linux-kernel - Re: [RFC PATCH] dm-csum: A new device mapper target that checks data integrity

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Date:	Fri, 26 Jun 2009 14:20:32 +0530
From:	SandeepKsinha <sandeepksinha@...il.com>
To:	Goswin von Brederlow <goswin-v-b@....de>
Cc:	Alberto Bertogli <albertito@...tiri.com.ar>,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com,
	linux-raid@...r.kernel.org, agk@...hat.com, neilb@...e.de
Subject: Re: [RFC PATCH] dm-csum: A new device mapper target that checks data 
	integrity

Hi,

I meant sectors and not blocks.

On Fri, Jun 26, 2009 at 12:56 PM, SandeepKsinha<sandeepksinha@...il.com> wrote:
> Hi Alberto,
>
> + * TODO: would it be better to have M1 and M2 apart, to improve the chances of
> + * recovery in case of a failure?
> + *
>
> How do you guarantee consistency in case of any lost writes? Your
> checksum might hold an updated value while your data block might be
> lost or written to a wrong destination?
>

sector in place of block.

> When implementing such integrity solutions, IMO, it is always
> advisable to handle such error conditions else this might lead to
> issues. Since, checksums  are very tightly coupled with the data and
> any misleading can be quite dangerous unlike parity which can be
> recovered.
>
> Calculate the data's CRC, and compare it to the one found in M1. If they
> + *    match, the reading is successful. If not, compare it to the one found in
> + *    M2. If they match, the reading is successful;
>
> Also, I hope by M1 and M2 you refer to the entry for a particular
> block in the respective IMD sector. What kind of mechanism do you use
> to determine which is younger?
> Is it the timestamp or some generation count?
>

for a particular sector and not block.

> I assume information is per_block_entry in the IMD sectors. Which I
> don't see in your implementation?
>  *
> + * The imd structure consists of:
> + *   - 16 bit CRC (CCITT) (big endian)
> + *   - 16 bit flags (big endian)
> + *   - 32 bit tag
>
> Correct me If I am missing something.
>
> On Fri, May 29, 2009 at 12:59 AM, Goswin von Brederlow<goswin-v-b@....de> wrote:
>> Now I need to get a new harddisk so I can test this without risking
>> live data. :)
>>
>> MfG
>>        Goswin
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
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>>
>
>
>
> --
> Regards,
> Sandeep.
>
>
>
>
>
>
> “To learn is to change. Education is a process that changes the learner.”
>



-- 
Regards,
Sandeep.





 	
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