Ext4 Design
From Ext4
Contents |
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;
};
