linux-kernel - Re: Need help increasing raid scan efficiency.

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Message-ID: <20250604114558.4d27abce@Zen-II-x12.niklas.com>
Date: Wed, 4 Jun 2025 11:45:58 -0400
From: David Niklas <simd@...mail.net>
To: Linux RAID <linux-raid@...r.kernel.org>
Cc: linux-kernel@...r.kernel.org
Subject: Re: Need help increasing raid scan efficiency.

I'm replying to everyone in the same email.

On Wed, 4 Jun 2025 08:34:35 -0400
"John Stoffel" <john@...ffel.org> wrote:
> >>>>> "David" == David Niklas <simd@...mail.net> writes:  
> 
> > My PC suffered a rather nasty case of HW failure recently where the
> > MB would break the CPU and RAM. I ended up with different data on
> > different members of my RAID6 array.  
> 
> Ouch, this is not good.  But you have RAID6, so it should be ok...
> 
> > I wanted to scan through the drives and take some checksums of
> > various files in an attempt to ascertain which drives took the most
> > data corruption damage, to try and find the date that the damage
> > started occurring (as it was unclear when exactly this began), and
> > to try and rescue some of the data off of the good pairs.  
> 
> What are you comparing the checksums too?  Just because you assemble
> drives 1 and 2 and read the filesystem, then assemble drives 3 and 4
> into another array, how do you know which checksum is correct if they
> differ?  

Once I find some files whose checksums differ, I can perform some
automated data tests to find which file is the intact one.

> > So I setup the array into read-only mode and started the array with
> > only two of the drives. Drives 0 and 1. Then I proceeded to try and
> > start a second pair, drives 2 and 3, so that I could scan them
> > simultaneously.  With the intent of then switching it over to 0 and
> > 2 and 1 and 3, then 0 and 3 and 1 and 2.  
> 
> I'm not sure this is really going to work how you think.... 
<snip>

I just think that I'll be able to read from all 4 drives but doing it in
2 arrays of 2 drives. Basically, I'll get a 2x speed increase over doing
it as 2 drives at a time.


On Wed, 4 Jun 2025 07:22:15 +0300
Jani Partanen <jiipee@...apeli.fi> wrote:
> On 03/06/2025 23.04, David Niklas wrote:
> > I think you misunderstood my original question, how do I assemble the
> > RAID6 pairs (RO mode) into two different arrays such that I can read
> > from them simultaneously?  
> 
> I dont think there is any other way to do what you want to do than use
> overlayfs. You may find some ideas from here:
> 
> https://archive.kernel.org/oldwiki/raid.wiki.kernel.org/index.php/Irreversible_mdadm_failure_recovery.html

Thanks for the idea. I'm not following why we setup the overlay but then
use mapper devices (which came from where?), with the mdadm commands.


On Tue, 3 Jun 2025 21:27:35 +0100
anthony <antmbox@...ngman.org.uk> wrote:
> On 03/06/2025 21:04, David Niklas wrote:
> > Searching online turned up raid6check.
> > https://unix.stackexchange.com/questions/137384/raid6-scrubbing- 
> > mismatch-repair
> > 
> > But the people there also pointed out that Linux's raid repair
> > operation only recalculates the parity. I would have thought that it
> > did a best of 3 option. I mean, that's a big part of why we have
> > RAID6 instead of RAID5, right?  
> 
>  From what I remember of raid6check, it actually does a proper raid 6
> calculation to recover the damaged data.
> 
<snip>
> I've done it slightly differently, I've got raid-5 sat on top of
> dm-integrity, so if a disk gets corrupted dm-integrity will simply
> return a read failure, and the raid doesn't have to work out what's
> been corrupted. I've got a different problem at the moment - my array
> has assembled itself as three spares, so I've got to fix that ... :-(
> 
> Cheers,
> Wol

Good to know. Thanks Wol. I hope you're able to get your drives up and
running again.


On Wed, 4 Jun 2025 10:59:21 +0200
Reindl Harald <h.reindl@...lounge.net> wrote:
<snip>
> > as I don't have enough room otherwise  
> 
> seriously?
> 
> an external 10 TB disk costs around 200 EUR
> an external 20 TB disk costs around 400 EUR
<snip>

Every time I upgraded the size of my array, I'd take the old disks and
use them as backup disks. Over time, it became a matter of not having
enough SATA ports, not a matter of costing too much. I was trying to
reuse disks instead of the disks being tossed out or collecting dust. I've
learned better now.


Thanks,
David