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List:       linux-raid
Subject:    Re: TLER / CCTL timeout handling
From:       Nebojsa Trpkovic <trx.lists () gmail ! com>
Date:       2010-03-25 0:48:40
Message-ID: 4BAAB2E8.2040104 () gmail ! com
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On 03/24/10 22:54, Stefan /*St0fF*/ Hübner wrote:
> Am 23.03.2010 23:10, schrieb Nebojsa Trpkovic:
>> Hello.
>>
>> I've found interesting text about TLER / CCTL
>> (http://en.wikipedia.org/wiki/Time-Limited_Error_Recovery )
> 
> You might want to read on inside the ATA8-ACS, section about SCT Transport.
>>
>> on desktop class drives:
>> http://forums.storagereview.com/index.php/topic/28333-tler-cctl/
>>
>> So, the question is:
>>
>> If I make my drive report back it failed to read the requested sector,
>> how that report will be handeled?
> 
> As far as I understood in recent answers to my questions: as expected.
>>
>> Will Linux software RAID be aware of that report and start some action
>> (rebuilding affected stripe or at least whole array, reallocating bad
>> sectors along the way) ?
>>
> Indeed.  Michael Tokarev answered to me on 2/9/10:
> "On failed _read_ it tries to
> reconstruct data from other disk drives and writes the reconstructed
> data back to the drive where read failed.  If the _write_ fails md will
> drop the disk."
> 
> This means: if read fails and the drive does not report back, the
> following reconstructing write calls will fail, too.  The disk gets
> dropped, because it (most probably) is still doing its error recovery on
> the former read request and by that not responding.
> 
> If you enable ERC read timeouts, it'll report a media error (or
> something similar), but honour the write request.  If you give the ERC
> write timeout a value that is not too small and also not too large (i.e.
> it shouldn't timeout the write-operation from the view of the kernel),
> it will either fix the pending sector, or reallocate it.  If the ERC
> write timeout value is too small, it'll very aggressively reallocate
> sectors - which should not be the intention, as there are very few spare
> sectors (compared to the amount of sectors in total - only a few thousand).
>>
>> Thank you.
>> Nebojsa Trpkovic
> 
> You're welcome, and all the best,
> Stefan
> 

Thank you very much!

This is great answer (explaining a lot of things) and great news
(there's nothing to worry/hack about).

So, now we (desktop drives users) just have to wait for smartmontools
5.40 or pull source from SVN and set some reasonable ERC read timeouts.

Is value of 7 seconds considered as reasonable ERC read timeout?

Nebojsa
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