For XFS, in normal operation, an extent is only allocated when data has to be written to disk. This is called delayed allocation. If we are extending a file by 50MB – that space is deducted from the total free space on the filesystem, but no decision on where to place that data is made until we start writing it out – due to memory pressure or the kernel automatically starts writing the dirty pages out (the sync once every 5 seconds on linux).

When an extent needs to be allocated, XFS looks it up in one of two b+trees it has of free space. There is one sorted by starting block number (so you can search for “an extent near here”) and one by size (so you can search for “an extent of x size”).

The ideal situation being that you want as large an extent as possible as close to the tail end of the file as possible (i.e. just making the current extent bigger).

The worst-case scenario is having to allocate extents to multiple files at once with all of them being written out synchronously (O_SYNC or memory pressure) as this will cause lots of small extents to be created.

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!).

memberdb/log.MYD:
 EXT: FILE-OFFSET      BLOCK-RANGE      AG AG-OFFSET        TOTAL
   0: [0..943]:        5898248..5899191  3 (36536..37479)     944
   1: [944..1023]:     6071640..6071719  3 (209928..210007)    80
   2: [1024..1127]:    6093664..6093767  3 (231952..232055)   104
   3: [1128..1279]:    6074800..6074951  3 (213088..213239)   152
   4: [1280..1407]:    6074672..6074799  3 (212960..213087)   128
   5: [1408..1423]:    6074264..6074279  3 (212552..212567)    16
memberdb/log.MYI:
 EXT: FILE-OFFSET      BLOCK-RANGE        AG AG-OFFSET        TOTAL
   0: [0..7]:          10165832..10165839  5 (396312..396319)     8

The interesting thing about this is that the log table grows very slowly. This table stores a bunch of debugging output for my memberdb applicaiton. It should possibly be a partitioned ARCHIVE table (and probably will in the future).

The thing about a file growing slowly over time is that it’s more likely to have more than 1 extent (I’ll examine why in the near future).

My InnoDB data and log files only have 1 extent.. I think I’ve done a xfs_fsr on my file system though.

(a little while ago I was writing a really long entry on everything possible. I realised that this would be a long read for people and that less people would look at it, so I’ve split it up).

This sprung out of doing work on the NDB disk data tree. Anything where efficient use of the filesystem is concerned tickles my fancy, so I went to have a look at what was going on.

Filesystems store what part of the disk belongs to what file in one of two ways. The first is to keep a list of every disk block (typically 4kb) that’s being used by the file. A 400kb file will have 100 block numbers. The second way is to store a range (extent). That is, a 400kb file could use 100 blocks starting at disk block number 1000.

XFS has a tool called xfs_bmap. It gives you a list of the extents allocated to a file.

So, let’s have a look at what it tells us about some recordings on my MythTV box.

myth@orpheus:~$ ls -lah myth-recordings/10_20050912183000_20050912190000.nuv
 -rw-r--r--  1 myth myth 452M 2005-09-12 19:00 myth-recordings/10_20050912183000_20050912190000.nuv
myth@orpheus:~$ xfs_bmap -v myth-recordings/10_20050912183000_20050912190000.nuv
myth-recordings/10_20050912183000_20050912190000.nuv:
 EXT: FILE-OFFSET       BLOCK-RANGE          AG AG-OFFSET             TOTAL
   0: [0..639]:         228712176..228712815  7 (21106232..21106871)    640
   1: [640..1663]:      83674040..83675063    2 (24358056..24359079)   1024
   2: [1664..923519]:   83675368..84597223    2 (24359384..25281239) 921856
   3: [923520..924031]: 84631272..84631783    2 (25315288..25315799)    512

Just to make things fun, this is all in 512byte blocks. But anyway, the real interesting thing is the number of extents. Ideally, every file would have one extent as this means that we avoid disk seeks – *the* most expensive disk operation.

XFS also provides the xfs_fsr tool (File System Repacker) that can defragment files (even on a mounted file system). On IRIX this used to run out of cron – fun when a bunch of machines hit a CXFS volume all at the same time.