Re: [linux-lvm] Problem with partially activate logical volume

From: Zdenek Kabelac <zdenek.kabelac@gmail.com>
To: LVM general discussion and development <linux-lvm@redhat.com>,
	Ken Bass <daytooner@gmail.com>
Subject: Re: [linux-lvm] Problem with partially activate logical volume
Date: Wed, 27 Jul 2022 21:56:27 +0200	[thread overview]
Message-ID: <8097ecbe-75dc-876a-a029-5ce4f0a24c72@gmail.com> (raw)
In-Reply-To: <CAH5g025eDd60jvU+_d7DLaPdsmK0TsNjB-jkewKg5FSNJMEmJw@mail.gmail.com>

Dne 25. 07. 22 v 16:48 Ken Bass napsal(a):
> (fwiw: I am new to this list, so please bear with me.)
> 
> Background: I have a very large (20TB) logical volume consisting of 3 drives. 
> One of those drives unexpectedloy died (isn't that always the case :-)). The 
> drive that failed happened to be the last PV. So I am assuming that there is 
> still 2/3 of the data still intact and, to some extent, recoverable. Although, 
> apparently the ext4 fs is not recognised.
> 
> I activated the LV partially (via -P). But running any utility on that (eg: 
> dumpe2fs, e2fsck, ...) I get many of these  in dmesg:
> 
> "Buffer I/O error on dev dm-0, logical block xxxxxxx, async page read."  The 
> thing is, the xxxxxxx block is on the missing drive/pv.
> 
> I have also tried some recovery software, but eventually get these same 
> messages, and the data recovered is not really useful.
> 
> Please help! How can I get passed that dmesg error, and move on. 14TB 
> recovered is better than 0.

Loosing such a large portion of device is always going to be a BIG problem.
Filesystem spreads metadata all over the place -  ExtX is somewhat better then 
BTree based FS like  XFS,BTRFS and may give you lot of your data back.

But that's why people should never underestimate how important is to keep 
reasonable fresh backups of their data - otherwise sooner or later there comes 
lesson like this one.

What you could try is to 'add' new PV to VG and use space for taking snapshot 
of your LV you want to repair - but this is somewhat complicated as you need 
to 'fix' VG first - which would ideally need to have some size of storage you 
just lost - but this gives you then fairly easy way forward.
(one way to do this is to use even 'virtual' storage over the loop back device 
- but that's for likely skilled admin.)

Lvm2 partial activation is designed to be used in a way to ACTIVATE LV - and 
copy the content to better/safe/secure location and there start to recover the 
storage.

Repairing storage in-place is usually straight road to hell - as there could 
be numerous way of recovery to try - but if your 1st. try actually destroys 
data even more, you can't retry with different strategy.

So depending on how much money and time you want to put into recovery of your 
data there are several different strategies possible  - considering storage 
space is relatively 'cheap' if your are data are really important.

Regards

Zdenek

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