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Date:   Thu, 28 Oct 2021 11:24:08 -0700
From:   Ira Weiny <>
To:     JeffleXu <>
Cc:     Vivek Goyal <>,
        "Darrick J. Wong" <>,
        Theodore Ts'o <>,,,
        "" <>,,,
        Christoph Hellwig <>,
        Dave Chinner <>
Subject: Re: [Question] ext4/xfs: Default behavior changed after per-file DAX

On Thu, Oct 28, 2021 at 01:52:27PM +0800, JeffleXu wrote:
> On 10/27/21 10:36 PM, Vivek Goyal wrote:
> > [snip]
> > 
> >>
> >> Is the biggest issue the lack of visibility to see if the device supports DAX?
> > 
> > Not necessarily. I think for me two biggest issues are.
> > 
> > - Should dax be enabled by default in server as well. If we do that,
> >   server will have to make extra ioctl() call on every LOOKUP and GETATTR
> >   fuse request. Local filesystems probably can easily query FS_XFLAGS_DAX
> >   state but doing extra syscall all the time will probably be some cost
> >   (No idea how much).
> I tested the time cost from virtiofsd's perspective (time cost of
> passthrough_ll.c:lo_do_lookup()):
> - before per inode DAX feature: 2~4 us
> - after per inode DAX feature: 7~8 us
> It is within expectation, as the introduction of per inode DAX feature,
> one extra ioctl() system call is introduced.
> Also the time cost from client's perspective (time cost of
> fs/fuse/dir.c:fuse_lookup_name())
> - before per inode DAX feature: 25~30 us
> - after per inode DAX feature: 30~35 us
> That is, ~15%~20% performance loss.
> Currently we do ioctl() to query the persitent inode flags every time
> FUSE_LOOKUP request is received, maybe we could cache the result of
> ioctl() on virtiofsd side, but I have no idea how to intercept the
> runtime modification to these persistent indoe flags from other
> processes on host, e.g. sysadmin on host, to maintain the cache consistency.

Do you really expect the dax flag to change on individual files a lot?  This in
itself is an expensive operation as the FS has to flush the inode.

> So if the default behavior of client side is 'dax=inode', and virtiofsd
> disables per inode DAX by default (neither '-o dax=server|attr' is

I'm not following what dax=server or dax=attr is?

> specified for virtiofsd) for the sake of performance, then guest won't
> see DAX enabled and thus won't be surprised. This can reduce the
> behavior change to the minimum.

What processes, other than virtiofsd have 'control' of these files?

I know that a sysadmin could come in and change the dax flag but I think that
is like saying a sys-admin can come in and change your .bashrc and your
environment is suddenly different.  We have to trust the admins not to do stuff
like that.  So I don't think admins are going to be changing the dax flag on
files out from under 'users'; in this case virtiofsd.  Right?

That means that virtiofsd could cache the status and avoid the performance
issues above correct?


> > 
> > - So far if virtiofs is mounted without any of the dax options, just
> >   by looking at mount option, I could tell, DAX is not enabled on any
> >   of the files. But that will not be true anymore. Because dax=inode
> >   be default, it is possible that server upgrade enabled dax on some
> >   or all the files.
> > 
> >   I guess I will have to stick to same reason given by ext4/xfs. That is
> >   to determine whether DAX is enabled on a file or not, you need to
> >   query STATX_ATTR_DAX flag. That's the only way to conclude if DAX is
> >   being used on a file or not. Don't look at filesystem mount options
> >   and reach a conclusion (except the case of dax=never).
> -- 
> Thanks,
> Jeffle

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