由于f2fs的log-structure特性，每次写一个数据块，需要相应更改direct node，NAT和SIT，尤其是NAT和SIT区域，可能仅仅需要修改一个entry几个字节的信息，就要重写整个page，这会严重降低文件系统的性能和SSD的使用寿命，因此，f2fs使用了journal的机制来减少NAT和SIT的写次数。所谓journal，其实就是把NAT和SIT的更改写到f2fs_summary_block中，当写checkpoint时，才把dirty的SIT和NAT区域回写。struct f2fs_summary_block {struct f2fs_summary entries[ENTRIES_IN_SUM];union {__le16 n_nats;__le16 n_sits;};/* spare area is used by NAT or SIT journals */union {struct nat_journal nat_j;struct sit_journal sit_j;};struct summary_footer footer;} __packed;struct nat_journal {struct nat_journal_entry entries[NAT_JOURNAL_ENTRIES];__u8 reserved[NAT_JOURNAL_RESERVED];} __packed;struct sit_journal {struct sit_journal_entry entries[SIT_JOURNAL_ENTRIES];__u8 reserved[SIT_JOURNAL_RESERVED];} __packed;
每个当前的segment在内存里面都有一个f2fs_summary_block的实例，f2fs使用了CURSEG_COLD_DATA和CURSEG_HOT_DATA的summary block，其中前者的block存储的是SIT的journal，后者存储的是NAT的journal，不过有个疑问是，f2fs支持2 log，当只有两个log时候，segment分别是data和node类型的，这时CURSEG_COLD_DATA和CURSEG_HOT_DATA就是同一个segment了，这样这两个journal不就冲突了？
journal的插入和查找，先搜索journal区域是否已经存在了相关的entry，如果存在了，则返回这个entry，否则，返回或者创建相关的entry。这样可以保证，频繁的NAT，SIT的项更改只会写到内存中。
int lookup_journal_in_cursum(struct f2fs_summary_block *sum, int type,unsigned int val, int alloc){int i;if (type == NAT_JOURNAL) {for (i = 0; i < nats_in_cursum(sum); i++) {if (le32_to_cpu(nid_in_journal(sum, i)) == val)return i;}if (alloc && nats_in_cursum(sum) < NAT_JOURNAL_ENTRIES)return update_nats_in_cursum(sum, 1);} else if (type == SIT_JOURNAL) {for (i = 0; i < sits_in_cursum(sum); i++)if (le32_to_cpu(segno_in_journal(sum, i)) == val)return i;if (alloc && sits_in_cursum(sum) < SIT_JOURNAL_ENTRIES)return update_sits_in_cursum(sum, 1);}return -1;}写checkpoint的时候会触发对SIT，NAT的flush操作，从而把journa区域的NAT，SIT项回写到SSD上：
void flush_sit_entries(struct f2fs_sb_info *sbi){struct sit_info *sit_i = SIT_I(sbi);unsigned long *bitmap = sit_i->dirty_sentries_bitmap;struct curseg_info *curseg = CURSEG_I(sbi, CURSEG_COLD_DATA);struct f2fs_summary_block *sum = curseg->sum_blk;unsigned long nsegs = TOTAL_SEGS(sbi);struct page *page = NULL;struct f2fs_sit_block *raw_sit = NULL;unsigned int start = 0, end = 0;unsigned int segno = -1;bool flushed;mutex_lock(&curseg->curseg_mutex);mutex_lock(&sit_i->sentry_lock);/* * "flushed" indicates whether sit entries in journal are flushed * to the SIT area or not. */flushed = flush_sits_in_journal(sbi);while ((segno = find_next_bit(bitmap, nsegs, segno + 1)) < nsegs) {struct seg_entry *se = get_seg_entry(sbi, segno);int sit_offset, offset;sit_offset = SIT_ENTRY_OFFSET(sit_i, segno);if (flushed)goto to_sit_page;offset = lookup_journal_in_cursum(sum, SIT_JOURNAL, segno, 1);if (offset >= 0) {segno_in_journal(sum, offset) = cpu_to_le32(segno);seg_info_to_raw_sit(se, &sit_in_journal(sum, offset));goto flush_done;}to_sit_page:if (!page || (start > segno) || (segno > end)) {if (page) {f2fs_put_page(page, 1);page = NULL;}start = START_SEGNO(sit_i, segno);end = start + SIT_ENTRY_PER_BLOCK – 1;/* read sit block that will be updated */page = get_next_sit_page(sbi, start);raw_sit = page_address(page);}/* udpate entry in SIT block */seg_info_to_raw_sit(se, &raw_sit->entries[sit_offset]);flush_done:__clear_bit(segno, bitmap);sit_i->dirty_sentries–;}mutex_unlock(&sit_i->sentry_lock);mutex_unlock(&curseg->curseg_mutex);/* writeout last modified SIT block */f2fs_put_page(page, 1);set_prefree_as_free_segments(sbi);}
flush_sits_in_journal会尝试把journal项对应的entry标记为脏的，，从而在页换出的时候触发对数据的回写f2fs有两个SIT（NAT）区域，一个是当前最新的数据，一个是当前checkpoint有效的数据，get_next_sit_page把更新的数据写在了最新的那个区域，并返回这个page。checkpoint完成后，最新区域成了checkpoint有效区域，而checkpoint有效区域成了更新的数据写入的区域。
人生的成功不过是在紧要处多一份坚持，
