Re: [RFC][PATCH] ovl: lazy copy up of data on first write

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Jan 18, 2019 at 05:33:36PM +0200, Amir Goldstein wrote:
> On Fri, Jan 18, 2019 at 4:43 PM Vivek Goyal <vgoyal@redhat.com> wrote:
> >
> > On Fri, Jan 18, 2019 at 03:45:19PM +0200, Amir Goldstein wrote:
> > > When metacopy feature is enabled, copy up only metadata when
> > > opening a file O_RDWR and defer copy up of data until the first
> > > write operation.
> > >
> >
> > Amir,
> >
> > What's the primary use case of lazy data copy up. Are there users who
> > open file O_RDWR but never write to it.
> 
> I don't know of all the cases, but AFAIK, MSOffice apps (over network share
> and maybe also LibreOffice) open the document file O_RDWR just to check
> if editing is allowed, but but they re-open the file read-only, because the
> document file is never written to. A temp file is written and moved over it.
> So copy up of an MSOffice document file data is never beneficial.
> 
> > Or we want to transfer latency of copy up from open to write.
> 
> Indeed. Chengguang reported that in their use case, the latency at open
> time is an issue.
> 
> >
> > Should this behavior be an opt in with another mount option (and not
> > be enabled automatically with metacopy=on).
> >
> 
> I can't think of one good reason for user to opt-in for copy up data on open.
> Or on a use case where latency on open is desired over latency on write.
> Can you?

IIUC, now if I open a file O_RDWR and only issue reads to file and never
issue writes, then for every read, I will open a lower file, finish read
and close fd. This is slower path. What's the performance penalty? If
this is significant, then it might make sense to opt-in for lazy copy
up behavior (instead of relying that application will open file O_RDONLY
if they never issue writes to it).

Thanks
Vivek