[dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

[david at lang.hm]

On Thu, 31 May 2007, Jens Axboe wrote:

> On Thu, May 31 2007, Phillip Susi wrote:
>> David Chinner wrote:
>>> That sounds like a good idea - we can leave the existing
>>> WRITE_BARRIER behaviour unchanged and introduce a new WRITE_ORDERED
>>> behaviour that only guarantees ordering. The filesystem can then
>>> choose which to use where appropriate....
>>
>> So what if you want a synchronous write, but DON'T care about the order?
>>   They need to be two completely different flags which you can choose
>> to combine, or use individually.
>
> If you have a use case for that, we can easily support it as well...
> Depending on the drive capabilities (FUA support or not), it may be
> nearly as slow as a "real" barrier write.

true, but a "real" barrier write could have significant side effects on 
other writes that wouldn't happen with a synchronous wrote (a sync wrote 
can have other, unrelated writes re-ordered around it, a barrier write 
can't)

David Lang