Ext4 Design

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  • Motivation
    • Ext3


Motivation for ext4

Ext3: default filesystem for many users, Reputation of dependability & compatibility

Scaling up to support large filesystems

Storage advancements

Increasing data storage requirements

Features requiring on-disk format change

    nanosec timestamps, fast EA, preallocation

Reliability wrt on-disk corruption

 * 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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