log mode COW

RFC Yan, Zheng 1 participant 1 message May 11, 2010 Btrfs file system

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Yan, Zheng

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2010-05-11 08:23 UTC #0 lore raw

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