Re: Can btrfs silently repair read-error in raid1

[cwillu <cwillu@cwillu.com>]

On Tue, May 8, 2012 at 1:36 AM, Fajar A. Nugraha <list@fajar.net> wrote:
> On Tue, May 8, 2012 at 2:13 PM, Clemens Eisserer <linuxhippy@gmail.com> wrote:
>> Hi,
>>
>> I have a quite unreliable SSD here which develops some bad blocks from
>> time to time which result in read-errors.
>> Once the block is written to again, its remapped internally and
>> everything is fine again for that block.
>>
>> Would it be possible to create 2 btrfs partitions on that drive and
>> use it in RAID1 - with btrfs silently repairing read-errors when they
>> occur?
>> Would it require special settings, to not fallback to read-only mode
>> when a read-error occurs?
>
> The problem would be how the SSD (and linux) behaves when it
> encounters bad blocks (not bad disks, which is easier).
>
> If it does "oh, I can't read this block. I just return an error
> immediately", then it's good.
>
> However, in most situation, it would be like "hmmm, I can't read this
> block, let me retry that again. What? still error? then lets retry it
> again, and again.", which could take several minutes for a single bad
> block. And during that time linux (the kernel) would do something like
> "hey, the disk is not responding. Why don't we try some stuff? Let's
> try resetting the link. If it doesn't work, try downgrading the link
> speed".
>
> In short, if you KNOW the SSD is already showing signs of bad blocks,
> better just throw it away.

The excessive number of retries (basically, the kernel repeating the
work the drive already attempted) is being addressed in the block
layer.

"[PATCH] libata-eh don't waste time retrying media errors (v3)", I
believe this is queued for 3.5