Ext4 Design

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Design for ext4

  • Ext3: default filesystem for many users, reputation of dependability & compatibility, leave existing ext3 users undisturbed, stable
  • Scaling up to support large filesystems: Storage advancements, Increasing data storage requirements, only large filesystem users move to ext4
  • Features requiring on-disk format change: nanosec timestamps, fast EA, preallocation
  • Reliability wrt on-disk corruption
  • 64 bit JBD split
  • Forward compatibility/upgradeability

On-Disk Structures

/*
 * This is the extent on-disk structure.
 * It's used at the bottom of the tree.
 */
struct ext4_extent {
        __le32  ee_block;       /* first logical block extent covers */
        __le16  ee_len;         /* number of blocks covered by extent */
        __le16  ee_start_hi;    /* high 16 bits of physical block */
        __le32  ee_start_lo;    /* low 32 bits of physical block */
};

/*
 * This is index on-disk structure.
 * It's used at all the levels except the bottom.
 */
struct ext4_extent_idx {
        __le32  ei_block;       /* index covers logical blocks from 'block' */
        __le32  ei_leaf_lo;     /* pointer to the physical block of the next *
                                 * level. leaf or next index could be there */
        __le16  ei_leaf_hi;     /* high 16 bits of physical block */
        __u16   ei_unused;
};
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