[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH 2/2] libxl: switch NetBSD image file handling to Qemu
On 28/11/12 16:29, Thor Lancelot Simon wrote: > On Wed, Nov 28, 2012 at 03:41:56PM +0100, Roger Pau Monn? wrote: >> On 28/11/12 14:26, Thor Lancelot Simon wrote: >>> >>> I am assuming there's some reason we want this, rather than handling >>> such requests in-kernel. But what is it? >> >> I should have marked this as "experimental", or something like this. I'm >> not sure if switching to Qdisk for all image file backends is needed. >> >From the Xen wiki (http://wiki.xen.org/wiki/Blktap) I've found that: >> >> "loop device had problems with flushing dirty pages (specifically, doing >> a large number of writes to an NFS-backed image don't result in the OOM >> killer going berserk)." >> >> I'm not sure if NetBSD is in the same situation, but if I remember >> correctly (haven't tried that in a long time), trying to use a disk file >> on a NFS share caused the NetBSD Dom0 kernel to crash. The PR for this >> issue is: http://gnats.netbsd.org/40726. > > It seems highly unlikely to me that a problem with the loop device driver > on Linux is tremendously relevant to NetBSD. > > But, even if there is some problem with vnd backed by NFS, that hardly > seems like it would be a good reason to make a change that reduces I/O > throughput for the *non* NFS-backed case by at least 20%. Why would one > keep disk images for guests, as a general rule, on NFS, rather than simply > doing the NFS mounts on the guests themselves, or using a more sensible > protocol like iSCSI? I have to assume most folks using files as disks are > storing them on local filesystems on the dom0, and wrecking performance > for that case to solve a problem with NFS that may or may not be hypothetical > seems like a very, very bad trade-off. Quite a lot of people use disk images on NFS, because it's probably the easiest shared storage that allows migration of guests. Even if disks on NFS was a remote possibility not used by much people I don't think it's acceptable to have a Dom0 crash when you try to create a guest with a disk on NFS, that is by far much more worse than performance degradation in my opinion. If this is not accepted, I will have to detect image format somewhere and use qemu when the format is different than raw. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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