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扩展属性如何存储和保存？

tag-icon • tag-icon filesystems files xattr
扩展属性如何存储和保存？

我有一个关于扩展文件属性的小问题。假设我用扩展属性中的元数据标记我的文件（例如，为了考虑完整性 - 但这对我的问题并不重要）。现在出现的问题：

  • 这些属性存储在哪里？我猜肯定不是在 inode 中，而是在什么位置 - 或者更好：结构？
  • 这些属性如何连接到文件？是否存在从属性结构到 inode 等的链接？
  • 复制/移动文件时会发生什么？我刚刚测试过，移动文件时，文件保持其属性。复制时，副本没有属性。所以我假设当将其刻录到 CD 或通过电子邮件发送文件时，它也会丢失其属性吗？

答案1

您问题的答案是特定于文件系统的。例如，对于 ext3，请查看fs/ext3/xattr.c，它包含以下描述：

  16 /*
  17  * 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.
  43  *
  44  * Locking strategy
  45  * ----------------
  46  * EXT3_I(inode)->i_file_acl is protected by EXT3_I(inode)->xattr_sem.
  47  * EA blocks are only changed if they are exclusive to an inode, so
  48  * holding xattr_sem also means that nothing but the EA block's reference
  49  * count can change. Multiple writers to the same block are synchronized
  50  * by the buffer lock.
  51  */

关于“属性如何连接”问题，链接是相反的，inode 有一个指向扩展属性的链接，分别参见 xattr.h 和 xattr.c 中的EXT3_XATTR_NEXT和。ext3_xattr_list_entries

回顾一下，这些属性链接到 inode，并且依赖于文件系统，因此，是的，在刻录 CD ROM 或通过电子邮件发送文件时，您将丢失这些属性。

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