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



On 3 March 2012 16:25, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> 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?

Doh! I finally get what you meant by this comment. Tapdisk -is- the
current default for xl.
xm uses loop and thus my mistake, sigh must learn to think before typing.

>
>>  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
>>
>>
>>
>
>

I did have a trawl through the linux kernel and linux-util repos last
night however and it appears that none of the direct-io/O_DIRECT
patches were ever merged.

Joseph.


-- 
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