On 12/26/19 10:54 PM, Chris Murphy wrote:
On Tue, Dec 24, 2019 at 2:56 PM Cameron Simpson <cs@xxxxxxxxxx> wrote:
Oh yes, one more thing. If you do the RAID1 thing: either make a shiny
new RAID1 and copy to it, or practice the transition with test drives.
Do not risk your high value data by trying to "in place RAID1" it.
I'm not sure if this is even possible with mdadm or lvm. For sure they
have no way of knowing which mirror is correct. But even if it's
possible, it's a bit complicated because it implies repartitioning in
order to make room for the necessary metadata area.
The "which copy is correct" problem is solvable:
1) you can create a 1-disk RAID1 on the partition with the correct data
(it will tell you your config is stupid but you can force it)
2) you then tell mdadm to change the number of drives to 2 (mdadm --grow);
this will be a 2-disk RAID1 working in 1-disk degraded mode
3) you then tell mdadm you have a new drive for that RAID1 (mdadm --add);
sync from 1st disk to 2nd will begin
The metadata area problem is a bit tricky, but it is not necessary
to repartition, just make your filesystem a bit smaller than it is now.
So:
a) unmount the filesystem
b) resize the filesystem to 100MB smaller (actually you need just a few kB,
but let's play very very safe, we will get the space back later)
c) create the RAID etc., the steps described above, so 1) 2) 3)
d) resize the filesystem without any size parameter (i.e. let it expand to
occupy the 99.9MB of extra space you have on the RAID device)
In any phase after 1) you can also mount the filesystem again (from /dev/md*),
since all the rest can be done on a mounted filesystem (no problem with d) too).
There are some things to notice:
- step b) requires a filesystem that supports shrinking; this can be done with
ext4 but it is not supported on xfs (BTW, this is why I refuse to consider xfs
a serious filesystem)
- step d) can be done on both a mounted or unmounted filesystem for ext4
but can only be done on a mounted fileystem ox xfs (another reason why I do not
like xfs)
- RAID creation in step 1) must be done with a --metadata option that forces the
metadata at the END of the space (so 0.90 or 1.0), since you are not going
to shift all your data forward to have space at the beginning
At the end of day, it can be done, but you really have to know what you are doing,
a small error can lead to a disaster. I would do that only on data I have a backup
already, or at least I would try all the procedure on a small test filesystem
before doing it on the real stuff.
Regards.
--
Roberto Ragusa mail at robertoragusa.it
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