Re: Plan: journalling fixes for WAPBL

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Thu, Sep 22, 2016 at 01:27:48AM +0200, Jarom??r Dole??ek wrote:
> 
> 3.2 use FUA (Force Unit Access) for commit record write
> This avoids need to issue even the second DIOCCACHESYNC, as flushing
> the disk cache is not really all that useful, I like the thread over
> at:
> http://yarchive.net/comp/linux/drive_caches.html
> Slightly less controversially, this would allow the rest of the
> journal records to be written asynchronously, leaving them to execute
> even after commit if so desired. It may be useful to have this
> behaviour optional. I lean towards skipping the disk cache flush as
> default behaviour however, if we implement write barrier for the
> commit record (see below).
> WAPBL would need to deal with drives without FUA, i.e fall back to cache flush.

I have never understood this business about needing FUA to implement
barriers.  AFAICT, for any SCSI or SCSI-like disk device, all that is
actually needed is to do standard writes with simple tags, and barrier
writes with ordered tags.  What am I missing?

I must have proposed adding a B_ARRIER or B_ORDERED at least five times
over the years.  There are always objections...

Thor