document shrinking LVMs

howto/lvm.md

@@ -156,3 +156,54 @@ should do the following:
  
         mount /srv
         service apache2 start
+
+# Shrinking
+
+ 1. unmount the filesystem:
+ 
+        umount /mnt
+      
+    If that's not possible because the filesystem is in use, you'll
+    need to stop processes using it. If that's possible (for example
+    when resizing `/`), you'll need to reboot in a separate operating
+    system first.
+
+ 2. forcibly check the filesystem:
+ 
+        e2fsck -fy /dev/sdX
+
+ 3. shrink the filesystem:
+ 
+        resize2fs /dev/sdX 5G
+
+ 4. shrink the logical volume, to reduce *to* 5G:
+ 
+        lvreduce -L 5G vg/mnt
+
+    to reduce *by* 5G:
+
+        lvreduce -L -5G vg/mnt
+
+    WARNING: make sure the resulting size matches exactly the one
+    specified in the `resize2fs` command above.
+
+ 5. tell ext to resize the filesystem *again*:
+
+        resize2fs /dev/sdX
+
+    NOTE: no size argument here!
+
+ 6. check the filesystem again:
+
+        e2fsck -fy /dev/sdX
+
+ 7. if you want to resize the underlying device (for example, if this
+    is a LVM inside a virtual machine on top of another LVM), you can
+    also shrink the parent logical volume, physical volume, and crypto
+    device (if relevant) at this point.
+
+        lvreduce -L 5G vg/hostname
+        pvresize /dev/sdY
+        cryptsetup resize DEVICE_LABEL
+
+    WARNING: this last step has not been tested.