk/by-id/<NEW PATH>
+sudo zpool status
+```
+
+## Initial Setup
+
+```
+sudo zpool create tank raidz1 sda sdb sdc
+sudo zpool set ashift=12 tank
+sudo zpool set autoexpand=on tank
+sudo zpool set compression=on tank
+```
+