The aim of log mode COW is solve extent tree performance issue when
free metadata space are scarce and fragmented. The basic idea is:
when cowing a non-shared block, we insert a log entry that maps
the new block to the old block into a special log tree, and do not
update the extent tree. After transaction is fully committed, copy
the new block back to the old location.
Log mode COW can be enabled/disabled by log_mode=%d mount option.
''0'' means disable log mode COW; ''1'' means
enable log for tree blocks
that are not reference counted; ''2'' means enable log for all
tree
blocks are not shared.
NOTE: The patch series base on current btrfs-unstable. They are for
test and review only. Enable log mode COW changes disk format.
Regards
Yan, Zheng
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