On 2015-09-17 20:34, Duncan wrote:
> Zygo Blaxell posted on Wed, 16 Sep 2015 18:08:56 -0400 as excerpted:
>
>> On Wed, Sep 16, 2015 at 03:04:38PM -0400, Vincent Olivier wrote:
>>>
>>> OK fine. Let it be clearer then (on the Btrfs wiki): nobarrier is an
>>> absolute no go. Case closed.
>>
>> Sometimes it is useful to make an ephemeral filesystem, i.e. a btrfs on
>> a dm-crypt device with a random key that is not stored.  This
>> configuration intentionally and completely destroys the entire
>> filesystem, and all data on it, in the event of a power failure.  It's
>> useful for things like temporary table storage, where ramfs is too
>> small, swap-backed tmpfs is too slow, and/or there is a requirement that
>> the data not be persisted across reboots.
>>
>> In other words, nobarrier is for a little better performance when you
>> already want to _intentionally_ destroy your filesystem on power
>> failure.
>
> Very good explanation of why it's useful to have such an otherwise
> destructive mount option even available in the first place.  Thanks! =:^)
>
The other reason, as has been pointed out in a different sub-thread, is 
that if you have a guaranteed good hardware RAID controller, which has a 
known good built in non-volatile write cache, and you turn off 
write-reordering, and you turn off the write-caches on all the connected 
hard drives, then it is relatively safe.  Of course, the chances of most 
people actually meeting all those conditions is pretty slim.