Re: [Xen-devel] xl block-attach vs block-detach

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

Please don't top post, it destroys the flow of the conversation.

On Fri, 2012-03-02 at 22:54 +0000, Joseph Glanville wrote:
> Hi,
> 
> As I understand it prefering tapdisk over loop+blkback has never been
> for performance reasons historically. (tapdisk2:aio does however
> exhibit very good performance)
> The primary reason that tapdisk was always recommended over file: is
> that the Linux file cache does very interesting things to your data
> and sync is returned to the blkback backend much sooner than the data
> actually resides safely on disk (which can sit in the linux disk cache
> for a sizeable amount of time if they machine has alot of ram).

Are you suggesting that the loop device doesn't support O_DIRECT and
will leave stuff dirty in the page cache even when direct access is
used? That is worth knowing!

> Unfortunately changing the default behavior to tapdisk

What exactly needs changing?

>  probably isn't
> viable at this time for a number of reasons - not least of which is
> the fact it is yet to be included in mainline.

tapdisk is not going to be included in mainline. The kernel side is
deemed to be non-upstreamble.

Someone is working on a fully userspace version of bkltap which we hope
will be ready soon.

Ian.

> However it would definitely be preferable in the long term - atleast
> from the perspective of data integrity and principle of least
> surprises.
> 
> Just my 2c.
> 
> Joseph.
> 
> On 3 March 2012 04:37, Stefano Stabellini
> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> > On Fri, 2 Mar 2012, Stefano Stabellini wrote:
> >> On Fri, 2 Mar 2012, Jan Beulich wrote:
> >> > > What would be the rationale behind using blkback+loop for "file:"?
> >> > > Backward compatibility?
> >> >
> >> > Yes.
> >> >
> >> > > Do you think it might break something for users if we change the 
> >> > > backend
> >> > > from xend to xl?
> >> >
> >> > This cannot be excluded, particularly because (just like me here)
> >> > users tend to do things you didn't expect them to when you write
> >> > the code.
> >>
> >> I see your point but actually that is quite an obvious bug, not a very
> >> subtle one that only happens in strange user configs.
> >>
> >
> > Scratch that: I have just tried on Linux 3.3 and the performance of
> > blkback with loopback is very good. We should use it whenever we can.
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxx
> > http://lists.xen.org/xen-devel
> 
> 
> 



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