On 2 Oct 2020, at 11:24, Dr. David Alan Gilbert <dgilbert@redhat.com> wrote:

* Fabiano Fidêncio (fidencio@redhat.com) wrote:
Snir,

Firstly, let me reply to one specific bit of your email and also add a question.

[...]

IIUC it's currently enabled by default (qemu), can it be disabled? if
not, is it planned to be configurable?

The current govmm codes only sets the "cache-size" when it's different
from zero:
https://github.com/intel/govmm/blob/master/qemu/qemu.go#L1256-L1258

So, setting virtio_fs_cache_size here
https://github.com/kata-containers/kata-containers/blob/2.0-dev/src/runtime/cli/config/configuration-qemu-virtiofs.toml.in#L113-L114
disables DAX.

A question I'd like to add to Snir's question is ...
Wouldn't it be safer to have `virtio_fs_cache_size = 0` as a default
value when using virtio-fs, at least till DAX gets solid?

I think that should work; the current qemu vhost-user-fs.c code has:

   DEFINE_PROP_SIZE("cache-size", VHostUserFS, conf.cache_size, 0),

so ommitting the property should give us 0 and cache disabled and
ommitting the property also means you work with upstream qemu's.

I do remember in older dev branches we had the default none-0
but finally found the error of our ways.

Do you have any idea of the timeline for the DAX issues to be resolved?
2.0 is a long-lived branch, presumably, so we want to have something
in place that works when DAX is here.

You may want to look at https://bugzilla.redhat.com/show_bug.cgi?id=1876915#c30


Dave


[...]

Best Regards,
-- 
Fabiano Fidêncio


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-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK


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