filefrag 实现原理
filefrag 实现原理
Linux 文件碎片
据说 Windows 需要碎片整理，而 Linux 不需要碎片整理。
不过 Linux 文件系统真的没有碎片吗？
文件碎片还是有的，只不过没有 Windows 那么明显。
在 Linux 发行版 CentOS 中，e2fsprogs 包有个 filefrag 的命令，可以用来查看文件在磁盘中存储的每个块信息。
当持续写入数据，然后执行 filefrag 命令查看该文件后，如下所示：
$ filefrag -v ./test_file
Filesystem type is: ef53
File size of ./test_file is 23865744 (5827 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 0: 37944.. 37944: 1:
1: 1.. 1: 111185506.. 111185506: 1: 37945:
2: 2.. 2: 111186961.. 111186961: 1: 111185507:
3: 3.. 3: 90212910.. 90212910: 1: 111186962:
4: 4.. 4: 111185000.. 111185000: 1: 90212911:
5: 5.. 5: 90213533.. 90213533: 1: 111185001:
6: 6.. 6: 90214427.. 90214427: 1: 90213534:
7: 7.. 7: 111187017.. 111187017: 1: 90214428:
8: 8.. 8: 111189013.. 111189013: 1: 111187018:
9: 9.. 9: 90213546.. 90213546: 1: 111189014:
10: 10.. 10: 111185543.. 111185543: 1: 90213547:
11: 11.. 11: 111188000.. 111188000: 1: 111185544:
12: 12.. 13: 90212927.. 90212928: 2: 111188001:
13: 14.. 14: 111187492.. 111187492: 1: 90212929:
14: 15.. 15: 111187496.. 111187496: 1: 111187493:
15: 16.. 31: 53424.. 53439: 16: 111187497:
16: 32.. 63: 65920.. 65951: 32: 53440:
17: 64.. 127: 89664.. 89727: 64: 65952:
18: 128.. 255: 68992.. 69119: 128: 89728:
19: 256.. 511: 112384.. 112639: 256: 69120:
20: 512.. 1023: 312320.. 312831: 512: 112640:
21: 1024.. 2047: 471040.. 472063: 1024: 312832:
22: 2048.. 3010: 858112.. 859074: 963: 472064:
23: 3011.. 3072: 979798.. 979859: 62: 859075:
24: 3073.. 4095: 922625.. 923647: 1023: 979860:
25: 4096.. 4732: 1562624.. 1563260: 637: 923648:
26: 4733.. 5826: 2016376.. 2017469: 1094: 1563261: eof
./test_file: 27 extents found
该测试文件存放的文件系统是 ext4，持续写入数据后，我们观察到：
刚开始占据 1 个文件系统块，写满 16 个块后，就开始 16 个连续块开始分配，直到 1024 个连续块。
从结果得知，写入删除频繁就容易造成了文件块不连续，也就产生了文件碎片。
filefrag 原理
Windows 的碎片整理工具是需要读取文件系统来发现文件是否碎片分布，而 Linux 提供了一个 ioctl(FS_IOC_FIEMAP) 的接口来给用户空间的应用程序查询文件块的分布。
其中，iocode FS_IOC_FIEMAP 定义在 /usr/include/linux/fs.h，所需要的结构体定义在 /usr/include/linux/fiemap.h。
有了前面这两个头文件，就可以实现一个 mini 版本的 filefrag。
filefrag 最小版本实现
C 语言版
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <linux/fs.h>
#include <linux/fiemap.h>
#define xstrerror() strerror(errno)
#define errlog(...) fprintf(stderr, __VA_ARGS__);
static void report_fiemap(int fd)
{
__u64 buf[2048];
struct fiemap_extent fm_last;
int i, rc, last = 0;
unsigned long flags = 0;
struct fiemap *fiemap = (struct fiemap *)buf;
struct fiemap_extent *fm_ext = &fiemap->fm_extents[0];
int count = (sizeof(buf) - sizeof(*fiemap)) / sizeof(struct fiemap_extent);
memset(fiemap, 0, sizeof(struct fiemap));
memset(&fm_last, 0, sizeof(fm_last));
do {
fiemap->fm_length = ~0ULL;
fiemap->fm_flags = flags;
fiemap->fm_extent_count = count;
rc = ioctl(fd, FS_IOC_FIEMAP, (unsigned long) fiemap);
if (rc < 0) {
errlog("ioctl fail, %s\n", xstrerror());
}
if (fiemap->fm_mapped_extents == 0) {
break;
}
for (i = 0; i < fiemap->fm_mapped_extents; i++) {
printf("logical: %lld, physical: %lld, length: %lld\n",
fm_ext[i].fe_logical, fm_ext[i].fe_physical, fm_ext[i].fe_length);
if (fm_ext[i].fe_flags & FIEMAP_EXTENT_LAST) {
last = 1;
}
fm_last = fm_ext[i];
}
fiemap->fm_start = (fm_ext[i - 1].fe_logical + fm_ext[i - 1].fe_length);
} while (last == 0);
}
int main(int argc, char *argv[])
{
int fd;
const char *filename;
if (argc != 2) {
printf("%s filepath \n", argv[0]);
exit(EXIT_SUCCESS);
}
filename = argv[1];
fd = open(filename, O_RDONLY);
if (fd == -1) {
errlog("open(%s) fail, %s\n", filename, strerror(errno));
}
report_fiemap(fd);
close(fd);
return 0;
}
执行输出如下：
# ./a.out ./foo
logical: 0, physical: 785387520, length: 49152
logical: 49152, physical: 785436672, length: 16384
logical: 65536, physical: 8323792896, length: 65536
logical: 131072, physical: 8323989504, length: 131072
logical: 262144, physical: 8324251648, length: 262144
logical: 524288, physical: 8324644864, length: 524288
logical: 1048576, physical: 8340840448, length: 1048576
logical: 2097152, physical: 8352956416, length: 2097152
logical: 4194304, physical: 8539602944, length: 49152
logical: 4243456, physical: 8539652096, length: 4145152
logical: 8388608, physical: 8506044416, length: 49152
logical: 8437760, physical: 8506093568, length: 2117632
logical: 10555392, physical: 8522547200, length: 1130496
logical: 11685888, physical: 8486912000, length: 1179648
logical: 12865536, physical: 8488718336, length: 1142784
logical: 14008320, physical: 8520151040, length: 1167360
logical: 15175680, physical: 8483827712, length: 1052672
logical: 16228352, physical: 8729436160, length: 548864
logical: 16777216, physical: 8787816448, length: 49152
logical: 16826368, physical: 8787865600, length: 880640
参考
https://www.kernel.org/doc/Documentation/filesystems/fiemap.txt