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attr.7: Document EA limits for Btrfs 

From experimentation, I found a 16kB limit on name + value +
overhead bytes. Digger deeper, I see that
https://btrfs.wiki.kernel.org/index.php/Project_ideas#Unlimited_extended_attributes
says:

    Unlimited extended attributes

    Not claimed — no patches yet — Not in kernel yet

    Currently size of value of an extended attribute must fit into
    inline space (~3900 on 4k leaf size), while other filesystems
    do not limit the size. Add a new b-tree item to hold the xattr
    value in extents.

And reading mkfs.btrfs(8) reveals that the default node (AKA leaf)
size is 16kB.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2015-04-22 10:38:31 +02:00
parent edb159da63
commit 7f4f24eaf1
1 changed files with 6 additions and 0 deletions
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@ -155,6 +155,12 @@ attribute information on disk are scalable.
In the JFS, XFS, and Reiserfs filesystem implementations,
the limit on bytes used in an EA value is the ceiling imposed by the VFS.
In the Btrfs filesystem implementation,
the total bytes used for the name, value, and implementation overhead bytes
is limited to the filesystem
.I nodesize
value (16kB by default).
.SH CONFORMING TO
Extended attributes are not specified in POSIX.1, but some other systems
(e.g., the BSDs and Solaris) provide a similar feature.