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

[Joseph Glanville <joseph.glanville@xxxxxxxxxxxxxx>]

On 4 March 2012 14:01, Joseph Glanville <joseph.glanville@xxxxxxxxxxxxxx> wrote:
> 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.
>
>
> --
> Founder | Director | VP Research
> Orion Virtualisation Solutions | www.orionvm.com.au | Phone: 1300 56
> 99 52 | Mobile: 0428 754 846

For interested parties this is the original patch series I was reffering to:
http://www.spinics.net/lists/linux-fsdevel/msg27514.html


There also seems to be a revival of this effort as of about a week ago:
https://lkml.org/lkml/2012/2/28/251

Kind regards,
Joseph.

-- 
Founder | Director | VP Research
Orion Virtualisation Solutions | www.orionvm.com.au | Phone: 1300 56
99 52 | Mobile: 0428 754 846

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel