7.3.2.2.4. Optimising for applications that use extended attributes

XFS can store small attributes directly in the inode if space is available in the inode. If the attribute fits into the inode, then it can be retrieved and modified without requiring extra I/O to retrieve separate attribute blocks. The performance differential between in-line and out-of-line attributes can easily be an order of magnitude slower for out-of-line attributes.

For the default inode size of 256 bytes, roughly 100 bytes of attribute space is available depending on the number of data extent pointers also stored in the inode. The default inode size is really only useful for storing a small number of small attributes.

Increasing the inode size at mkfs time can increase the amount of space available for storing attributes in-line. A 512 byte inode size increases the space available for attributes to roughly 350 bytes; a 2 KB inode has roughly 1900 bytes of space available.

There is, however, a limit on the size of the individual attributes that can be stored in-line - there is a maximum size limit of 254 bytes for both the attribute name and the value (that is, an attribute with a name length of 254 bytes and a value length of 254 bytes will stay in-line). Exceeding these size limits forces the attributes out of line, even if there would have been enough space to store all the attributes in the inode.