On Fri, Mar 31, 2023 at 10:39:23AM -0400, Peter Xu wrote: > On Fri, Mar 31, 2023 at 08:56:01AM +0100, Daniel P. Berrangé wrote: > > On Thu, Mar 30, 2023 at 06:01:51PM -0400, Peter Xu wrote: > > > On Thu, Mar 30, 2023 at 03:03:20PM -0300, Fabiano Rosas wrote: > > > > From: Nikolay Borisov <nbori...@suse.com> > > > > > > > > Implement 'fixed-ram' feature. The core of the feature is to ensure that > > > > each ram page of the migration stream has a specific offset in the > > > > resulting migration stream. The reason why we'd want such behavior are > > > > two fold: > > > > > > > > - When doing a 'fixed-ram' migration the resulting file will have a > > > > bounded size, since pages which are dirtied multiple times will > > > > always go to a fixed location in the file, rather than constantly > > > > being added to a sequential stream. This eliminates cases where a vm > > > > with, say, 1G of ram can result in a migration file that's 10s of > > > > GBs, provided that the workload constantly redirties memory. > > > > > > > > - It paves the way to implement DIO-enabled save/restore of the > > > > migration stream as the pages are ensured to be written at aligned > > > > offsets. > > > > > > > > The feature requires changing the stream format. First, a bitmap is > > > > introduced which tracks which pages have been written (i.e are > > > > dirtied) during migration and subsequently it's being written in the > > > > resulting file, again at a fixed location for every ramblock. Zero > > > > pages are ignored as they'd be zero in the destination migration as > > > > well. With the changed format data would look like the following: > > > > > > > > |name len|name|used_len|pc*|bitmap_size|pages_offset|bitmap|pages| > > > > > > What happens with huge pages? Would page size matter here? > > > > > > I would assume it's fine it uses a constant (small) page size, assuming > > > that should match with the granule that qemu tracks dirty (which IIUC is > > > the host page size not guest's). > > > > > > But I didn't yet pay any further thoughts on that, maybe it would be > > > worthwhile in all cases to record page sizes here to be explicit or the > > > meaning of bitmap may not be clear (and then the bitmap_size will be a > > > field just for sanity check too). > > > > I think recording the page sizes is an anti-feature in this case. > > > > The migration format / state needs to reflect the guest ABI, but > > we need to be free to have different backend config behind that > > either side of the save/restore. > > > > IOW, if I start a QEMU with 2 GB of RAM, I should be free to use > > small pages initially and after restore use 2 x 1 GB hugepages, > > or vica-verca. > > > > The important thing with the pages that are saved into the file > > is that they are a 1:1 mapping guest RAM regions to file offsets. > > IOW, the 2 GB of guest RAM is always a contiguous 2 GB region > > in the file. > > > > If the src VM used 1 GB pages, we would be writing a full 2 GB > > of data assuming both pages were dirty. > > > > If the src VM used 4k pages, we would be writing some subset of > > the 2 GB of data, and the rest would be unwritten. > > > > Either way, when reading back the data we restore it into either > > 1 GB pages of 4k pages, beause any places there were unwritten > > orignally will read back as zeros. > > I think there's already the page size information, because there's a bitmap > embeded in the format at least in the current proposal, and the bitmap can > only be defined with a page size provided in some form. > > Here I agree the backend can change before/after a migration (live or > not). Though the question is whether page size matters in the snapshot > layout rather than what the loaded QEMU instance will use as backend.
IIUC, the page size information merely sets a constraint on the granularity of unwritten (sparse) regions in the file. If we didn't want to express page size directly in the file format we would need explicit start/end offsets for each written block. This is less convenient that just having a bitmap, so I think its ok to use the page size bitmap > > > If postcopy might be an option, we'd want the page size to be the host > > > page > > > size because then looking up the bitmap will be straightforward, deciding > > > whether we should copy over page (UFFDIO_COPY) or fill in with zeros > > > (UFFDIO_ZEROPAGE). > > > > This format is only intended for the case where we are migrating to > > a random-access medium, aka a file, because the fixed RAM mappings > > to disk mean that we need to seek back to the original location to > > re-write pages that get dirtied. It isn't suitable for a live > > migration stream, and thus postcopy is inherantly out of scope. > > Yes, I've commented also in the cover letter, but I can expand a bit. > > I mean support postcopy only when loading, but not when saving. > > Saving to file definitely cannot work with postcopy because there's no dest > qemu running. > > Loading from file, OTOH, can work together with postcopy. Ahh, I see what you mean. > Right now AFAICT current approach is precopy loading the whole guest image > with the supported snapshot format (if I can call it just a snapshot). > > What I want to say is we can consider supporting postcopy on loading in > that we start an "empty" QEMU dest node, when any page fault triggered we > do it using userfault and lookup the snapshot file instead rather than > sending a request back to the source. I mentioned that because there'll be > two major benefits which I mentioned in reply to the cover letter quickly, > but I can also extend here: > > - Firstly, the snapshot format is ideally storing pages in linear > offsets, it means when we know some page missing we can use O(1) time > looking it up from the snapshot image. > > - Secondly, we don't need to let the page go through the wires, neither > do we need to send a request to src qemu or anyone. What we need here > is simply test the bit on the snapshot bitmap, then: > > - If it is copied, do UFFDIO_COPY to resolve page faults, > - If it is not copied, do UFFDIO_ZEROPAGE (e.g., if not hugetlb, > hugetlb can use a fake UFFDIO_COPY) > > So this is a perfect testing ground for using postcopy in a very efficient > way against a file snapshot. Yes, that's an nice unexpected benefit of this fixed ram file format. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|