Re: Proposal: B_ARRIER (addresses wapbl performance?)

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

On Thu, Oct 30, 2008 at 01:28:21PM -0700, Bill Stouder-Studenmund wrote:
> On Wed, Oct 29, 2008 at 05:51:09PM -0400, Thor Lancelot Simon wrote:
>
> The problem is that this won't help. Ordered tags will relate the 
> sequencing of commands relative to each other. The journal, however, 
> doesn't care about the relative ordering of operations, it wants to know 
> when the writes to the journal have hit stable storage.
> 
> The key problem is that, on SCSI disks with the write cache enabled, a
> write command can complete by writing to the cache.

But SCSI disks don't lie like this unless explicitly configured to, and
with proper use of tags, there is no need to configure them that way;
there is no performance benefit.

Never mind that if they have power protection, it's still safe to do so.

So from my point of view, this does precisely what WAPBL needs -- render
it just as safe as a non-journalled filesystem, or safer, while radically
improving performance.

Thor