f2fs系列之六：checkpoint

最新推荐文章于 2024-12-25 23:49:29 发布

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最新推荐文章于 2024-12-25 23:49:29 发布

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文章标签：移动开发数据结构与算法

原文链接：http://blog.51cto.com/xiamachao/2349108

本文详细介绍了f2fs文件系统的checkpoint机制，包括相关数据结构、主要工作内容以及触发检查点的时机。检查点主要用于维护data、node和metadata的一致性，确保SIT/NAT信息准确写回。触发条件包括空闲segment不足、恢复操作和trim操作等。

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f2fs 的checkpoint 维护data、node和meta data(SIT,NAT)的数据一致性，把一起写到SSA区域的数据分别写回到SIT/NAT区域。

checkpoint 相关数据结构

super block区域里记录了checkpoint （CP)的起始block address，以及checkpoint 区域segment 的数量：

struct f2fs_super_block {
        __le32 magic;                   /* Magic Number */
        __le16 major_ver;               /* Major Version */
        __le16 minor_ver;               /* Minor Version */
        __le32 log_sectorsize;          /* log2 sector size in bytes */
        __le32 log_sectors_per_block;   /* log2 # of sectors per block */
        __le32 log_blocksize;           /* log2 block size in bytes */
        __le32 log_blocks_per_seg;      /* log2 # of blocks per segment */
        __le32 segs_per_sec;            /* # of segments per section */
        __le32 secs_per_zone;           /* # of sections per zone */
        __le32 checksum_offset;         /* checksum offset inside super block */
        __le64 block_count;             /* total # of user blocks */
        __le32 section_count;           /* total # of sections */
        __le32 segment_count;           /* total # of segments */
        __le32 segment_count_ckpt;      /* # of segments for checkpoint */
        __le32 segment_count_sit;       /* # of segments for SIT */
        __le32 segment_count_nat;       /* # of segments for NAT */
        __le32 segment_count_ssa;       /* # of segments for SSA */
        __le32 segment_count_main;      /* # of segments for main area */
        __le32 segment0_blkaddr;        /* start block address of segment 0 */
        __le32 cp_blkaddr;              /* start block address of checkpoint */
                .............

check point 包含的信息如下：

#define F2FS_CP_PACKS           2       /* # of checkpoint packs */

struct f2fs_checkpoint {
        __le64 checkpoint_ver;          /* checkpoint block version number */
        __le64 user_block_count;        /* # of user blocks */
        __le64 valid_block_count;       /* # of valid blocks in main area */
        __le32 rsvd_segment_count;      /* # of reserved segments for gc */
        __le32 overprov_segment_count;  /* # of overprovision segments */
        __le32 free_segment_count;      /* # of free segments in main area */

        /* information of current node segments */
        __le32 cur_node_segno[MAX_ACTIVE_NODE_LOGS];
        __le16 cur_node_blkoff[MAX_ACTIVE_NODE_LOGS];
        /* information of current data segments */
        __le32 cur_data_segno[MAX_ACTIVE_DATA_LOGS];
        __le16 cur_data_blkoff[MAX_ACTIVE_DATA_LOGS];
        __le32 ckpt_flags;              /* Flags : umount and journal_present */
        __le32 cp_pack_total_block_count;       /* total # of one