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Date: Fri, 25 Mar 2011 12:38:28 +0100 From: Szeredi Miklos <miklos@...redi.hu> To: Ric Wheeler <rwheeler@...hat.com> Cc: Valerie Aurora <valerie.aurora@...il.com>, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk, Christoph Hellwig <hch@...radead.org> Subject: Re: Union mounts comparison with overlay file system prototype? On Thu, Mar 24, 2011 at 2:43 PM, Ric Wheeler <rwheeler@...hat.com> wrote: > Can one or both of you summarize what we union mounts and overlay do better > or worse? Do we need both or just one? The semantics are very similar, the differences are in the implementation. Union mounts: - whiteout/opaque/fallthrough support in filesystems - whiteout operation is atomic - no dentry and inode duplication - copy up on lookup and readdir - does not support union of two read-only trees - merged directory stored in upper tree Overlayfs - whiteout/opaque as xattrs - whiteout operation is not atomic - dentry and inode duplication(*) - only copy up on modification - supports union of two read-only trees - merged directory not cached(**) (*) it's possible to eliminate inode duplication of non-directories with some VFS modifications (**) caching should be possible to do -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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