Re: [PATCH] barrier patch set

[Bartlomiej Zolnierkiewicz]

On Saturday 20 of March 2004 18:10, Chris Mason wrote:
> On Sat, 2004-03-20 at 12:05, Bartlomiej Zolnierkiewicz wrote:
> > On Saturday 20 of March 2004 17:32, Chris Mason wrote:
> > > On Sat, 2004-03-20 at 11:23, Bartlomiej Zolnierkiewicz wrote:
> > > > > > - why are we doing pre-flush?
> > >
> > > The journaled filesystems need this.  We need to make sure that before
> > > we write the commit block for a transaction, all the previous log
> > > blocks we're written are safely on media.  Then we also need to make
> > > sure the commit block is on media.
> >
> > For low-level driver it shouldn't really matter whether sectors to be
> > written are the commit block for a transaction or the previous log blocks
> > and in the current implementation it does matter.
>
> As Jens said, it depends on how you define barrier ;-)  I define it as
> this io will be written after all the previous io and before any later
> io.   It was originally written with scsi tags in mind as well, the FS
> side was the same for both.

Yes, thanks for explaining this.

I took a quick look at fs/jbd/ and now I think I understand the way barriers
currently work.  I assume that SCSI handles barriers by ordered tags, right?

> In the end, I'm not that picky though, any reasonable setup that gets
> the blocks on media is fine.

Yep.

Regards,
Bartlomiej

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