Re: questions about softraid limitations

["Janos Haar" <janos.haar@netcenter.hu>]

----- Original Message ----- 
From: "David Greaves" <david@dgreaves.com>
To: "Janos Haar" <janos.haar@netcenter.hu>
Cc: <linux-raid@vger.kernel.org>
Sent: Sunday, May 18, 2008 3:00 PM
Subject: Re: questions about softraid limitations


> Janos Haar wrote:
>> The actually best way is this:
>>
>> blockdev --setro /dev/SOURCE
>> blockdev --setra  0 /dev/SOURCE
>> ddrescue -v -b 4096 -B 1024 -e 1 -y 0 /dev/SOURCE /dev/TARGET (if you
>> dont use a real clean disk with full of zeros, you need the -A option 
>> too)
>>
>> + need some kernel patch to disable all retry on the source disk.
> Is that something you could share?

Aaa, i made it myself about 1 year before on 2.6.18, but unfortunately at 
the moment the source is already deleted, and i dont save the diff. :-(
I am not a good C programmer, but takes about 1 hour. ;-)

>
> It sounds like something that would be worth including in the libata error
> handling to be controlled by something like 
> /sys/block/sdX/device/ioerr_retry_count

This is a great idea! :)
But not just for the ide drives!
I think this is good for all block device!
(usb, scsi, ide, sata...)

Oops.
And an old problem comes again:
On IDE drives with read errors, sometimes drop the DMA.
My script can be re-enable it sometimes, but not the real good solution....
If this can be fixed too, this will be great! :-)

>
> I'd be interested in playing with it and sending to Tejun Heo if you don't 
> have
> time.
>
>> The best recovery history is in 3 steps:
>>
> [snip]
>>
>> There is no easy solution. :-(
>
> OK - I'll put this up on the wiki sometime.
>
>>> Hmm - I wonder if things like ddrescue could work with the md bitmaps
>>> to improve
>>> this situation?
>>> Is this related to David Lethe's recent request?
>>
>> I think ddrescue is for copy/rescue the data, not for processing it.
>> But can log the errors... ;-)
> Yes - I meant to take the ddrescue output and manipulate the bitmap on the
> copied drive to mark certain parts as 'bad'.
> Ideally this could be done in the event of a dual-failure so that you 
> could
> re-add 2 failed drives and the bitmaps could indicate.
>
> This may requires something closer to a bytemap but...

This is good in this way. :-)

Cheers,

Janos

>
>
> David