On Thu, Aug 19, 2010 at 01:24:07AM +0200, Bodo Eggert wrote: > Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > > On Tue, 17 Aug 2010, Valerie Aurora wrote: > > >> > - hard links to make sure a separate inode is not necessary for each > >> > whiteout/fallthrough entry > >> > >> The problem with hard links is that you run into hard link limits. I > >> don't think we can do hard links for whiteouts and fallthrus. Each > >> whiteout or fallthru will cost an inode if we implement them as > >> extended attributes. This cost has to be balanced against the cost of > >> implementing them as dentries, which is mainly code complexity in > >> individual file systems. > > Not knowing the details, I'd suggest to implement a generic function to > create an attributed inode and let the fs override it to create an > unlinked-file-dentry instead. > > Benefit: All fs supporting extended attributes will be able to support > whiteout. If the fs has other means of supporting whiteout, they may fake > the attribute. Yeah, I think that's the way to go. > Possible problems: > - Having two ways of reporting a whiteout? Or can it be reported using a > (static) fake inode? They are going to look the same at the VFS level and higher. > - How do you un-whiteout while (not) having an overlaying fs? The current version of whiteout support always hides DT_WHT dentries from userspace. Perhaps a start is to only hide DT_WHT entries when the file system is union mounted. Applications usually ignore all dentries with d_ino == 0 so it might not cause problems. Right now, you have to remove whiteouts offline using fsck. > > get_unlinked_inode() is a great idea. But I feel that individual > > inodes for each fallthrough is excessive. It'll make the first > > readdir() really really expensive and wastes a lot of disk and memory > > for no good reason. > > > > Not sure how to fix the hard link limits problem though... > > Do a hardlink if you can create a hard link, otherwise use a fresh inode > and use that for the next hardlink(s). Bleah! Then you have a code path that is only tested when you hit LINK_MAX. Sounds like a recipe for bugs for me. -VAL -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
