Hi Ted, After all these years, I may have finally noticed a better approach to the NFS directory cookie problem in HTree. Let me summarize the issue for those not privy to its horrors: * The Single Unix Specification defines semantics for directory traversal whereby it is permitted to create and delete directory entries simultaneously with a process that reads the directory entries and does something with them. * SUS specifies that a sequence of all the files in a directory will be returned by the readdir call, and though it leaves unspecified whether a simultaneously deleted or created directory entry will appear in the sequence, it does seem to require that no entry appear more than once and that no entry that existed before the during the entire directory traversal be omitted. (This is not directly stated, but can be inferred from the definition of directory stream.) http://www.opengroup.org/onlinepubs/009695399/functions/readdir.html The implied requirement that no pre-existing directory entry be returned multiple times or be omitted poses problems for modern btree directory indexing schemes, which like to move batches of entries around on disk as new entries are created or old ones deleted. Whatever the indexing scheme, it is necessary to come up with some stable enumeration of directory entries that satisfies the (implied) SUS requirement. Enter NFS with a monumental design blunder that makes this exceptionally difficult to achieve for any modern directory indexing scheme. You summarize the issues authoritavely here: http://lkml.org/lkml/2007/4/7/159 One nit: NFS v2 provides 31 bits worth of telldir cookie, not 32. (But you knew that) I see that you have been busy explaining the gory details to Chris, as you once explained them to me: http://oss.oracle.com/pipermail/btrfs-devel/2008-January/000460.html The code to deal with the telldir cookie problem in HTree was written by one Theodore Y Ts'o in 2002, and consists of some hundreds of lines of clever RB tree hacking that miraculously works so reliably that none of the tens of millions of Ext3 users has had any complaint for years. Now six years after the fact, I think there is a nicer way. This is something of a duh: if HTree refrains from splitting leaves while a directory traversal is in progress then a simple linear traversal can be used in classic Ext2/UFS fashion, with the logical file address of each directory entry serving as the telldir cookie. The idea is to disable leaf splitting during directory traversal and use an alternate leaf overflow mechanism in its place. To handle a create into a full leaf when splitting is disabled, a new leaf block is created having the same hash index as the current one and inserted before the full leaf in the hash tree to hold the new entry and any subsequent creates into the same hash bucket. The lookup probe has to be modified slightly to traverse all leaves with the same hash index. Or maybe this already works. HTree could opportunistically re-distribute the entries later when no directory listing is in progress, or it could just leave things as they are. The fallback is quite unlikely to occur in practice, and the alternative storage format is only very slightly less efficient to process on lookup, and does not consume any extra space. A possible heuristic for disabling node splitting: disable on each opendir until a matching number of closedirs are received, unless there was a telldir in which case it stays disabled until a following readdir returns NULL, followed by matching closedirs. A new feature bit would be needed should the new overflow code path ever be hit for a given directory (unlikely...) in which case an older HTree-enabled ext3 would fall back to linear scan if somebody manages to write a volume under an ungraded kernel then reboot to an older kernel. There would also be a flag in the HTree index root block that does not affect the volume as a whole. I think that the way we did it, any unknown HTRee flag just causes a fallback to linear scan. (If so, this would be a nice example of forward compatibility gone right.) Now the sticky bit where the brain damaged NFS semantics really get to take their best kick at us. This mechanism has to work across NFS server reboots. We can notice from the flag in the Htree index root block that a particular directory went into an "NFS mode" that was never properly completed. But how can we know when to take the directory out of NFS mode and get on with a slightly more efficient version of life? Well. There are two kinds of NFS servers available on Linux: 1) kernel and 2) userspace. The kernel version is... part of the kernel, and so we can always know when it has mounted our volume. We could give it a minute or two to reconnect to the client that was doing the readdir, then assume the readdir will never resume. Otherwise, if the readdir does resume in that time, the client will get exactly the SUS semantics and things will work properly even if the server reboots again. The user space server is another story. I have no idea how to detect its presence, and I doubt we need to care, because it already suffers from fatal flaws such as: "some awkwardnesses in the semantics (for instance, moving a file to a different directory will render its file handle invalid)" http://packages.debian.org/sid/nfs-user-server Nobody should use this server, and if they do, they simply risk dropouts and repeated filenames in a directory listing if they go out of their way to abuse it. What you would have to do to break the above heuristic for the user space NFS server: * NFS client starts a directory listing * Server reboots after retrieving some dirents * NFS client creates some new files in the directory, splitting a btree block * Client side ls omits or repeats a few names. Sob. * Moral of the story is: use knfsd if you want your NFS server to respect reboot semantics properly, or to operate properly at all. If the new semantics are acceptable, the payoff for adopting this simpler method is: * Can use a more efficient HTree hash because we do not need to be so paranoid about bucket levelling to avoid rare cases that might break the RB tree approach. * We get to use the nice shiny alternative hash mechanism that we put considerable effort into building but so far have never used. * Considerably more efficient directory traversal. * Delete will now happen in physical create order and therefore not thrash the inode table, which is a problem that HTree can currently hit with large directories ant tight cache. * Several hundred lines of special purpose code removed. (Replaced by maybe 50 different lines of special purpose code.) Drawbacks: * Adds a new feature to knfsd to force it to advertise its presence to filesystems. On the other hand, this might be seen as a feature useful to other filesystems (btrfs and jfs come to mind). * Breaks the user space NFS server worse than it already is. But it is already fatally broken, so do we care? Regards, Daniel -- 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
