Re: Possible issue with software RAID 1 in case of disks with different speed

["John Stoffel" <john@stoffel.org>]

>>>>> "Christian" == Christian Focke-Kiss <christian.focke-kiss@mail.fernfh.ac.at> writes:

> I am using Debian stable for years (currently, release 13; before, 12,
> 11, 10, etc. starting with 3.0, if my long-time backups are complete).

Same here, though with different backup tools

> For some years now, I am using software RAID with two RAID 1 sets with
> three 8TB disks each, one for my production data (/home, /var/lib,
> /var/www, etc.) and the second one for a nightly backup of my
> production data (three disks each because HDDs failed quite frequently
> then, and recovery was slow).

I like this, three way mirrors are cheap insurance. 

> Initially, I used external disks connected mainly via USB 3.x, and also
> via USB 2.0, when I ran out of USB ports. Reason for USB was that I
> couldn't/wouldn't afford a server with slots for accomodating six 3.5"
> HDDs.

There's the problem, USB connections are notoriously crappy.  It would
be better to go with eSATA or some other more reliable transport.  

> After migrating the failed 3.5" HDDs to 2.5" SSDs one-by-one over
> the years, I finally migrated five disks into a rack-mount server
> and connected the sixth disk via USB 3.x (I couldn't migrate all six
> disks because one slot was still occupied by the boot disk).

> I run nightly rsync jobs ('managed' by Back In Time) to backup
> everything from /dev/md0 (boot) to /dev/md1 (production) and then to
> /dev/md2 (backup).

> Now, as long as the sixth disk was still connected via USB 3.x, the
> rsync job and a kworker job were 'blocked' after some hours of
> rsync- ing, and the console displayed some 'sync' errors, and I had
> to press Ctrl+Alt+Del to reboot the system because login didn't work
> anymore.

What did you see in the logs?  I.e. did you kick off the logs and then
maybe send the logs to another system (if possible) or to the console
which you could then switch the display to, so you might have a chance
of looking at things?  

> I flagged the USB 3.x disk 'write-mostly' and 'nofailfast' but this
> didn't resolve the issue.

I suspect your external USB enclosure is crap.  I've never found a
good one in my experience.  It's too prone to losing connectivity, or
having a crappy controller chip which just doesn't handle long term
writes well, etc. 

> Only after I added two NVMe SSDs as boot disks and migrated the
> sixth SSD into the sixth slot, everything runs fine.

It's the USB connection, not the disk or RAID.  

> Conclusion:
> I suspect software RAID 1 has issues if one disk of a three disk RAID 1
> set is significantly slower than the other two disks.

Logs?  Warnings?  

Glad to hear it's up, but in the future, just use eSATA or regular
SATA in your case.  I've got some old big cases with 8 drive bays for
3.5" disks and it works great.  I've thought about server case with
2.5" disks that could be hot-swapped, but in reality for a home
system, I don't need it up five 9s, I can handle downtime.  

But thanks for the anecdote!
John