disk space allocation (part 3: storing extents on disk)

Here I’m going to talk about how file systems store what part of the disk a part of the file occupies. If your database files are very fragmented, performance will suffer. How much depends on a number of things however.

XFS can store some extents directly in the inode (see xfs_dinode.h). If I’m reading things correctly, this can be 2 extents per fork (data fork and attribute fork). If more than this number of extents are needed, a btree is used instead.

HFS/HFS+ can store up to 8 extents directly in the catalog file entry (see Apple TechNote 1150 – which was updated in March 2004 with information on the journal format). If the file has more than 8 extents, a lookup then needs to be done into the extents overflow file. Interestingly enough, in MacOS X 10.4 and above (i think it was 10.4… may have been 10.3 as well) if a file is less than 20MB and has more than 8 extents, on an open, the OS will automatically try to defragment that file. Arguably you should just fix your allocation strategy, but hey – maybe this does actually help.

File systems such as ext2, ext3 and reiserfs just store a list of block numbers. In the case of ext2 and ext3, the futher into a file you are, the more steps are required to find the disk block number associated with that block in the file.

So what does an extent actually look like? Well, for XFS, the following excerpt from xfs_bmap_btree.h is interesting:

#define ISUNWRITTEN(x) ((x)->br_state == XFS_EXT_UNWRITTEN)

typedef struct xfs_bmbt_irec
xfs_fileoff_t br_startoff; /* starting file offset */
xfs_fsblock_t br_startblock; /* starting block number */
xfs_filblks_t br_blockcount; /* number of blocks */
xfs_exntst_t br_state; /* extent state */
} xfs_bmbt_irec_t;

It’s also rather self explanetry. Holes (for sparse files) in XFS don’t have extents, and an extent doesn’t have to have been written to disk. This allows you to preallocate space in chunks without having written anything to it. Reading from an unwritten extent gets you zeros (otherwise it would be a security hole!).

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