|
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] 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. -- 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
|
 |
Lists.xenproject.org is hosted with RackSpace, monitoring our |