Re: [PATCH 01/21] block: Add atomic write operations to request_queue limits

[John Garry]

On 04/10/2023 22:00, Bart Van Assche wrote:
> > We only care about *PF. The *N variants were cut from the same cloth as
> > TRIM and UNMAP.
>
> Hi Martin,
>
> Has the following approach been considered? RWF_ATOMIC only guarantees 
> atomicity. Persistence is not guaranteed without fsync() / fdatasync().

This is the approach taken. Please consult the proposed man pages, where 
we say that persistence is not guaranteed without 
O_SYNC/O_DSYNC/fsync()/fdatasync()

The only thing which RWF_ATOMIC guarantees is that the write will not be 
torn.

If you see 2.1.4.2.2 Non-volatile requirements in the NVMe spec, it 
implies that the FUA bit or a flush command is required for persistence.

In 4.29.2 Atomic write operations that do not complete in SBC-4, we are 
told that atomic writes may pend in the device volatile cache and no 
atomic write data will be written if a power failure causes loss of data 
from the write.

> I think this would be more friendly towards battery-powered devices
> (smartphones). On these devices it can be safe to skip fsync() / 
> fdatasync() if the battery level is high enough.

Thanks,
John