filesystems - 如何将 "extended attributes"存储在 ext4 inode block 中，因为我们有 inode 额外大小 "i_extra_isize"

Question

根据

struct ext4_inode {
  .....
  __le16  i_extra_isize;
  .....
}

鉴于我使用的 inode 大小 = 256 字节，我看到这可以用于存储扩展属性。几个问题:

1. 我如何知道我的 FS inode 大小是 256 字节还是 128 字节？
2. 如何设置小扩展属性(<10 字节)并将其存储在额外区域中？ (原因:性能很重要)，因为我相信不会有额外的 IO block 读取为连续的。 Fileystem/C lib 对此有何要求？典型的系统调用“setattr”会这样做吗？我一直在使用命令行“setfattr”进行设置，但不确定它是否会转到 inode 128 字节之后分配的额外区域。
3. 在 128 字节 inode 之后映射额外字节的内存结构是什么，这是我要存储我的小扩展属性的地方。

Answer 1

How do I know if my FS inode size is 256 or 128 bytes ?

$ sudo tune2fs -l /dev/sda2  | egrep -i 'inode size'
Inode size:               256

How do I set the small extended attributes

Ext3/4 支持可配置的 inode 大小(来自 -I 选项 mkfs.ext{3,4} cmd 参数)，但默认 inode 大小为 128 字节。 Ext4 将默认为 256 字节。这是容纳一些额外字段(如纳秒时间戳或 inode 版本控制)所必需的，inode 的剩余空间将用于存储足够小以适应该空间的扩展属性。此外，您可能还想查看实现的函数 fs/ext4/file.c:struct inode_operations{.setattr,.getattr}。

What's the Fileystem/C lib call for that?

引用 API 的 setxattr(), lsetxattr(), fsetxattr()用于以编程方式设置扩展属性。使用命令 attr、lsattr、chattr 测试设置文件的扩展属性。你也可能想要 look into

Whats the in-memory structure for mapping the extra bytes after 128 bytes of inode, the place where I want to store my small extended attributes.

引用fs/ext4/xattr.c

”

* Extended attributes are stored directly in inodes (on file systems with
 18  * inodes bigger than 128 bytes) and on additional disk blocks. The i_file_acl
 19  * field contains the block number if an inode uses an additional block. All
 20  * attributes must fit in the inode and one additional block. Blocks that
 21  * contain the identical set of attributes may be shared among several inodes.
 22  * Identical blocks are detected by keeping a cache of blocks that have
 23  * recently been accessed.
 24  *
 25  * The attributes in inodes and on blocks have a different header; the entries
 26  * are stored in the same format:
 27  *
 28  *   +------------------+
 29  *   | header           |
 30  *   | entry 1          | |
 31  *   | entry 2          | | growing downwards
 32  *   | entry 3          | v
 33  *   | four null bytes  |
 34  *   | . . .            |
 35  *   | value 1          | ^
 36  *   | value 3          | | growing upwards
 37  *   | value 2          | |
 38  *   +------------------+
 39  *
 40  * The header is followed by multiple entry descriptors. In disk blocks, the
 41  * entry descriptors are kept sorted. In inodes, they are unsorted. The
 42  * attribute values are aligned to the end of the block in no specific order.

”