[GSoC 2018: XFS support] Week #3, #4 and #5

Blog post by abx1 on Mon, 2018-05-28 22:19

Hey everyone,

I was not able to put in complete effort into GSoC, as I was having exams in my University during the past few weeks. I’ll be regular with the updates starting with this one.

I was encountering issues related to header files and xfs_shell wasn’t bulding properly. It was the problem in JamFile that FS_SHELL wasn’t added as an additional define. Doing so made sure that fs-shell wrappers are properly loaded.

Implementing superblock

I had to revise the on-disk structure of XFS and read through the implementation. I started with the implementation of XFS superblock, which is needed for identify_partition. Here is implementation

struct xfs_sb {
    uint32            sb_magicnum;
    uint32            sb_blocksize;
    uint64            sb_dblocks;
    uint64            sb_rblocks;
    uint64            sb_rextents;
    uuid_t            sb_uuid;
    uint64            sb_logstart;
    xfs_ino_t         sb_rootino;
    xfs_ino_t         sb_rbmino;
    xfs_ino_t         sb_rsumino;
    uint32            sb_rextsize;
    uint              sb_agblocks;
    uint32            sb_agcount;
    uint32            sb_rbmblocks;
    uint32            sb_logblocks;
    uint16            sb_versionnum;
    uint16            sb_sectsize;
    uint16            sb_inopblock;
    uint16            sb_inodesize;
    char              sb_fname[12];
    uint8             sb_blocklog;
    uint8             sb_sectlog;
    uint8             sb_inodelog;
    uint8             sb_inopblog;
    uint8             sb_agblklog;
    uint8             sb_rextslog;
    uint8             sb_inprogress;
    uint8             sb_imax_pct;
    uint64            sb_icount;
    uint64            sb_ifree;
    uint64            sb_fdblocks;
    uint64            sb_frextents;
    xfs_ino_t         sb_uquotino;
    xfs_ino_t         sb_gquotino;
    uint16            sb_qflags;
    uint8             sb_flags;
    uint8             sb_shared_vn;
    uint32            sb_inoalignmt;
    uint32            sb_unit;
    uint32            sb_width;
    uint8             sb_dirblklog;
    uint8             sb_logsectlog;
    uint16            sb_logsectsize;
    uint32            sb_logsunit;
    uint32            sb_features2;

Explanation for few are as follows

  • sb_magicnum - value = 0x58465342, identifies the filesystem
  • sb_blocksize - usually 4096 bytes
  • sb_dblocks - no of blocks available for the data
  • sb_rblocks - no of blocks on real-time device
  • sb_uuid - Universal Unique ID for teh filesystem
  • sb_logstart - (internal log) first block number for journaling log
  • sb_rootino - 128 when using blocksize is 4096 bytes
  • sb_agblocks - size of allocation group (in blocks)
  • sb_agcount - count of AG
  • sb_sectsize - disk sector size in bytes
  • sb_inodesize - size of inode in bytes
  • sb_inopblock - no of inodes in block
  • sb_icount - count for no of inodes allocated on the filesystem(only in the first superblock)
  • sb_ifree - count of free inodes on the filesystem
  • sb_fdblocks - count of free data blocks on the filesystem

Thanks Abhinand