Andreas Jellinghaus wrote:
Hi Edward,
Hello.
I saw your mail on btrfs ml with the grub patches and the notes
how to deal with btrfs.
can you explain how grub and btrfs work exactly?
I read the grub manual at
http://www.gnu.org/software/grub/manual/html_node/Bootstrap-
tricks.html#Bootstrap-tricks
so I wonder: does btrfs provide a "boot loader area" similar
to ffs and reiserfs, where grub places the stage 1.5 code
and stage 1 can read it?
Yes.
or does grub find out the sectors of the stage 1.5 file and
put the list of those into the stage 1 file (first sector address
of stage 1.5 file) and stage 1.5 first sector (list of all
other sectors)?
Grub doesn't make the blocklist for stage1_5
Optionally grub makes the blocklist for stage2 (in particular
when stage1.5 can not be embedded for some reasons).
Note, that it is important to embed and use btrfs_stage1_5.
First, because btrfs has defragmentator, which can make the
blocklist out of date (so that you'll need to reinstall grub).
Second, I am not sure, if the blocklist will be composed
correctly in the case when stage2 locates in btrfs volume
(I didn't look at the blocklist specification: it can happen that
grub installer makes an assumption that all sectors of stage2
locate on the same device).
and what would be the proper way to make boot from a raid1 device?
so that if one disk fails the other can boot?
device (hd0) /dev/sda
root (hd0,X)
setup (hd0)
device (hd0) /dev/sdb
root (hd0,X)
setup (hd0)
Perhaps, it will work. But officially grub doesn't
understand software raid, and I am not familiar
with raid1 specifications, so... ;)
if data is written to mbr only and the sectors between mbr and
the first partition, this could work. but if data is written to
mbr and to btrfs (either a "boot loader area" or changes to the
stage 1.5 file), then that data can only contain the valid block
lists for one of the two hard disks - which shouldn't be a problem
if drives are the same and have 100% the same geometry and partitioning.
so I wonder, and it would be great if thise fine details would be
documented for btrfs somewhere, as there is very little information
about them (the situation isn't better with other filesystems either).
thanks for your help!
Regards, Andreas
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