HDD Offline Unrecoverable Returned to Normal Value ?


georgez

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I have a unmanaged WD 2TB which had over 100 offline unrecoverable sectors, it passed the preclear 3 times without any errors, so I installed it as unmanaged just in case I need to replace a disk immediately. Today I received a notification mail from unRAID which read:

 

Event: unRAID device sdr SMART message [198]

Subject: Notice [uNRAID-48T] - offline uncorrectable returned to normal value

Description: WDC_WD20EARX-00PASB0_WD-WMAZA9344705 (sdr)

Importance: normal

 

So I checked the SMART result again, and now the offline unrecoverable sector is 0, how come unrecoverable sectors being recovered any way?

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Just making sure I understand you, when you say 'unmanaged', do you mean unassigned to the array?

 

I believe those messages are associated with unassigned drives by their drive symbol, which can change from boot to boot, making them unreliable.  (I've requested that that be changed to associate them only with their serial numbers.)  Are you sure the drive currently at sdr is the same drive you were concerned about?

 

SMART attribute 198 is somewhat redundant to other attributes, so I don't worry too much about it.  It's just one way that bad sectors are identified, but once found, they will show up also in other attributes such as 197 Current Pending Sectors.  If a Preclear was successful, they are gone, whether the SMART report shows 198 returned to zero or not.  The critical one is 197, it MUST return to zero.

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Thanks for your reply, RobJ. Yeah I meant 'unassigned' (and I misspelled 'uncorrectable' as 'unrecoverable'), and I'm pretty sure the drive is the right one because that's the only unassigned drive in my box. With your explanations, now I know I can count on the perclear result and let go the 198 message. However, I'm still curious to know, if a sector is marked as 'uncorrectable', can it still be recovered / corrected?

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When the next write to that sector occurs, such as during a Preclear, the drive then tests the sector and makes a decision about it.  If it decides it's a good sector, it stores the new data there and considers it recovered.  If it decides it is not good enough to reliably store data, then it's 'reallocated', and a spare sector is mapped into its slot.  Whenever you read or write that sector in the future, the drive intercepts the request and gives you the assigned spare instead, but transparently so you can't tell the difference.  You would see #5 Reallocated_sector_ct go up by one, and if it has it, #196 Reallocated_Event_Count would increase by one also.  #197 Current_Pending_Sector would decrease by one, and #198 Offline_Uncorrectable *might* go down by one.  SMART handling isn't consistent between different drives.

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I see. So when a bad sector (I mean really bad) is recovered, it's just remapped to another sector. If I understand correctly, there are some reserved sectors in every HDD, and when a bad sector is found, it's remapped to a reserved one. What if all these reserved sectors are used up, any further bad sectors will definitely be marked as bad and not possible to be recovered?

 

I just raised another question: are there any bad sectors remapped in a brand new HDD? I wildly guess there are, the factory just hides them.

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I see. So when a bad sector (I mean really bad) is recovered, it's just remapped to another sector. If I understand correctly, there are some reserved sectors in every HDD, and when a bad sector is found, it's remapped to a reserved one. What if all these reserved sectors are used up, any further bad sectors will definitely be marked as bad and not possible to be recovered?

Most modern drives have a couple thousand spare sectors.  But, if you ever saw that many reallocated sectors, you really should have thrown out the drive long before.

 

Reallocated sectors by themselves are not a bad thing per se, nor an indication that the drive is necessarily dying.  But, when they begin to increase beyond whatever your comfort level is on the number you allow, you should replace the drive.  Many users around here feel like any reallocated sector is a cause to throw out the drive.  Myself a few don't bother me so long as they stay stable (I'm old enough to remember MFM/RLL drives that had the labels on them with the bad sectors listed from the factory, and in those days you wouldn't trust a drive that didn't have any bad sectors listed)

 

I just raised another question: are there any bad sectors remapped in a brand new HDD? I wildly guess there are, the factory just hides them.

I would actually think the answer to that is no.  And that's simply because I have received replacement drives under warranty from various manufacturers that are showing reallocated sectors already.
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