Re: [libvirt] [Qemu-devel] QEMU interfaces for image streaming and post-copy block migration

On 09/07/2010 10:02 AM, Kevin Wolf wrote: > Am 07.09.2010 16:49, schrieb Anthony Liguori: > >>> Shouldn't it be a runtime option? You can use the very same image with >>> copy-on-read or copy-on-write and it will behave the same (execpt for >>> performance), so it's not an inherent feature of the image file. >>> >>> >> The way it's implemented in QED is that it's a compatible feature. This >> means that implementations are allowed to ignore it if they want to. >> It's really a suggestion. >> > Well, the point is that I see no reason why an image should contain this > suggestion. There's really nothing about an image that could reasonably > indicate "use this better with copy-on-read than with copy-on-write". > > It's a decision you make when using the image. > Copy-on-read is, in many cases, a property of the backing file because it suggests that the backing file is either very slow or potentially volatile.

IOW, let's say I'm an image distributor and I want to provide my images in a QED format that actually streams the image from an http server. I could provide a QED file without a copy-on-read bit set but I'd really like to convey this information as part of the image. You can argue that I should provide a config file too that contained the copy-on-read flag set but you could make the same argument about backing files too.

>> So yes, you could have a run time switch that overrides the feature bit >> on disk and either forces copy-on-read on or off. >> >> Do we have a way to pass block drivers run time options? >> > We'll get them with -blockdev. Today we're using colons for format > specific and separate -drive options for generic things. > That's right. I think I'd rather wait for -blockdev.

>> You need to understand the cluster boundaries in order to optimize the >> metadata updates. Sure, you can expose interfaces to the block layer to >> give all of this info but that's solving the same problem for doing >> block level copy-on-write. >> >> The other challenge is that for copy-on-read to be efficiently, you >> really need a format that can distinguish between unallocated sectors >> and zero sectors and do zero detection during the copy-on-read >> operation. Otherwise, if you have a 10G virtual disk with a backing >> file that's 1GB is size, copy-on-read will result in the leaf being 10G >> instead of ~1GB. >> > That's a good point. But it's not a reason to make the interface > specific to QED just because other formats would probably not implement > it as efficiently. You really can't do as good of a job in the block layer because you have very little info about the characteristics of the disk image.