Ext4 Design

Revision as of 00:18, 5 May 2009

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

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

• FOSS.in 2006 ext4 paper
• OLS 2005 ext3 presentation
• OLS 2005 ext3 paper
• Ext3 for large filesystems (LWN June 12, 2006)