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docs: Document notes on LVM cache booting

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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>
This commit is contained in:
Michael Chang 2020-03-19 13:56:14 +08:00 committed by Daniel Kiper
parent 0454b04453
commit 5e5a47b8a7
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docs/grub.texi
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@ -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