Hot Swapping disks

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stever3
Posts: 4
youtube meble na wymiar Warszawa
Joined: Tue Jan 23, 2018 1:35 am

Hot Swapping disks

Post by stever3 »

Have an AS6104T with 4 2TB disks in it, configured as RAID 5, one of the disks is failing so I would like to replace it, couple of questions:

1-Can I hot swap the disk?
2-Should I turn the NAS off before replacing the disk, or can I just leave it on and do the swap?
3-Can I change the disk capacity or does the new one need to be the same size as the old one?

Thanks for any insight you can provide.
Antonios
Posts: 82
Joined: Tue Jun 27, 2017 1:05 pm

Re: Hot Swapping disks

Post by Antonios »

Let it fail and drop out of the array, or pull it out, same result.

MUST be 100% sure no other HDD has the slightest problem when the array runs in 'degraded' mode, this time might be considerable even if you swap the HDD immediately, the rebuild takes a long time and until it finishes successfully no data is safe.

Therefore, if time allows, if there is available storage space elsewhere etc, get a FULL backup first.

Your situation is kinda tricky because if the HDD had failed already then there is only one way to go, replace and hope for no power outage during rebuild etc, but now the HDD still works and a plan for selecting the best time frame for a successful rebuild must take account even local weather conditions and such that could cause power outages etc.

New HDD must be same or larger, otherwise it won't rebuild. To expand total volume capacity you need to replace all other HDDs one by one, rebuilding array after each replacement.

BTW, what kind / make of HDD was this? How old? What order (position) in the array? All HDDs were the same age in there so predicting others might fail soon?
stever3
Posts: 4
Joined: Tue Jan 23, 2018 1:35 am

Re: Hot Swapping disks

Post by stever3 »

Thanks for the info, The failing disk is a WD 2 TB, the one oddball in my system, Others are 2TB WD Reds. It's not in bad shape yet but shows Current_Pending_Sector count shows 8 bad, May just leave it for now until it fails.
Antonios
Posts: 82
Joined: Tue Jun 27, 2017 1:05 pm

Re: Hot Swapping disks

Post by Antonios »

Agree, borderline case with 'if it ain't broke...'

Wonder if such a suspect HDD could be cloned outside of the NAS while NAS is off, then the clone used in the NAS, I mean it would be a different HDD, different s/n, different type maybe, but holding the correct data, would the NAS object to that change at startup and drop the cloned HDD out of the array? I sure would like to test that in some newly setup RAID 5 array with no real data on it.

Myself I would prefer this cloning scenario instead of having to run a RAID 5 array for any amount of time in 'degraded' mode, let alone trying to rebuild it, I mean this is why RAID 6 was invented. Unless of course I had no choice, that is the suspect HDD was dead already.

What bothers me is the case where although running the array in degraded mode goes ok, but when attempting rebuild the NAS will have to read in sequence all sectors on all remaining HDDs, including rarely used sectors belonging to files of no importance that are never accessed and if a problem arises in such an 'unimportant' file, one that was never accessed so that specific problem went unnoticed, well, ...tough luck!

Well, I suppose this is why they came up with Background Media Scan for very large HDDs, but I hate that too, for a quiet home NAS application that is, mainly because BMS does not allow HDDs to spin down ever, let alone the constant clicking of the head actuators every few seconds.
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