From 5e5a47b8a789c9e74265720be64352284978cfdb Mon Sep 17 00:00:00 2001 From: Michael Chang Date: Thu, 19 Mar 2020 13:56:14 +0800 Subject: [PATCH] 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 Reviewed-by: Daniel Kiper --- docs/grub.texi | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+) diff --git a/docs/grub.texi b/docs/grub.texi index 83979af38..8e6f9acec 100644 --- 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