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@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? 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
