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NeddySeagoon
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PostPosted: Fri Dec 15, 2017 6:11 pm    Post subject: Raid Recovery Reply with quote

Team,

I have a broken raid5. It was rebuilding and dropped another device.
Google says I need ts do mdadm --create --assume-clean with all the bits in the right plcace. It also says that I might need an old mdadm to get the finer points right
mdadm-3.1.5 which looks about the right age won't build. The log is https://paste.pound-python.org/show/ZuTz02y0qTWzPb7k5VXV/

I'm down to a 2008 netbook and mobile tethering right now.

What am I missing to build mdadm from 2011?
.
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eccerr0r
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PostPosted: Fri Dec 15, 2017 6:44 pm    Post subject: Reply with quote

I'm very afraid of --create with any RAID, especially one that I'm trying to recover.
Are you sure you can't force assemble?

The major() and minor() macros appear to have changed in the newest glibc.

[EDITED] It looks like you now need to include <sys/sysmacros.h> to get major() and minor() but this will be deprecated soon.
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NeddySeagoon
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PostPosted: Fri Dec 15, 2017 9:30 pm    Post subject: Reply with quote

eccerr0r,

The story gets worse. --assemble --force might have worked but I had a power cut and the failed drives came back as spares.
That means that they lost their slot info.

I've already tried --create --assume-clean, that's how I know I need an old mdadm.
Of course, it might already be too late ..

The fallback firewall failed when I needed it, so I've had to fall further back but its an advance on tethering.

Thank you for the include <sys/sysmacros.h>. I'll try it out after I've had some sleep.
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frostschutz
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PostPosted: Fri Dec 15, 2017 9:40 pm    Post subject: Reply with quote

Do you have any mdadm --examine output of the raid in its original configuration...?

--create is a last ditch - best done on overlays. You never need and should never use an old version of mdadm.

https://unix.stackexchange.com/a/131927/30851

https://raid.wiki.kernel.org/index.php/Recovering_a_failed_software_RAID#Making_the_harddisks_read-only_using_an_overlay_file
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NeddySeagoon
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PostPosted: Sun Dec 17, 2017 3:36 pm    Post subject: Reply with quote

frostschutz,

mdadm --examine once the raid had dropped two drives and after the power cut, when they came back as spares.

I really wish I had known about the overlay technique sooner. Still, that's experience, you get it just after you needed it.
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NeddySeagoon
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PostPosted: Sat Dec 23, 2017 7:47 pm    Post subject: Reply with quote

I got may raid back and all the logical volumes it contains. It needs two replacement drives.

As frostschutz says, --create --assume-clean is very last ditch. Don't do it unless you are getting some hand holding.
When a new raid set is created, lots of default values to --create are used. When its new, you don't really care.
When its a recreate, you care very much and need to discover these to feed to create.
The defaults change over the years. too.

I did not use an old mdadm. I poked about for the options to get the raid metadata to point to my existing data.

Anyway, --create --assume-clean is doing it wrong and I was lucky.
I'm not out of the woods yet. Some KVMs boot and some don't but the raid set is read only while I see what's what.

I guess I should use an overlay to see what the damage will be.
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frostschutz
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PostPosted: Sat Dec 23, 2017 8:07 pm    Post subject: Reply with quote

for some reason, I never saw your dec 17 post until now ... so sorry for not replying to invisible post :wink:

glad you got it sorted
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eccerr0r
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PostPosted: Sat Dec 23, 2017 8:14 pm    Post subject: Reply with quote

I'm worried that we may have to go RAID6 or something to deal with the multidisk failure (or somehow more frequent backups!)

I was playing with RAID6 a bit with my retired 500G disks (because of unreliable disks), and it seems that there's a fairly substantial speed degradation. Not sure if I'd see different results with good disks or not, but it's definitely worse than than the same RAID5 without the second parity disk.
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NeddySeagoon
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PostPosted: Sat Dec 23, 2017 9:53 pm    Post subject: Reply with quote

eccerr0r,

Relocated sectors on rotating rust can lead to a significant slowdown as there is extra head movement and/or rotational latency to perform the remap every time a remapped sector is read or written.
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John R. Graham
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PostPosted: Sat Dec 23, 2017 11:31 pm    Post subject: Reply with quote

eccerr0r wrote:
...but it's definitely worse than than the same RAID5 without the second parity disk.
Is this with Software RAID6? I haven't noticed the same thing with my Adaptec RAID controllers.

- John
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eccerr0r
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PostPosted: Sun Dec 24, 2017 6:45 am    Post subject: Reply with quote

Yes, software RAID6.

It was still about the rate of a single disk, not like it's less than one disk. I was getting better than one disk's bandwidth overall with RAID5. Write is pretty abysmal, less than one disk. I should doublecheck however. This was from memory which is known to be faulty :)

I was using multiple controllers - using the onboard intel and a silicon image PCI card. There is a PCI bottleneck on this machine but the disks aren't very fast to begin with.
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