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roots: reimplement transid_max_nocache using extent tree root

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ROOT_TREE contains the ROOT_ITEM for EXTENT_TREE.  Every modification
(that we care about) to a btrfs must go through EXTENT_TREE, and must
modify the page in ROOT_TREE pointing to the root of EXTENT_TREE...
which makes that a very good source for the filesystem transid.

Remove the loop and the root lookups, and just look at one item for
max_transid.

Also note that every caller of transid_max_nocache() immediately
feeds the return value to m_transid_re.update(), so don't do that
inside transid_max_nocache().

Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This commit is contained in:
Zygo Blaxell 2018-10-30 23:31:11 -04:00
parent 0e8b591232
commit 9a97699dd9
1 changed files with 11 additions and 18 deletions
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29
src/bees-roots.cc
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@ -198,19 +198,15 @@ uint64_t
BeesRoots::transid_max_nocache()
{
uint64_t rv = 0;
uint64_t root = BTRFS_FS_TREE_OBJECTID;
BEESNOTE("Calculating transid_max (" << rv << " as of root " << root << ")");
BEESTRACE("Calculating transid_max...");
rv = btrfs_get_root_transid(root);
// XXX: Do we need any of this? Or is
// m_transid_re.update(btrfs_get_root_transid(BTRFS_FS_TREE_OBJECTID)) good enough?
BEESNOTE("Calculating transid_max");
BEESTRACE("Calculating transid_max");
// We look for the root of the extent tree and read its transid.
// Should run in O(1) time and be fairly reliable.
BtrfsIoctlSearchKey sk;
sk.tree_id = BTRFS_ROOT_TREE_OBJECTID;
sk.min_type = sk.max_type = BTRFS_ROOT_BACKREF_KEY;
sk.min_objectid = root;
sk.min_type = sk.max_type = BTRFS_ROOT_ITEM_KEY;
sk.min_objectid = sk.max_objectid = BTRFS_EXTENT_TREE_OBJECTID;
while (true) {
sk.nr_items = 1024;
@ -220,21 +216,18 @@ BeesRoots::transid_max_nocache()
break;
}
// We are just looking for the highest transid on the filesystem.
// We don't care which object it comes from.
for (auto i : sk.m_result) {
sk.next_min(i);
if (i.type == BTRFS_ROOT_BACKREF_KEY) {
if (i.transid > rv) {
BEESLOGDEBUG("transid_max root " << i.objectid << " parent " << i.offset << " transid " << i.transid);
BEESCOUNT(transid_max_miss);
}
root = i.objectid;
}
if (i.transid > rv) {
rv = i.transid;
}
}
}
m_transid_re.update(rv);
// transid must be greater than zero, or we did something very wrong
THROW_CHECK1(runtime_error, rv, rv > 0);
return rv;
}