Any RAID experts here?

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Incindre

117 posts

Master Geek

#189515 2-Jan-2016 19:31

I've been having quite a lot of difficulty in creating a RAID 10 array in Ubuntu Server 14.04 with Webmin, mainly because of issues with the GPT tables on the HDD's and indeterminable Webmin foibles.

As per this thread I've deleted my RAID in Webmin and repaired the partition tables for each disk using gdisk.

Before I dive into MDADM and start throwing the --create command into play I'd like to put a few issues to bed. I'll post up different questions in this thread to keep it all neat and self-contained.

QUESTION: 1!

One of the things that's bugging the hell out of me is: while trying to fix an issue, I unplugged all the HDD's, and when reconnected they reordered themselves, as far as linux lists the devices.

So where I had /dev/sda/ as my bootdrive plugged into SATA-1, and /dev/sdb-sde (my data drives) all neatly mapped as per the SATA ports on the motherboard, now my bootdrive (separate from the raid business) is showing up as /dev/sdb and I've no idea which of the other drives is which! Therefore if I get a drive failure I won't be able to immediately identify the drive by correlating the missing /dev/sdx code to 'the second drive down' in my case.

Does anyone know how to assign these drive letters to specific devices? Or any system which will make it easy for me to identify which drive is the culprit and swap em' in case of a failure? I'm not just talking hard failure where it's obvious from the BIOS, I mean if I need to fail a drive because of SMART errors etc.

Any of your pro tips would be greatly appreciated!

Gozer

184 posts

Master Geek

#1460360 2-Jan-2016 20:23

This seems to be prevalent when mixing sata and pata devices.
Using UUID in /etc/fstab makes the system always mount the partitions as you want them mounted.
Another option is to give the partitions a label (can be done easily with gparted, of course from a live disk). Then you use the label in /etc/fstab.
https://help.ubuntu.com/community/Fstab

Incindre

117 posts

Master Geek

#1460402 2-Jan-2016 21:52

Gozer: This seems to be prevalent when mixing sata and pata devices.
Using UUID in /etc/fstab makes the system always mount the partitions as you want them mounted.
Another option is to give the partitions a label (can be done easily with gparted, of course from a live disk). Then you use the label in /etc/fstab.
https://help.ubuntu.com/community/Fstab

Bear with a noobie here, at the moment my drives have a GPT table but no partition. Therefore when I run <sudo blkid> the only UUID's I get are for the three partitions on my boot drive.
Am I right in thinking that, as per this, the correct thing to do is to create 'symlinks' for each piece of hardware rather than mess with UUID's?

Mark

1653 posts

Uber Geek

#1460407 2-Jan-2016 21:57

I'm not so great with Linux LVM, but you could make a few notes and stick them to the outside of the computer :-)

Once you've created something it'll look something like this :

root@127.0.0.1:/dev# cat /dev/mdstat

cat: /dev/mdstat: No such file or directory

root@127.0.0.1:/dev# mdadm --detail /dev/md1

/dev/md1:

Version : 1.0

Creation Time : Thu May 12 19:27:22 2011

Raid Level : raid5

Array Size : 5854396032 (5583.19 GiB 5994.90 GB)

Used Dev Size : 1951465344 (1861.06 GiB 1998.30 GB)

Raid Devices : 4

Total Devices : 4

Persistence : Superblock is persistent

Update Time : Sat Jan 2 21:38:53 2016

State : clean

Active Devices : 4

Working Devices : 4

Failed Devices : 0

Spare Devices : 0

Layout : left-symmetric

Chunk Size : 128K

Name : N4200PRO:1 (local to host N4200PRO)

UUID : 4c987154:45a0e7d7:846bd7a4:75f94e10

Events : 233796

Number Major Minor RaidDevice State

5 8 2 0 active sync /dev/sda2

4 8 18 1 active sync /dev/sdb2

2 8 34 2 active sync /dev/sdc2

3 8 50 3 active sync /dev/sdd2

The bottom few lines will show you what devices are in the setup, then pull the serial numbers of those devices with :


root@127.0.0.1:/dev# hdparm -i /dev/sda

/dev/sda:

 Model=WDC WD20EZRX-00D8PB0 , FwRev=80.00A80, SerialNo= WD-WMC4N0L611U7
 Config={ HardSect NotMFM HdSw>15uSec SpinMotCtl Fixed DTR>5Mbs FmtGapReq }
 RawCHS=16383/16/63, TrkSize=0, SectSize=0, ECCbytes=0
 BuffType=unknown, BuffSize=0kB, MaxMultSect=16, MultSect=?0?
 CurCHS=16383/16/63, CurSects=16514064, LBA=yes, LBAsects=268435455
 IORDY=on/off, tPIO={min:120,w/IORDY:120}, tDMA={min:120,rec:120}
 PIO modes: pio0 pio3 pio4 
 DMA modes: mdma0 mdma1 mdma2 
 UDMA modes: udma0 udma1 udma2 
 AdvancedPM=no WriteCache=enabled
 Drive conforms to: Unspecified: ATA/ATAPI-1 ATA/ATAPI-2 ATA/ATAPI-3 ATA/ATAPI-4 ATA/ATAPI-5 ATA/ATAPI-6 ATA/ATAPI-7

 * signifies the current active mode

Then if/when a device fails just run the hdparm command again on the surviving members and don't pull them out :-)

Clunky... and there is bound to be a smarter way of doing it, but I'm a lazy storage admin and the arrays do it all for me now!

Incindre

117 posts

Master Geek

#1460429 2-Jan-2016 23:34

Nice Mark, slightly ghetto but it will work in a pinch!

Still, I'd like to hear people's opinion on symlinking, as it seems to be a more reliable way of having your drives do what you want them to do every single time... but the internets aren't providing definitive answers on whether or not this is even possible with MDADM. Does MDADM even support this functionality, or does it only speak in the language of '/dev/sdx'?

I'll be RAIDing the entire disks together rather than creating an array of partitions, which I didn't make clear in the first post sorry.

openmedia

3325 posts

Uber Geek

Trusted

#1460432 2-Jan-2016 23:40

MDADM software raid doesn't really care if the drives change name, as long as they have the correct UUID in their RAID meta information.

I personally always put a partition table down for raid or LVM.

GPT on disks + single partition correctly aligned + Raid (I'd use Raid 6 over Raid 10 for future expansion) then LVM.

Generally known online as OpenMedia, now working for Red Hat APAC as a Technology Evangelist and Portfolio Architect. Still playing with MythTV and digital media on the side.

Incindre

117 posts

Master Geek

#1460568 3-Jan-2016 12:15

openmedia: MDADM software raid doesn't really care if the drives change name, as long as they have the correct UUID in their RAID meta information.

I personally always put a partition table down for raid or LVM.

GPT on disks + single partition correctly aligned + Raid (I'd use Raid 6 over Raid 10 for future expansion) then LVM.

So MDADM will use UUID's automatically without any prompting/added configuration?