Re: qemu-img cache modes with Linux cgroup v1

["Richard W.M. Jones" <rjones@redhat.com>]

On Mon, Jul 31, 2023 at 11:40:36AM -0400, Stefan Hajnoczi wrote:
> 3. Using buffered I/O because O_DIRECT is not universally supported?
> 
> If you can't use O_DIRECT, then qemu-img could be extended to manage its
> dirty page cache set carefully. This consists of picking a budget and
> writing back to disk when the budget is exhausted. Richard Jones has
> shared links covering posix_fadvise(2) and sync_file_range(2):
> https://lkml.iu.edu/hypermail/linux/kernel/1005.2/01845.html
> https://lkml.iu.edu/hypermail/linux/kernel/1005.2/01953.html
> 
> We can discuss qemu-img code changes and performance analysis more if
> you decide to take that direction.

There's a bit more detail in these two commits:

  https://gitlab.com/nbdkit/libnbd/-/commit/64d50d994dd7062d5cce21f26f0e8eba0e88c87e
  https://gitlab.com/nbdkit/nbdkit/-/commit/a956e2e75d6c88eeefecd967505667c9f176e3af

In my experience this method is much better than using O_DIRECT,
it has much fewer sharp edges.

By the way, this is a super-useful tool for measuring how much of the
page cache is being used to cache a file:

  https://github.com/Feh/nocache

Rich.

-- 
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