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

[Joseph Glanville <joseph.glanville@xxxxxxxxxxxxxx>]

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).

Unfortunately changing the default behavior to tapdisk 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.
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.
>
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