Re: md road-map: 2011

[Giovanni Tessore <giotex@texsoft.it>]

On 02/17/2011 11:58 AM, Keld Jørn Simonsen wrote:
> On Thu, Feb 17, 2011 at 11:45:35AM +0100, David Brown wrote:
>> On 17/02/2011 02:04, Keld Jørn Simonsen wrote:
>>> On Thu, Feb 17, 2011 at 01:30:49AM +0100, David Brown wrote:
>>>> On 17/02/11 00:01, NeilBrown wrote:
>>>>> On Wed, 16 Feb 2011 23:34:43 +0100 David Brown<david.brown@hesbynett.no>
>>>>> wrote:
>>>>>
>>>>>> I thought there was some mechanism for block devices to report bad
>>>>>> blocks back to the file system, and that file systems tracked bad block
>>>>>> lists.  Modern drives automatically relocate bad blocks (at least, they
>>>>>> do if they can), but there was a time when they did not and it was up to
>>>>>> the file system to track these.  Whether that still applies to modern
>>>>>> file systems, I do not know - they only file system I have studied in
>>>>>> low-level detail is FAT16.
>>>>> When the block device reports an error the filesystem can certainly
>>>>> record
>>>>> that information in a bad-block list, and possibly does.
>>>>>
>>>>> However I thought you were suggesting a situation where the block device
>>>>> could succeed with the request, but knew that area of the device was of
>>>>> low
>>>>> quality.
>>>> I guess that is what I was trying to suggest, though not very clearly.
>>>>
>>>>> e.g. IO to a block on a stripe which had one 'bad block'.  The IO should
>>>>> succeed, but the data isn't as safe as elsewhere.  It would be nice if we
>>>>> could tell the filesystem that fact, and if it could make use of it. But
>>>>> we
>>>>> currently cannot.   We can say "success" or "failure", but we cannot say
>>>>> "success, but you might not be so lucky next time".
>>>>>
>>>> Do filesystems re-try reads when there is a failure?  Could you return
>>>> fail on one read, then success on a re-read, which could be interpreted
>>>> as "dying, but not yet dead" by the file system?
>>> This should not be a file system feature. The file system is built upon
>>> the raid, and in mirrorred raid types like raid1 and raid10, and also
>>> other raid types, you cannot be sure which specific drive and sector the
>>> data was read from - it could be one out of many (typically two) places.
>>> So the bad blocks of a raid is a feature of the raid and its individual
>>> drives, not the file system. If it was a property of the file system,
>>> then the fs should be aware of the underlying raid topology, and know if
>>> this was a parity block or data block of raid5 or raid6, or which
>>> mirror instance of a raid1/10 type which  was involved.
>>>
>> Thanks for the explanation.
>>
>> I guess my worry is that if md layer has tracked a bad block on a disk,
>> then that stripe will be in a degraded mode.  It's great that it will
>> still work, and it's great that the bad block list means that it is
>> /only/ that stripe that is degraded - not the whole raid.
> I am proposing that the stripe not be degraded, using a recovery area for bad
> blocks on the disk, that goes together with the metadata area.
>
>> But I'm hoping there can be some sort of relocation somewhere
>> (ultimately it doesn't matter if it is handled by the file system, or by
>> md for the whole stripe, or by md for just that disk block, or by the
>> disk itself), so that you can get raid protection again for that stripe.
> I think we agree in hoping:-)

IMHO the point is that this feature (Bad Block Log) is a GREAT feature 
as it just helps in keeping track of the health status of the underlying 
disks, and helps A LOT in recovering data from the array when a 
unrecoverable read error occurs (now the full array goes offline). Then 
something must be done proactively to repair the situation, as it means 
that a disk of the array has problems and should be replaced. So, first 
it's worth to make a backup of the still alive array (getting some read 
error when the bad blocks/stripes are encountered [maybe using ddrescue 
or similar]), then replace the disk, and reconstruct the array; after 
that a fsck on the filesystem may repair the situation.

You may argue that the unrecoverable read error come from just very few 
sector of the disk, and it's not worth to replace it (personally I would 
replace also on very few ones), as there are still many reserverd 
sectors for relocation on the disk. Then a simple solution would just be 
to zero-write the bad blocks in the Bad Block Log (the data is gone 
already): if the write succedes (disk uses reserved sectors for 
relocation), the blocks are removed from the log (now they are ok); then 
fsck (hopefully) may repair the filesystem. At this point there are no 
more md read erros, maybe just filesystem errors (the array is clean, 
the filesystem may be not, but notice that nothing can be done to avoid 
filesystem problems, as there has been a data loss; only fsck may help).

Regards

-- 
Cordiali saluti.
Yours faithfully.

Giovanni Tessore


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