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authorSteven Whitehouse <swhiteho@redhat.com>2009-08-25 13:44:04 +0100
committerSteven Whitehouse <swhiteho@redhat.com>2009-08-25 13:44:04 +0100
commitb6ed2e03df1e2c6ee41cf0e2e2699f2410671916 (patch)
tree9b9f4c5670f620f88aba9668b7fee9bd07eb8642 /include
parentd34843d0c4a20872f3f3bfb510328bd043b939ff (diff)
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GFS2: Add explanation of extended attr on-disk format
Some useful info regarding the on-disk representation of GFS2 extended attributes. Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Diffstat (limited to 'include')
-rw-r--r--include/linux/gfs2_ondisk.h22
1 files changed, 22 insertions, 0 deletions
diff --git a/include/linux/gfs2_ondisk.h b/include/linux/gfs2_ondisk.h
index c56b4bce56d0..b80c88dedbbb 100644
--- a/include/linux/gfs2_ondisk.h
+++ b/include/linux/gfs2_ondisk.h
@@ -333,6 +333,28 @@ struct gfs2_leaf {
/*
* Extended attribute header format
+ *
+ * This works in a similar way to dirents. There is a fixed size header
+ * followed by a variable length section made up of the name and the
+ * associated data. In the case of a "stuffed" entry, the value is
+ * inline directly after the name, the ea_num_ptrs entry will be
+ * zero in that case. For non-"stuffed" entries, there will be
+ * a set of pointers (aligned to 8 byte boundary) to the block(s)
+ * containing the value.
+ *
+ * The blocks containing the values and the blocks containing the
+ * extended attribute headers themselves all start with the common
+ * metadata header. Each inode, if it has extended attributes, will
+ * have either a single block containing the extended attribute headers
+ * or a single indirect block pointing to blocks containing the
+ * extended attribure headers.
+ *
+ * The maximim size of the data part of an extended attribute is 64k
+ * so the number of blocks required depends upon block size. Since the
+ * block size also determines the number of pointers in an indirect
+ * block, its a fairly complicated calculation to work out the maximum
+ * number of blocks that an inode may have relating to extended attributes.
+ *
*/
#define GFS2_EA_MAX_NAME_LEN 255