Erez Zadok wrote: > > > Jamie Lokier wrote: > > > Phillip Lougher wrote: > > > If I read the patches correctly, when a file page is written to, only > > > that page gets copied into the page cache and locked, the other pages > > > continue to be read off disk from cramfs? With Unionfs a page write > > > causes the entire file to be copied up to the r/w tmpfs and locked into > > > the page cache causing unnecessary RAM overhead. > > Yes, unionfs does copyup whole files, but it doesn't lock the entire file > into the page cache. But I agree, that copying up large files to a tmpfs > partition adds more memory pressure, at least temporarily (until pdflush > kicks in). 1: I'm thinking systems which have union-over-cramfs probably don't have swap at all... 2: It's a problem when you modify a very large file, even on a fast PC with plenty of RAM. LVM snapshots might be better for this sort of thing. > > Ok, so why not fix that in unionfs? An option so that holes in the > > overlay file let through data from the underlying file sounds like it > > would be generally useful, and quite easy to implement. > > If I understand you right, you want to copyup one page at a time, right? > That's not nearly as easy as one might imagine. First, you can't do it on > file systems which don't support holes. Second, holes is a file-systems > specific implementation issue, and the knowledge of holes AFAIC, is hidden > from the VFS (IIRC, FreeBSD has a specific "zfod" page flag, which is turned > on when the VM has a page that came out of a f/s hole). True, although the new FIEMAP ioctl is supposed to make holes more filesystem independent, when they are supported. > You'll need a way to tell if a given page was copied up or not, and > distinguish b/t pages which are naturally filled with zeros vs. those which > came from f/s holes. Metadata. Don't you have other metadata anyway, like whiteouts? :-) > Copyup is also providing persistency: you can copyup to a persistent f/s > such as ext2. So you'll need a bitmap or some sort of record that will > survive file system remount and system reboot; such a bitmap will have to > tell which pages of a file have been copied up or not. Yes. > I'm not saying it's not possible, but it's to do this page-wise caching at a > stackable layer than inside a native f/s such as ext2. Now, if there was a > generic VFS op that allowed me to query a file system whether a page it a > given file is a hole or not, then unionfs would be able to do page-wise > copyup easily. See FIEMAP. Is it any use? -- Jamie -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
