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 Post subject: RAID failure diagnostic
PostPosted: Tue Feb 26, 2013 9:37 am 
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Joined: Tue Feb 26, 2013 9:24 am
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Hi all, long-time lurker and first-time poster.

I have had a NSA-221 for a while now running 2x 250GB drives in a RAID1 array with no problems on firmware 4.40. I recently upgraded to a pair of 3TB Seagate ST3000DM001 drives and for a few days after installation things were fine. However after those few days the NAS became unresponsive (couldn't log into web UI or access SMB shares) so I rebooted it. After the reboot the RAID array was reported as being degraded. Both disks are reported as "healthy" in the SMART software add-on however.

I attempted to repair the RAID array but it gave me an error saying something along the lines of the second hard drive must be equal or greater in size to the first hard drive. I tried the "failed" drive in a Ubuntu VM with ZFS file system support and the data appeared to be there, so I wiped all the partitions, rebuilt the RAID volume and restored from a backup.

However three or four days after doing this the RAID array failed again, with the same symptoms.

My question is; is this symptomatic of a hard drive failure (considering the healthy SMART status and fact I can read the drive's data in Ubuntu) or is it just buggy RAID software in the NAS? I would ordinarily blame the drive, but the NAS *is* extraordinarily buggy so I don't really trust it not to waste my time.

Are there any good tools that would diagnose a drive failure - I was thinking of using Seagate's SeaTools with the drive in a USB disk caddy but wonder if it would get confused with the ZFS-based RAID on the drive.

Any advice gratefully received...


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PostPosted: Tue Feb 26, 2013 9:50 am 
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Just an addendum: I'm not sure whether it's ZFS or XFS. One of the two ;-)


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PostPosted: Tue Feb 26, 2013 2:07 pm 
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Joined: Mon Jun 16, 2008 10:45 am
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XFS. ZFS is a much more complicated FS.

Have you tried a single disk without RAID1? I seem to remember there was an issue with XFS and Oxnas. Just cannot remember what. By testing a single disk you can rule out the (extra) complexity of raid. If it is also instable with a single disk, I would recommend to convert the filesystem to ext3.


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PostPosted: Tue Feb 26, 2013 2:18 pm 
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Joined: Tue Feb 26, 2013 9:24 am
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Thanks for the reply.

XFS is what Zyxel defaults the file system on the volume to; I wasn't aware you could define a seperate filesystem and have it work properly. I do want to maintain a RAID1 array if possible to help keep my data secure but I guess if I have to have a primary JBOD (?) volume on one disk with, say, a weekly backup to another JBOD volume on the other disk that would be more or less acceptable until I can buy myself a NAS that can do RAID properly :)

It was also quite stable with XFS/RAID1 with my previous 250GB drives. Could it be less stable for bigger disk capacities?


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PostPosted: Tue Feb 26, 2013 6:33 pm 
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Joined: Mon Jun 16, 2008 10:45 am
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dotdavid wrote:
XFS is what Zyxel defaults the file system on the volume to; I wasn't aware you could define a seperate filesystem and have it work properly.
You can swap around disks between various ZyXEL NASses, and keep your data. So the NAS has to be down-compatible. So far ZyXEL has used reiserfs, xfs, ext3 and ext4 for their data partitions, when I remember well. I *think* any filesystem will be accepted, as long as the kernel supports it.

Quote:
I do want to maintain a RAID1 array if possible to help keep my data secure
In that case you should *not* use RAID1. Raid is about availability, not about security.
Raid protects you against failure of a single disk. But that is all. It doesn't protect you against:
  • Accidentally deletion of files (#1 in data loss).
  • All disks are in the same unit, which mean one hazard threatens them all:
    • A failing power supply which last voltage peak toasts all connected harddisks (or at least enough to break your array).
    • Theft.
    • Fire.
    • Dropping the whole disk unit, killing all disks.
Further, because most raid arrays start with n equal disks, and all disks are subject to the same wear, the odds that a 2nd disk will die while rebuilding an array after a failing first one, breaking the array, is bigger than you like, and is getting bigger.
It gets bigger because disks are getting bigger, while their MTBF and throughput doesn't grow as fast. This means that syncing an array takes more time, which is more time for a 2nd disk to fail.

For security you need backups.

Quote:
It was also quite stable with XFS/RAID1 with my previous 250GB drives. Could it be less stable for bigger disk capacities?
Well, yes. Bigger disks mean bigger filesystem structures, bigger sector numbers, ... In your case the sector numbers have exceeded a 32 bits number, which can open a whole new area of bugs.


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PostPosted: Tue Feb 26, 2013 7:39 pm 
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Joined: Tue Feb 26, 2013 9:24 am
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Yeah I have an external USB 3TB HDD for backups, not offsite but there's only so much I can do ;-)

I think I'm going to give it a try in an EXT3 JBOD configuration with one disk the master and the other disk having regular backups done on it, in addition to the USB drive. As you say I don't need high availability. Hopefully taking RAID and XFS out of the equation will make things more stable...


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