Re: Enable the skip_copy feature will results in data integrity issue in raid5 degraded mode.

[Shaohua Li <shli@kernel.org>]

On Mon, Feb 13, 2017 at 05:07:45PM +0800, Chien Lee wrote:
> Hello,
> 
> 
> Recently we find a bug about skip_copy feature in raid5 degraded mode.
> In the beginning, we enable the skip_copy feature to speed up system’s
> write performance. But when the system has database read/write I/O
> continually in raid5 degraded mode, the Mongo DB will detect the
> checksum error and generate related debug log. The following is the
> testing detail.
> 
> 
> a. Enable skip_copy
>     --> Checksum error logs from Mongo DB
> 
> 2017-02-06T11:54:56.537+0800 E STORAGE  [conn7] WiredTiger (0)
> [1486353296:537114][52:0x7f98396a4700],
> file:collection-110-3235234017846331078.wt,  WT_CURSOR.next: read
> checksum error for 4096B block at offset 61440: calculated block
> checksum of 1363526237 doesn't match expected checksum of 2969711960
> 
> 
> b. Disable skip_copy
>     --> Mongo DB has no checksum error.
> 
> 
> We've pretty sure that it must be a bug by our repeated database I/O
> testing. When skip_copy feature is enabled, the raid5/raid6 always
> causes the mongo DB checksum error in degraded mode less than one
> hour. On the contrary, it will never cause this abnormal situation
> when the skip_copy feature is disabled. Besides, because the skip_copy
> feature only affects the write action instead of read action, I think
> it should be the write action in degraded mode while skip_copy feature
> is enabled cause this bug.
> 
> 
> Please kindly provide us some help or idea about the root cause and solution.

Thanks for the reporting, I'll look at it. In the meaning time, do you have a
quick way which I can use to reproduce the issue?

Thanks,
Shaohua