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 extent allocation, preallocation
  • Reliability wrt on-disk corruption
  • 64 bit JBD split
  • Forward compatibility/upgradeability
  • Multiblock allocation
  • Delayed allocation
  • 1st Class Quota Support (Implementation in progress)
  • Large allocation blocks (Not yet implemented; in design phase)

Ext4 Extents

The core of the Ext4 FS is the support for extents. An extent is simply a set of blocks which are logically contiguous within the file and also on the underlying block device. Most contemporary filesystems put considerable effort into allocating contiguous blocks for files as a way of making I/O operations faster, so blocks which are logically contiguous within the file often are also contiguous on-disk. As a result, storing the file structure as extents should result in significant compression of the file's metadata, since a single extent can replace a large number of block pointers. The reduction in metadata should enable faster access as well.

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;

External References

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