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greggerca

Bad sectors on one pool member

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Hi there,

I saw a previous thread about putting a damaged disk back into the pool, but my question is slightly different...

I have a disk that StableBit Scanner says has some damage. It's a small amount of damage vs. the whole disk, but of course, it's part of a big file, so that's likely toast.

 

What happens now?

 

What happens with DrivePool when a bad file is either "fixed" or becomes corrupt?

Does the bad file get replicated to other pool members?

Or does DrivePool figure out the disk has damage and overwrites the bad file?

Should I leave the bad file in place (rename it so it stays put over the bad disk area) and copy a good file over to that folder?

 

I've run chkdsk /f /b on the drive a few times (it takes around 18 hours), and Scanner is trying to "recover" the file - but I think that's going to fail.

 

If chkdsk has done its process on some bad sectors, is the drive OK for a while? Or will DrivePool keep trying to write back to those sectors?

 

Thanks...

 

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This is a complicated topic. 

 

 

The damaged sectors on the disk are ones that StableBit Scanner found were unreadable.  This could be an intermittent issue (such as communication issues with the disk), or it could be damage on the drive. 

 

Either way, if this is occuring, most likely you will not be able to properly read any affected files.  In fact, if you try copying a file that has been effected by the damage, you will most likely get an error while doing so.  

I've experienced this more than a few times, actually. 

 

 

As for "what happens now?", that depends.  

 

My recommendation would be to immediately remove  the disk from the pool.  And unless you've disabled the StableBit Scanner balancer in StableBit DrivePool, that is exactly what the software will be trying to do. 

But you mention the corrupted/damaged file.... As I said, you will most likely recieve errors when trying to move the file. This includes with the balancing engine, as we actually use the same API that WIndows Explorer uses to move and copy files. 

 

So a bad file should not get moved. 

 

 

Additionally, if you remove the disk from the pool, you will need to use the "Force Damaged Disk removal" option. This skips bad files, rather than erroring out on them and stopping the removal.  Once this has completed, StableBit DrivePool runs a check on the pool, and will reduplicate files, if needed.  So if the bad file is duplicated, it will reduplicate the affected file from the other copy on the pool. 

 

 

As for for the file recovery, StableBit Scanner tries to read it multiple times, using different "head positioning profiles" to see if we can grab the file.  If it does, you'll get the whole file back.  Otherwise, it will fail, and no file. 

Whether this succeeds or fails depends heavily on the disk. 

 

 

 

As for the CHKDSK /b pass, this may or may not recover the data.  However, if the disk has any SMART errors as well, I would highly recommend RMAing the disk rather than putting it back into use.  

The unreadable sectors should not occur and indicate a serious problem. 

 

 

 

I think I've covered all of your questions, but if I didn't, or didn't cover them to your satisfaction, don't hesitate to let me know. 

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Thank you for your reply!

I think you covered almost everything. I didn't remove the disk from the pool yet, and Scanner is working at the disk. So, I'm not sure if I should stop that process, remove the disk, and then try Scanner again.

 

The file sucks to have damaged, but the world won't end without it.

 

Out of curiosity, is Scanner doing the same work as chkdsk /b?

It seems like you need to run chkdsk several times for it to catch everything. So I'm wondering if I need to do both chkdsk AND let Scanner do its thing, or just use one or the other?

 

Thanks again!

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Thank you for your reply!

I think you covered almost everything. I didn't remove the disk from the pool yet, and Scanner is working at the disk. So, I'm not sure if I should stop that process, remove the disk, and then try Scanner again.

 

Either way will net you the same result, actually.   However, during disk removal the pool is read only.  In this case, that may be a good idea, to ensure nothing else gets written to the disk. 

 

And again, you'll want/need to use the "Force damaged disk removal" option, or it will error out on the problem file(s). 

 

 

 

The file sucks to have damaged, but the world won't end without it.

 

It always sucks, regardless of the file, as it makes it a PITA to deal with.  But better to know before you need the file. 

 

 

Out of curiosity, is Scanner doing the same work as chkdsk /b?

It seems like you need to run chkdsk several times for it to catch everything. So I'm wondering if I need to do both chkdsk AND let Scanner do its thing, or just use one or the other?

 

Yes and no.  StableBit Scanner is looking at the disk. CHKDSK looks at the volume/partition.  We can actually find issues in the disk structure, and will report it.  

 

In fact, the manual outlines the type of damage that we can detect: 

http://stablebit.com/Support/Scanner/2.X/Manual?Section=File%20Recovery

 

Some of this stuff won't be caught but CHKDSK, but a majority will.

 

 

However, the "/b" flag for CHKDSK does does a lot of the same things that the surface scan does.  it checks all of the disk, used sections and not, makes sure it's readable, and repairs the damage or marks it as bad sectors. 

 

The big thing is that "/b" writes to the disk.  if the data can't be recovered, it's lost.   StableBit Scanner ONLY reads from the disk.  So even if we fail, you may still be able to recover the data (either with a CHKDSK pass or data recovery tools). 

 

 

 

The big thing is, it shouldn't happen regardless.  And when it does, it usually is due to a defect on the disk, and it may be better to replace the disk *now*, then keep using it (eg waiting for it to get worse, because it most likely will). 

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