Re: scrub backing device

[ching <lsching17@gmail.com>]

it may be a mess that if there is 2 btrfs partition (1 via bcache, 1 via dm) with same UUID appear at the same time.

https://btrfs.wiki.kernel.org/index.php/Gotchas

Currently, i think removing cache device would be simpler.


On May 12, 2016 7:57:48 AM GMT+08:00, Marc MERLIN <marc@merlins.org> wrote:
>On Thu, May 12, 2016 at 07:34:26AM +0800, ching wrote:
>> for example
>https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-scrub
> 
>btrfs scrub is designed to make sure all the blocks and checksums are
>consistent.
>Ching makes a good point that it would be helpful to have a way to
>access the underlying backing device directly to ensure it is
>consistent.
>
>I think currently you can use dmsetup with an offset to get the real
>filesystem past the bcache header, mount that read only somewhere and
>scrub that.
>Not ideal because scrub will not be able to fix issues that are
>fixable,
>but at least it should report errors.
>
>Would that work?
>
>Marc
>
>> On May 12, 2016 3:13:22 AM GMT+08:00, Eric Wheeler
><bcache@lists.ewheeler.net> wrote:
>> >On Wed, 11 May 2016, ching lu wrote:
>> >
>> >> The read cache prevent the program to scrub the backing device
>> >directly.
>> >> 
>> >> if a few bits flipped in the backing device, the cache may still
>> >> return healthy data for a while.
>> >> 
>> >> Must i remove cache device before scrubbing?
>> >
>> >What do you mean by scrub?
>> >
>> >--
>> >Eric Wheeler
>> >
>> >> 
>> >> 
>> >> On Wed, May 11, 2016 at 9:15 AM, Eric Wheeler
>> ><bcache@lists.ewheeler.net> wrote:
>> >> > On Fri, 29 Apr 2016, ching lu wrote:
>> >> >
>> >> >> Hello,
>> >> >>
>> >> >> i want to develop a script to perform data scrubbing
>periodically.
>> >> >
>> >> > I think you want to set dirty_percent to 0 and
>> >cache_mode=writethrough.
>> >> > Then wait a long time for dirty_percent to actually reach 0.
>> >> >
>> >> > --
>> >> > Eric Wheeler
>> >> >
>> >> >
>> >> >>
>> >> >> For my use case, i think it is meaningless to scrub cached
>data.
>> >> >>
>> >> >> If i change the cache mode to "none", will it turn off the read
>> >cache too?
>> >> >>
>> >> >> Furthermore, will this invalidate cached data? (i do not want
>to
>> >"warm
>> >> >> up" the cache again after the scrubbing)
>> >> >>
>> >> >> Thanks in advance,
>> >> >> c
>> >> >> --
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