On 04/06/2012 04:09 AM, Mark Fasheh wrote:
> Currently btrfs has a limitation on the maximum number of hard links an
> inode can have. Specifically, links are stored in an array of ref
> items:
>
> struct btrfs_inode_ref {
> __le64 index;
> __le16 name_len;
> /* name goes here */
> } __attribute__ ((__packed__));
>
> The ref arrays are found via key triple:
>
> (inode objectid, BTRFS_INODE_EXTREF_KEY, parent dir objectid)
>
> Since items can not exceed the size of a leaf, the total number of links
> that can be stored for a given inode / parent dir pair is limited to under
> 4k. This works fine for the most common case of few to only a handful of
> links. Once the link count gets higher however, we begin to return EMLINK.
>
>
> The following patches fix this situation by introducing a new ref item:
>
> struct btrfs_inode_extref {
> __le64 parent_objectid;
> __le64 index;
> __le16 name_len;
> __u8 name[0];
> /* name goes here */
> } __attribute__ ((__packed__));
>
> Extended refs behave differently from ref arrays in several key areas.
>
> Each extended refs is it's own item so there is no ref array (and
> therefore no limit on size).
>
> As a result, we must use a different addressing scheme. Extended ref keys
> look like:
>
> (inode objectid, BTRFS_INODE_EXTREF_KEY, hash)
>
> Where hash is defined as a function of the parent objectid and link name.
>
> This effectively fixes the limitation, though we have a slightly less
> efficient packing of link data. To keep the best of both worlds then, I
> implemented the following behavior:
>
> Extended refs don't replace the existing ref array. An inode gets an
> extended ref for a given link _only_ after the ref array has been filled. So
> the most common cases shouldn't actually see any difference in performance
> or disk usage as they'll never get to the point where we're using an
> extended ref.
>
> It's important while reading the patches however that there's still the
> possibility that we can have a set of operations that grow out an inode ref
> array (adding some extended refs) and then remove only the refs in the
> array. I don't really see this being common but it's a case we always have
> to consider when coding these changes.
>
> Right now there is a limitation for extrefs in that we're not handling the
> possibility of a hash collision. There are two ways I see we can deal with
> this:
>
> We can use a 56-bit hash and keep a generation counter in the lower 8
> bits of the offset field. The cost would be an additional tree search
> (between offset <hash>00 and <hash>FF) if we don't find exactly the name we
> were looking for.
>
> An alternative solution to dealing with collisions could be to emulate the
> dir-item insertion code - specifically something like insert_with_overflow()
> which will stuff multiple items under one key. I tend to prefer the idea of
> simply including a generation in the key offset however since it maintains
> the 1:1 relationship of keys to names which turns out to be much nicer to
> code for in my honest opinion. Also none of the code which iterates the tree
> looking for refs would have to change as the only difference is in the key
> offset and not in the actual item structure.
>
>
> Testing wise, the patches are in an intermediate state. I've debugged a fair
> bit but I'm certain there's gremlins lurking in there. The basic namespace
> operations work well enough (link, unlink, etc). I've done light testing of
> my changes in backref.c by exercising BTRFS_IOC_INO_PATHS. The changes in
> tree-log.c need the most review and testing - I haven't really figured out a
> great way to exercise the code in tree-log yet (suggestions would be
> great!).
>
For the log recover test, I used to sysrq+b to make sure our log remains on disk.
Will also test this patchset sooner or later.
thanks,
liubo
>
> Finally, these patches are based off Linux v3.3.
> --Mark
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