Re: Citation Needed: BTRFS Failure Resistance

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2019-05-23 12:24, Chris Murphy wrote:
> On Thu, May 23, 2019 at 5:19 AM Austin S. Hemmelgarn
> <ahferroin7@gmail.com> wrote:
>>
>> On 2019-05-22 14:46, Cerem Cem ASLAN wrote:
>>> Could you confirm or disclaim the following explanation:
>>> https://unix.stackexchange.com/a/520063/65781
>>>
>> Aside from what Hugo mentioned (which is correct), it's worth mentioning
>> that the example listed in the answer of how hardware issues could screw
>> things up assumes that for some reason write barriers aren't honored.
>> BTRFS explicitly requests write barriers to prevent that type of
>> reordering of writes from happening, and it's actually pretty unusual on
>> modern hardware for those write barriers to not be honored unless the
>> user is doing something stupid (like mounting with 'nobarrier' or using
>> LVM with write barrier support disabled).
> 
> 'man xfs'
> 
>         barrier|nobarrier
>                Note: This option has been deprecated as of kernel
> v4.10; in that version, integrity operations are always performed and
> the mount option is ignored.  These mount options will be removed no
> earlier than kernel v4.15.
> 
> Since they're getting rid of it, I wonder if it's sane for most any
> sane file system use case.
> 
As Adam mentioned, it's mostly volatile storage that benefits from this. 
  For example, on the systems where I have /var/cache configured as a 
separate filesystem, I mount it with barriers disabled because the data 
there just doesn't matter (all of it can be regenerated easily) and it 
gives me a few percent better performance.  In essence, it's the mostly 
same type of stuff where you might consider running ext4 without a 
journal for performance reasons.

In the case of XFS, it probably got removed to keep people who fancy 
themselves to be power users but really have no clue what they're doing 
from shooting themselves in the foot to try and get some more performance.

IIRC, the option originally got added to both XFS and ext* because early 
write barrier support was a bigger performance hit than it is today, and 
BTRFS just kind of inherited it.