On Wed, Oct 12, 2011 at 2:31 PM, Kevin Wolf <kw...@redhat.com> wrote: > Am 12.10.2011 14:51, schrieb Stefan Hajnoczi: >>> Also a bit in l2 offset to say "there is no l2 table" cause all >>> clusters in l2 are contiguous so we avoid entirely l2. Obviously this >>> require an optimization step to detect or create such condition. >> >> There are several reserved L1 entry bits which could be used to mark >> this mode. This mode severely restricts qcow2 features though: how >> would snapshots and COW work? Perhaps by breaking the huge cluster >> back into an L2 table with individual clusters? Backing files also >> cannot be used - unless we extend the sub-clusters approach and also >> keep a large bitmap with allocated/unallocated/zero information. >> >> A mode like this could be used for best performance on local storage, >> where efficiently image transport (e.g. scp or http) is not required. >> Actually I think this is reasonable, we could use qemu-img convert to >> produce a compact qcow2 for export and use the L2-less qcow2 for >> running the actual VM. >> >> Kevin: what do you think about fleshing out this mode instead of >> sub-clusters? > > I'm hesitant to something like this as it adds quite some complexity and > I'm not sure if there are practical use cases for it at all. > > If you take the current cluster sizes, an L2 table contains 512 MB of > data, so you would lose any sparseness. You would probably already get > full allocation just by creating a file system on the image. > > But even if you do have a use case where sparseness doesn't matter, the > effect is very much the same as allowing a 512 MB cluster size and not > changing any of the qcow2 internals.
I guess I'm thinking of the 512 MB cluster size situation, because we'd definitely want a cow bitmap in order to keep backing files and sparseness. > (What would the use case be? Backing files or snapshots with a COW > granularity of 512 MB isn't going to fly. That leaves only something > like encryption.) COW granularity needs to stay at 64-256 kb since those are reasonable request sizes for COW. Stefan
