Re: DM-RAID1 data corruption

[Takahiro Yasui <tyasui@redhat.com>]

Hi Mikulas,

I know this data corruption issue can happen. To make this
condition easily, I stopped dmeventd and injected an error
to leg 0, then this issue happened in my environment.

The problem is leg 0 is always the default mirror without
checking any information. To store the information which
leg is the default mirror might solve this issue.

Thanks,
Taka


> Hi
> 
> This is the scenario of data corruption that I was talking about:
> 
> Mirror has two legs, 0 and 1 and a log. Disk 0 is the default.
> 
> A write is propagated to both legs. The write fails on leg 0 and succeeds 
> on leg 1.
> 
> The function "write_callback" puts the bio to "failure" list (if 
> errors_handled was true). It also wakes userspace.
> 
> do_failures pops the bios from ms->log_failure and calls dm_rh_mark_nosync 
> on them to mark the region nosync. dm_rh_mark_nosync completes the bio 
> with success.
> 
> *the computer crahes* (before the userspace daemon had a chance to run)
> 
> On next reboot, disk is 0 revived (suppose that it temporarily failed 
> because of a loose cable, overheating, insufficient power or so, and the 
> condition is repaired), raid1 sees set bit in the dirty bitmap and starts 
> copying data from disk 0 to disk 1.
> 
> The result: write bio was ended as succes, but the data was lost. For 
> databases, this might have bad consequences - committed transactions being 
> forgotten.
> 
> -
> 
> If the above scenario can't happen, pls. describe why.
> 
> What would be a possible way to fix this?
> 
> Delay all bios until the userspace code removes the failed mirror?
> Or store the number of the default mirror in the log?
> 
> Mikulas
> 
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