Re: [RFE] Please, add optional RAID1 feature (= chunk checksums) to make it more robust

[Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>]

Please keep discussion on list.  This is probably an MUA issue.  Happens
to me on occasion when I hit "reply to list" instead of "reply to all".
 vger doesn't provide a List-Post: header so "reply to list" doesn't
work and you end up replying to the sender.

On 7/22/2012 5:11 PM, Jaromir Capik wrote:
>>> I admit, that the problem could lie elsewhere ... but that doesn't
>>> change anything on the fact, that the data became corrupted without
>>> me noticing that.
>>
>> The key here I think is "without me noticing that".  Drives normally
>> cry
>> out in the night, spitting errors to logs, when they encounter
>> problems.
>>  You may not receive an immediate error in your application,
>>  especially
>> when the drive is a RAID member and the data can be shipped
>> regardless
>> of the drive error.  If you never check your logs, or simply don't
>> see
>> these disk errors, how will you know there's a problem?
> 
> Hello Stan.
> 
> I used to periodically check logs as well as S.M.A.R.T. attributes.
> And I believe I've already mentioned two of the cases and how
> I finally discovered the issues. Moreover I switched from manual
> checking to receiving emails from monitoring daemons. And even
> if you receive such email, it usually takes some time to replace
> the failing drive. That time window might be fatal for your data
> if junk is read from one of the drives and when it's followed
> by a write. Such write would destroy the second correct copy ...
> 
>>
>> Likewise, if the checksumming you request is implemented in md/RAID1,
>> and your application never sees a problem when a drive heads South,
>> and
>> you never check your logs and thus don't see the checksum errors...
> 
> You wouldn't have to ... because the corrupted chunks would be 
> immediately resynced with good data and you'll REALLY get some errors
> in the logs if the harddrive or controller or it's driver doesn't
> produce them for whatever reason.
> 
>>
>> How is this new checksumming any better than the current situation?
>>  The
>> drive is still failing and you're still unaware of it.
> 
> Do you believe, that other reasons of silent data corruptions simply
> do not exist? Try to imagine a case, when the correct data aren't
> written at all to one of the drives due to a bug in the drive's firmware
> or due to a bug in the controller design or due to a bug in the
> controller driver or due to other reasons. Such bug could be tiggered
> by anything ... it could be a delay in the read operation when the
> sector is not well readable or any race condition, etc. Especially
> new devices and their very first versions are expected to be buggy.
> Checksuming would prevent them all and would make the whole
> I/O really bulletproof. 

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