Re: mismatch_cnt again

[Bill Davidsen <davidsen@tmr.com>]

Piergiorgio Sartor wrote:
> Hi,
>
>   
>> But unless your drive firmware is broken the drive with only ever give
>> the correct data or an error. Smart has a counter for blocks that have
>> gone bad and will be fixed pending a write to them:
>> Current_Pending_Sector.
>>
>> The only way the drive should be able to give you bad data is if
>> multiple bits toggle in such a way that the ECC still fits.
>>     
>
> Not really, I've disks which are *perfect* in smart sense
> and nevertheless I had mistmatch count.
> This was a SW problem, I think now fixed, in RAID-10 code.
>
>   
IIRC there still is an error in raid-1 code, in that data is written to 
multiple drives without preventing modification of the memory between 
writes. As I understand Neil's explanation, this happens (a) when memory 
is being changed rapidly and frequently via memory mapped files, or (b) 
writing via O_DIRECT, or (c) when raid-1 is being used for swap. I'm not 
totally sure why the last one, but I have always seem mismatches on swap 
in a system which is actually swapping. What is more troubling is that 
if I do a hibernate, which writes to swap, and then force a boot from 
other media to a Live-CD, doing a check of the swap array occasionally 
shows a mismatch. That doesn't give me a secure feeling, although I have 
never had an issue in practice, I was just curious.

> This means that, yes, there could be mismatches, without
> any warning, from other sources than disks.
> And these could be anywhere in the system.
> I already mentioned, time ago, a cabling problem which was
> leading to a similar result: wrong data on different disks,
> without any warning or error from the HW layer.
>
> That is why it is important to know *where* the mismatch
> occurs and, if possible, in which device component.
> If it is an empty part of the FS, no problem, if it
> belongs to a specific file, then it would be possible
> to restore/recreate it.
>
> Of course, a tool will be needed telling which file is
> using a certain block of the device.
>   

There are tools which claim to do that, or list blocks used in a given 
file, which is not nearly as useful, but easier to do.

-- 
Bill Davidsen <davidsen@tmr.com>
  "We can't solve today's problems by using the same thinking we
   used in creating them." - Einstein