Ext4 Design
From Ext4
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− | + | === Design for ext4 === | |
* Ext3: default filesystem for many users, reputation of dependability & compatibility, leave existing ext3 users undisturbed, stable | * Ext3: default filesystem for many users, reputation of dependability & compatibility, leave existing ext3 users undisturbed, stable | ||
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* Forward compatibility/upgradeability | * Forward compatibility/upgradeability | ||
* Multiblock allocation | * Multiblock allocation | ||
− | * Delayed allocation | + | * [[DelayedAllocation|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. | 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 === | |
<pre> | <pre> | ||
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− | + | === External References === | |
− | + | ||
− | [http://ext2.sourceforge.net/2005-ols/2005-ols-ext3-presentation.pdf] | + | * [https://foss.in/2006/cfp/slides/ext4-foss.pdf FOSS.in 2006 ext4 paper] |
− | [http://ext2.sourceforge.net/2005-ols/2005-ext3-paper.pdf] | + | * [http://ext2.sourceforge.net/2005-ols/2005-ols-ext3-presentation.pdf OLS 2005 ext3 presentation] |
− | [http://lwn.net/Articles/187321/] | + | * [http://ext2.sourceforge.net/2005-ols/2005-ext3-paper.pdf OLS 2005 ext3 paper] |
+ | * [http://lwn.net/Articles/187321/ Ext3 for large filesystems] (LWN June 12, 2006) |
Revision as of 00:18, 5 May 2009
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
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; };