Extents in “ext3” file system
My Ubuntu Linux ship with ext3 file-system. This FS is very similar to classical model explained in UNIX OS. A file is logically arranged in a set of blocks, managed through an array of block pointers. In ext3, each inode has an array of fifteen elements. Twelve elements of this array point to a disk block. Usually, a disk block is configured to 4KB. Thus twelve such 4KB blocks (L0/ level 0) could be pointed by these array entries (i.e. a file of 48BK is always contained in L0 blocks).
As soon as a new block is allocated for this file, thirteenth element of this array comes to play. It is called L1/level1 block and keep pointers to 1024 L0 blocks. Thirteenth request creates an entry in L1 blocks. Fourteenth and fifteenth entry serve L2 and L3 blocks.
Instead of allocating a block at a time, Linux has optimized the way disk blocks are allocated for a file with “extents”. An extent is a contiguous set of disk blocks. Kernel allocates an extent of blocks.
Lets create a file and see how does Linux handle block-allocation:
1 #include <fcntl.h>
2 int main()
3 {
4 int i;
5 char buf[4096];
6
7 memset(buf, ‘a’, 4096);
8 int fd = open(“foo.txt”, O_CREAT|O_WRONLY|O_TRUNC);
9
10 for (i =0; i < 2; i++) {
11 write(fd, buf, 4096);
12 }
13 write(fd, “end”, 3);
14 close(fd);
15 }
We have created a 8192 +3 = 8195 bytes file.
kanaujia@ubuntu:~/Desktop/ToKeep/cprogs$ ls -l foo.txt
———- 1 kanaujia kanaujia 8195 2011-10-16 10:42 foo.txt
How much space this file occupy on disk?
kanaujia@ubuntu:~/Desktop/ToKeep/cprogs$ du -h !$
du -h foo.txt
12K foo.txt
That means I need three disk blocks of 4KB to store this file. Now let’s see how Linux allot these block in a single extent. The ioctl() call has FS_IOC_FIEMAP flag that provides facility to get access to this information from user space. A file extent map structure is defined as follows:
struct fiemap {
28 __u64 fm_start; /* logical offset (inclusive) at
29 * which to start mapping (in) */
30 __u64 fm_length; /* logical length of mapping which
31 * userspace wants (in) */
32 __u32 fm_flags; /* FIEMAP_FLAG_* flags for request (in/out) */
33 __u32 fm_mapped_extents; /* number of extents that were mapped (out) */
34 __u32 fm_extent_count; /* size of fm_extents array (in) */
35 __u32 fm_reserved;
36 struct fiemap_extent fm_extents[0]; /* array of mapped extents (out) */
37};
A simple C program to fill-up this structure would fetch you extent information.
For my file, I got the following data:
kanaujia@ubuntu:~/Desktop/ToKeep/cprogs$ ./fiemap ./foo.txt
File ./foo.txt has 1 extents:
# Logical Physical Length Flags
0: 0000000000000000 0000000000000000 0000000000003000 0007
We have only one extent to accommodate this file. This extent spans three disk block as the length is 0x3000 or 12KB.
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