On Fri, Jan 18, 2019 at 4:43 PM Vivek Goyal <vgoyal@xxxxxxxxxx> 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? There is no contract with metacopy=on that states when a file is copied up as meta and when it is copied up as data. If there was a contract, it would probably state that only data modification operations SHOULD copy up data and open is not a data modification operation (it was just the last opportunity to do anything before stacked file ops). Thanks, Amir.
