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Re: [Qemu-devel] [RFC PATCH 0/7] virtio-fs: shared file system for virtu
From: |
jiangyiwen |
Subject: |
Re: [Qemu-devel] [RFC PATCH 0/7] virtio-fs: shared file system for virtual machines3 |
Date: |
Tue, 8 Jan 2019 14:08:24 +0800 |
User-agent: |
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1 |
On 2018/12/27 3:08, Vivek Goyal wrote:
> On Sat, Dec 22, 2018 at 05:27:28PM +0800, jiangyiwen wrote:
>> On 2018/12/11 1:31, Dr. David Alan Gilbert (git) wrote:
>>> From: "Dr. David Alan Gilbert" <address@hidden>
>>>
>>> Hi,
>>> This is the first RFC for the QEMU side of 'virtio-fs';
>>> a new mechanism for mounting host directories into the guest
>>> in a fast, consistent and secure manner. Our primary use
>>> case is kata containers, but it should be usable in other scenarios
>>> as well.
>>>
>>> There are corresponding patches being posted to Linux kernel,
>>> libfuse and kata lists.
>>>
>>> For a fuller design description, and benchmark numbers, please see
>>> Vivek's posting of the kernel set here:
>>>
>>> https://marc.info/?l=linux-kernel&m=154446243024251&w=2
>>>
>>> We've got a small website with instructions on how to use it, here:
>>>
>>> https://virtio-fs.gitlab.io/
>>>
>>> and all the code is available on gitlab at:
>>>
>>> https://gitlab.com/virtio-fs
>>>
>>> QEMU's changes
>>> --------------
>>>
>>> The QEMU changes are pretty small;
>>>
>>> There's a new vhost-user device, which is used to carry a stream of
>>> FUSE messages to an external daemon that actually performs
>>> all the file IO. The FUSE daemon is an external process in order to
>>> achieve better isolation for security and resource control (e.g. number
>>> of file descriptors) and also because it's cleaner than trying to
>>> integrate libfuse into QEMU.
>>>
>>> This device has an extra BAR that contains (up to) 3 regions:
>>>
>>> a) a DAX mapping range ('the cache') - into which QEMU mmap's
>>> files on behalf of the external daemon; those files are
>>> then directly mapped by the guest in a way similar to a DAX
>>> backed file system; one advantage of this is that multiple
>>> guests all accessing the same files should all be sharing
>>> those pages of host cache.
>>>
>>> b) An experimental set of mappings for use by a metadata versioning
>>> daemon; this mapping is shared between multiple guests and
>>> the daemon, but only contains a set of version counters that
>>> allow a guest to quickly tell if its metadata is stale.
>>>
>>> TODO
>>> ----
>>>
>>> This is the first RFC, we know we have a bunch of things to clear up:
>>>
>>> a) The virtio device specificiation is still in flux and is expected
>>> to change
>>>
>>> b) We'd like to find ways of reducing the map/unmap latency for DAX
>>>
>>> c) The metadata versioning scheme needs to settle out.
>>>
>>> d) mmap'ing host files has some interesting side effects; for example
>>> if the file gets truncated by the host and then the guest accesses
>>> the mapping, KVM can fail the guest hard.
>>>
>>> Dr. David Alan Gilbert (6):
>>> virtio: Add shared memory capability
>>> virtio-fs: Add cache BAR
>>> virtio-fs: Add vhost-user slave commands for mapping
>>> virtio-fs: Fill in slave commands for mapping
>>> virtio-fs: Allow mapping of meta data version table
>>> virtio-fs: Allow mapping of journal
>>>
>>> Stefan Hajnoczi (1):
>>> virtio: add vhost-user-fs-pci device
>>>
>>> configure | 10 +
>>> contrib/libvhost-user/libvhost-user.h | 3 +
>>> docs/interop/vhost-user.txt | 35 ++
>>> hw/virtio/Makefile.objs | 1 +
>>> hw/virtio/vhost-user-fs.c | 517 ++++++++++++++++++++
>>> hw/virtio/vhost-user.c | 16 +
>>> hw/virtio/virtio-pci.c | 115 +++++
>>> hw/virtio/virtio-pci.h | 19 +
>>> include/hw/pci/pci.h | 1 +
>>> include/hw/virtio/vhost-user-fs.h | 79 +++
>>> include/standard-headers/linux/virtio_fs.h | 48 ++
>>> include/standard-headers/linux/virtio_ids.h | 1 +
>>> include/standard-headers/linux/virtio_pci.h | 9 +
>>> 13 files changed, 854 insertions(+)
>>> create mode 100644 hw/virtio/vhost-user-fs.c
>>> create mode 100644 include/hw/virtio/vhost-user-fs.h
>>> create mode 100644 include/standard-headers/linux/virtio_fs.h
>>>
>>
>> Hi Dave,
>>
>> I encounter a problem after running qemu with virtio-fs,
>>
>> I find I only can mount virtio-fs using the following command:
>> mount -t virtio_fs /dev/null /mnt/virtio_fs/ -o
>> tag=myfs,rootmode=040000,user_id=0,group_id=0
>> or mount -t virtio_fs /dev/null /mnt/virtio_fs/ -o
>> tag=myfs,rootmode=040000,user_id=0,group_id=0,dax
>>
>> Then, I want to know how to use "cache=always" or "cache=none", even
>> "cache=auto", "cache=writeback"?
>>
>> Thanks,
>> Yiwen.
>
> Hi Yiwen,
>
> As of now, cache options are libfuse daemon options. So while starting
> daemon, specify "-o cache=none" or "-o cache=always" etc. One can not
> specify caching option at virtio-fs mount time.
>
> Thanks
> Vivek
>
> .
>
Ok, I get it, thanks.
Yiwen.
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