Auto da alloc

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auto_da_alloc(*) Many broken applications don't use fsync() when noauto_da_alloc replacing existing files via patterns such as fd = open("foo.new")/write(fd,..)/close(fd)/ rename("foo.new", "foo"), or worse yet, fd = open("foo", O_TRUNC)/write(fd,..)/close(fd). If auto_da_alloc is enabled, ext4 will detect the replace-via-rename and replace-via-truncate patterns and force that any delayed allocation blocks are allocated such that at the next journal commit, in the default data=ordered mode, the data blocks of the new file are forced to disk before the rename() operation is committed. This provides roughly the same level of guarantees as ext3, and avoids the "zero-length" problem that can happen when a system crashes before the delayed allocation blocks are forced to disk.


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