今天遇到一个很奇怪的问题，在一台linux执行df -h查看磁盘使用情况，发现卡成狗，于是着手去解决：
1、通过strace df -h追踪是卡在什么位置
[root@new_web ~]# strace df -h
execve("/bin/df", ["df", "-h"], [/* 22 vars */]) = 0
brk(0) = 0x2307000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd1715f4000
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=37353, ...}) = 0
mmap(NULL, 37353, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fd1715ea000
close(3) = 0
open("/lib64/libc.so.6", O_RDONLY) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0p\356\1\344=\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1926520, ...}) = 0
mmap(0x3de4000000, 3750152, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x3de4000000
mprotect(0x3de418a000, 2097152, PROT_NONE) = 0
mmap(0x3de438a000, 20480, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x18a000) = 0x3de438a000
mmap(0x3de438f000, 18696, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x3de438f000
close(3) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd1715e9000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd1715e8000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd1715e7000
arch_prctl(ARCH_SET_FS, 0x7fd1715e8700) = 0
mprotect(0x3de438a000, 16384, PROT_READ) = 0
mprotect(0x3de3a1f000, 4096, PROT_READ) = 0
munmap(0x7fd1715ea000, 37353) = 0
brk(0) = 0x2307000
brk(0x2328000) = 0x2328000
open("/usr/lib/locale/locale-archive", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=99158576, ...}) = 0
mmap(NULL, 99158576, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fd16b756000
close(3) = 0
open("/etc/mtab", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=366, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd1715f3000
read(3, "/dev/vda1 / ext4 rw,barrier=0 0 "..., 4096) = 366
read(3, "", 4096) = 0
close(3) = 0
munmap(0x7fd1715f3000, 4096) = 0
statfs("/", {f_type="EXT2_SUPER_MAGIC", f_bsize=4096, f_blocks=10320720, f_bfree=8614131, f_bavail=8089869, f_files=2621440, f_ffree=2405901, f_fsid={-102338008, -1862172196}, f_namelen=255, f_frsize=4096}) = 0
open("/usr/share/locale/locale.alias", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=2512, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd1715f3000
read(3, "# Locale name alias data base.\n#"..., 4096) = 2512
read(3, "", 4096) = 0
close(3) = 0
munmap(0x7fd1715f3000, 4096) = 0
open("/usr/share/locale/en_US.UTF-8/LC_MESSAGES/coreutils.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en_US.utf8/LC_MESSAGES/coreutils.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en_US/LC_MESSAGES/coreutils.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en.UTF-8/LC_MESSAGES/coreutils.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en.utf8/LC_MESSAGES/coreutils.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en/LC_MESSAGES/coreutils.mo", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=435, ...}) = 0
mmap(NULL, 435, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fd1715f3000
close(3) = 0
statfs("/sys", {f_type="SYSFS_MAGIC", f_bsize=4096, f_blocks=0, f_bfree=0, f_bavail=0, f_files=0, f_ffree=0, f_fsid={0, 0}, f_namelen=255, f_frsize=4096}) = 0
statfs("/dev/pts", {f_type="DEVPTS_SUPER_MAGIC", f_bsize=4096, f_blocks=0, f_bfree=0, f_bavail=0, f_files=0, f_ffree=0, f_fsid={0, 0}, f_namelen=255, f_frsize=4096}) = 0
statfs("/dev/shm", {f_type=0x1021994, f_bsize=4096, f_blocks=2041596, f_bfree=2041596, f_bavail=2041596, f_files=2041596, f_ffree=2041595, f_fsid={0, 0}, f_namelen=255, f_frsize=4096}) = 0
statfs("/proc/sys/fs/binfmt_misc", {f_type=0x42494e4d, f_bsize=4096, f_blocks=0, f_bfree=0, f_bavail=0, f_files=0, f_ffree=0, f_fsid={0, 0}, f_namelen=255, f_frsize=4096}) = 0
statfs("/data", {f_type="EXT2_SUPER_MAGIC", f_bsize=4096, f_blocks=129015406, f_bfree=46849538, f_bavail=40295938, f_files=32768000, f_ffree=32749794, f_fsid={346710647, -22441262}, f_namelen=255, f_frsize=4096}) = 0
### 卡在此处
statfs("/opt", {f_type=0xff534d42, f_bsize=0, f_blocks=0, f_bfree=0, f_bavail=0, f_files=0, f_ffree=0, f_fsid={0, 0}, f_namelen=4096, f_frsize=0}) = 0
### 卡在此处
statfs("/data1", {f_type="EXT2_SUPER_MAGIC", f_bsize=4096, f_blocks=129015406, f_bfree=128955560, f_bavail=122401960, f_files=32768000, f_ffree=32767852, f_fsid={1328044912, -550095550}, f_namelen=255, f_frsize=4096}) = 0
statfs("/data2", {f_type="EXT2_SUPER_MAGIC", f_bsize=4096, f_blocks=129015406, f_bfree=128964770, f_bavail=122411170, f_files=32768000, f_ffree=32767982, f_fsid={-1295730252, -1655731663}, f_namelen=255, f_frsize=4096}) = 0
open("/usr/lib64/gconv/gconv-modules.cache", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=26060, ...}) = 0
mmap(NULL, 26060, PROT_READ, MAP_SHARED, 3, 0) = 0x7fd1715ec000
close(3) = 0
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 2), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd1715eb000
write(1, "Filesystem Size Used Avail"..., 49Filesystem Size Used Avail Use% Mounted on
) = 49
write(1, "/dev/vda1 40G 6.6G 31G"..., 40/dev/vda1 40G 6.6G 31G 18% /
) = 40
write(1, "tmpfs 7.8G 0 7.8G"..., 47tmpfs 7.8G 0 7.8G 0% /dev/shm
) = 47
write(1, "/dev/vdb 493G 314G 154G"..., 44/dev/vdb 493G 314G 154G 68% /data
) = 44
write(1, "/dev/vdc 493G 234M 467G"..., 45/dev/vdc 493G 234M 467G 1% /data1
) = 45
write(1, "/dev/vdd 493G 198M 467G"..., 45/dev/vdd 493G 198M 467G 1% /data2
) = 45
close(1) = 0
munmap(0x7fd1715eb000, 4096) = 0
close(2) = 0
exit_group(0) = ?
观察的时候发现追踪到/opt的时候卡住了
但是df -h的结果来看，确实没有/opt的挂载信息，于是进入/opt，发现也是奇卡无比
于是使用 mount |column -t 查看所有的挂载信息
[root@new_web /]# mount | column -t /dev/vda1 on / type ext4 (rw,barrier=0) proc on /proc type proc (rw) sysfs on /sys type sysfs (rw) devpts on /dev/pts type devpts (rw,gid=5,mode=620) tmpfs on /dev/shm type tmpfs (rw) none on /proc/sys/fs/binfmt_misc type binfmt_misc (rw) /dev/vdb on /data type ext4 (rw) //192.168.2.6/replication on /opt type cifs (rw,username=dba,password=0987654) /dev/vdc on /data1 type ext4 (rw) /dev/vdd on /data2 type ext4 (rw)
发现/opt挂载了远程nfs系统，卸载/opt
umount /opt
再次执行df -h 发现秒开！！
问题原因：
此处的NFS系统已经不使用了（挂了），但是它的挂载信息还在，导致文件系统一直在查找，所以就一直卡在那里