docs: Document notes on LVM cache booting

Add notes on LVM cache booting to the GRUB manual to help user understanding
the outstanding issue and status.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

diff --git a/docs/grub.texi b/docs/grub.texi
index 83979af381d80edb7325b323e80d9d27fbb4c217..8e6f9acecfa1dc89a9834d2e633fb5af2809a02f 100644 (file)
--- a/docs/grub.texi
+++ b/docs/grub.texi
@@ -894,6 +894,7 @@ magic.
 @menu
 * General boot methods::        How to boot OSes with GRUB generally
 * Loopback booting::            Notes on booting from loopbacks
+* LVM cache booting::           Notes on booting from LVM cache logical volume
 * OS-specific notes::           Notes on some operating systems
 @end menu
 
@@ -991,6 +992,26 @@ way. Please consider alternative boot methods like copying all files
 from the image to actual partition. Consult your OS documentation for
 more details
 
+@node LVM cache booting
+@section Booting from LVM cache logical volume
+
+The LVM cache logical volume is the logical volume consisting of the original
+and the cache pool logical volume. The original is usually on a larger and
+slower storage device while the cache pool is on a smaller and faster one. The
+performance of the original volume can be improved by storing the frequently
+used data on the cache pool to utilize the greater performance of faster
+device.
+
+GRUB boots from LVM cache logical volume merely by reading it's original
+logical volume so that dirty data in cache pool volume is disregarded. This is
+not a problem for "writethrough" cache mode as it ensures that any data written
+will be stored both on the cache and the origin LV. For the other cache mode
+"writeback", which delays writing from the cache pool back to the origin LV to
+boost performance, GRUB may fail to boot in the wake of accidental power outage
+due to it's inability to assemble the cache device for reading the required
+dirty data left behind. The situation will be improved after adding full
+support to the LVM cache logical volume in the future.
+
 @node OS-specific notes
 @section Some caveats on OS-specific issues