On Tue, Oct 04, 2022 at 01:57:50AM +0200, Denis V. Lunev wrote: > On 10/3/22 21:53, Stefan Hajnoczi wrote: > > On Fri, Sep 30, 2022 at 05:24:11PM +0800, Ming Lei wrote: > > > ublk-qcow2 is available now. > > Cool, thanks for sharing! > yep > > > > So far it provides basic read/write function, and compression and snapshot > > > aren't supported yet. The target/backend implementation is completely > > > based on io_uring, and share the same io_uring with ublk IO command > > > handler, just like what ublk-loop does. > > > > > > Follows the main motivations of ublk-qcow2: > > > > > > - building one complicated target from scratch helps libublksrv APIs/functions > > > become mature/stable more quickly, since qcow2 is complicated and needs more > > > requirement from libublksrv compared with other simple ones(loop, null) > > > > > > - there are several attempts of implementing qcow2 driver in kernel, such as > > > ``qloop`` [2], ``dm-qcow2`` [3] and ``in kernel qcow2(ro)`` [4], so ublk-qcow2 > > > might useful be for covering requirement in this field > There is one important thing to keep in mind about all partly-userspace > implementations though: > * any single allocation happened in the context of the > userspace daemon through try_to_free_pages() in > kernel has a possibility to trigger the operation, > which will require userspace daemon action, which > is inside the kernel now. > * the probability of this is higher in the overcommitted > environment > > This was the main motivation of us in favor for the in-kernel > implementation. CCed Josef Bacik because the Linux NBD driver has dealt with memory reclaim hangs in the past. Josef: Any thoughts on userspace block drivers (whether NBD or ublk) and how to avoid hangs in memory reclaim? Stefan
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