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

[Stefan Bader]

> The order that these are expected by the filesystem to hit stable
> storage are:
>
> 1. block 4 and 10 on stable storage in any order
> 2. barrier block X on stable storage
> 3. block 5 and 20 on stable storage in any order
>
> The point I'm trying to make is that in XFS,  block 5 and 20 cannot
> be allowed to hit the disk before the barrier block because they
> have strict order dependency on block X being stable before them,
> just like block X has strict order dependency that block 4 and 10
> must be stable before we start the barrier block write.
>

That would be the exactly how I understand Documentation/block/barrier.txt:

"In other words, I/O barrier requests have the following two properties.
1. Request ordering
...
2. Forced flushing to physical medium"

"So, I/O barriers need to guarantee that requests actually get written
to non-volatile medium in order."

Stefan