Re: Problem with disk

[Ric Wheeler <ric@emc.com>]

Tejun Heo wrote:

> Ric Wheeler wrote:
>
>> I think that MD will do the right thing if the IO terminates with an 
>> error condition.  If the error is silent (and that can happen during 
>> a write), then it clearly cannot recover.
>
>
> The condition I've described results in silent loss of data.  
> Depending on type and implementation, LLDD might be able to detect the 
> condition (PHY RDY status changed for SATA), but the event happens 
> after the affected writes are completed successfully.  For example,
>
> 1. fs issues writes for block #x, #y and then barrier #b.
> 2. #x gets written to the write-back cache and completed successfully
> 3. power glitch occurs while #y is in progress.  LLDD detects the 
> condition, recovers the drive and retries #y.
> 4. #y gets written to the write-back cache and completed successfully
> 4. barrier #b gets executed and #y gets written to the media, but #x 
> is lost and nobody knows about it.

The promise that you get from the barrier is pretty simple - after a 
successful one, all IO's that have been submitted before then are on 
platter if the barrier works.

In your example, if you mean power glitch as in power loss, x will be 
lost (and probably lots of other write cache state), but the application 
should expect it (or add extra barriers)....

>
> I'm worried about the problem because, with libata, hotplug is 
> becoming available to the masses and when average Joe hot plugs a new 
> drive into his machine which has $8 power supply (really, they sell 
> 300w ATX power at 8000 KRW which is about $8), this is going to 
> happen.  I had a pretty decent power supply from a reputable maker but 
> I still got hit by the problem.

Not sure that I understand exactly how a glitch (as opposed to a full 
loss) would cause x to get lost - the drive firmware should track the 
fact that x was in the write cache and not destaged to platter.

>
> Maybe the correct approach is to establish a warm-plug protocol.  
> Kernel provides a way to plug IOs and user helper program plugs all 
> IOs until the new device settles.
>
> Thanks.
>