EXT4 design
Ext3系统已有许实用的缺省状态，有良好的兼容性，可靠性。
Ext4更支持大的文件系统，对大文件支持性更好。
磁盘上的一些结构变了：纳秒级时间戳、extent分配、预分配
写在磁盘崩溃也可以保证可靠性
64bit的JBD
向前兼容文件系统/文件系统可升级
多块分配
延迟分配
第一配额指标的支持 1st Class Quota Support
大的块分配 Large allocation blocks
EXT4 Extents
extent对应的是磁盘上和逻辑上都连续的空间，Extents的设计使得更少的元数据管理更多的数据，所以可以做到更快的访问。
Extents在磁盘上的数据结构
/*extent bottom of the tree 的结构*/
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 */
}
/*extent 除开bottom of the tree 的结构*/
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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