如果我从本地 LAN（192.168.100.0/24）访问 MySQL 服务器，它运行良好。如果我尝试从另一个 LAN（本例中为 192.168.113.0/24）访问它，它会挂起很长时间才能提供结果。以 的形式SHOW PROCESSLIST;显示此过程Sleep，为State空。
如果我strace -p此过程我得到以下输出（23512 是相应 mysqld 进程的 PID）：
Process 23512 attached - interrupt to quit
restart_syscall(<... resuming interrupted call ...>) = 1
fcntl(10, F_GETFL) = 0x2 (flags O_RDWR)
fcntl(10, F_SETFL, O_RDWR|O_NONBLOCK) = 0
accept(10, {sa_family=AF_INET, sin_port=htons(51696), sin_addr=inet_addr("192.168.113.4")}, [16]) = 33
fcntl(10, F_SETFL, O_RDWR) = 0
rt_sigaction(SIGCHLD, {SIG_DFL, [CHLD], SA_RESTORER|SA_RESTART, 0x7f9ce7ca34f0}, {SIG_DFL, [CHLD], SA_RESTORER|SA_RESTART, 0x7f9ce7ca34f0}, ) = 0
getpeername(33, {sa_family=AF_INET, sin_port=htons(51696), sin_addr=inet_addr("192.168.113.4")}, [16]) = 0
getsockname(33, {sa_family=AF_INET, sin_port=htons(3306), sin_addr=inet_addr("192.168.100.190")}, [16]) = 0
open("/etc/hosts.allow", O_RDONLY) = 64
fstat(64, {st_mode=S_IFREG|0644, st_size=580, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f9ce9839000
read(64, "# /etc/hosts.allow: list of host"..., 4096) = 580
read(64, "", 4096) = 0
close(64) = 0
munmap(0x7f9ce9839000, 4096) = 0
open("/etc/hosts.deny", O_RDONLY) = 64
fstat(64, {st_mode=S_IFREG|0644, st_size=880, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f9ce9839000
read(64, "# /etc/hosts.deny: list of hosts"..., 4096) = 880
read(64, "", 4096) = 0
close(64) = 0
munmap(0x7f9ce9839000, 4096) = 0
getsockname(33, {sa_family=AF_INET, sin_port=htons(3306), sin_addr=inet_addr("192.168.100.190")}, [16]) = 0
fcntl(33, F_SETFL, O_RDONLY) = 0
fcntl(33, F_GETFL) = 0x2 (flags O_RDWR)
setsockopt(33, SOL_SOCKET, SO_RCVTIMEO, "\36\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", 16) = 0
setsockopt(33, SOL_SOCKET, SO_SNDTIMEO, "<\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", 16) = 0
fcntl(33, F_SETFL, O_RDWR|O_NONBLOCK) = 0
setsockopt(33, SOL_IP, IP_TOS, [8], 4) = 0
setsockopt(33, SOL_TCP, TCP_NODELAY, [1], 4) = 0
futex(0x7f9cea5c9564, FUTEX_WAKE_OP_PRIVATE, 1, 1, 0x7f9cea5c9560, {FUTEX_OP_SET, 0, FUTEX_OP_CMP_GT, 1}) = 1
futex(0x7f9cea5c6fe0, FUTEX_WAKE_PRIVATE, 1) = 1
poll([{fd=10, events=POLLIN}, {fd=12, events=POLLIN}], 2, -1) = 1 ([{fd=10, revents=POLLIN}])
fcntl(10, F_GETFL) = 0x2 (flags O_RDWR)
fcntl(10, F_SETFL, O_RDWR|O_NONBLOCK) = 0
accept(10, {sa_family=AF_INET, sin_port=htons(51697), sin_addr=inet_addr("192.168.113.4")}, [16]) = 31
fcntl(10, F_SETFL, O_RDWR) = 0
rt_sigaction(SIGCHLD, {SIG_DFL, [CHLD], SA_RESTORER|SA_RESTART, 0x7f9ce7ca34f0}, {SIG_DFL, [CHLD], SA_RESTORER|SA_RESTART, 0x7f9ce7ca34f0}, ) = 0
getpeername(31, {sa_family=AF_INET, sin_port=htons(51697), sin_addr=inet_addr("192.168.113.4")}, [16]) = 0
getsockname(31, {sa_family=AF_INET, sin_port=htons(3306), sin_addr=inet_addr("192.168.100.190")}, [16]) = 0
open("/etc/hosts.allow", O_RDONLY) = 33
fstat(33, {st_mode=S_IFREG|0644, st_size=580, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f9ce9839000
read(33, "# /etc/hosts.allow: list of host"..., 4096) = 580
read(33, "", 4096) = 0
close(33) = 0
munmap(0x7f9ce9839000, 4096) = 0
open("/etc/hosts.deny", O_RDONLY) = 33
fstat(33, {st_mode=S_IFREG|0644, st_size=880, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f9ce9839000
read(33, "# /etc/hosts.deny: list of hosts"..., 4096) = 880
read(33, "", 4096) = 0
close(33) = 0
munmap(0x7f9ce9839000, 4096) = 0
getsockname(31, {sa_family=AF_INET, sin_port=htons(3306), sin_addr=inet_addr("192.168.100.190")}, [16]) = 0
fcntl(31, F_SETFL, O_RDONLY) = 0
fcntl(31, F_GETFL) = 0x2 (flags O_RDWR)
setsockopt(31, SOL_SOCKET, SO_RCVTIMEO, "\36\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", 16) = 0
setsockopt(31, SOL_SOCKET, SO_SNDTIMEO, "<\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", 16) = 0
fcntl(31, F_SETFL, O_RDWR|O_NONBLOCK) = 0
setsockopt(31, SOL_IP, IP_TOS, [8], 4) = 0
setsockopt(31, SOL_TCP, TCP_NODELAY, [1], 4) = 0
futex(0x7f9cea5c9564, FUTEX_WAKE_OP_PRIVATE, 1, 1, 0x7f9cea5c9560, {FUTEX_OP_SET, 0, FUTEX_OP_CMP_GT, 1}) = 1
futex(0x7f9cea5c6fe0, FUTEX_WAKE_PRIVATE, 1) = 1
poll([{fd=10, events=POLLIN}, {fd=12, events=POLLIN}], 2, -1^C <unfinished ...>
Process 23512 detached
此输出会重复，直到发送答案。请求可能需要长达 15 分钟才能得到处理。在本地 LAN 中，这仅是几毫秒的时间。
为什么会这样？我该如何进一步调试？
[编辑]
tcpdump显示了大量这样的内容：
14:49:44.103107 IP cassandra-test.mysql > 192.168.X.6.64626: Flags [S.], seq 1434117703, ack 1793610733, win 14600, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0
14:49:44.135187 IP 192.168.X.6.64625 > cassandra-test.mysql: Flags [P.], seq 106:145, ack 182, win 4345, length 39
14:49:44.135293 IP cassandra-test.mysql > 192.168.X.6.64625: Flags [P.], seq 182:293, ack 145, win 115, length 111
14:49:44.167025 IP 192.168.X.6.64624 > cassandra-test.mysql: Flags [.], ack 444, win 4280, length 0
14:49:44.168933 IP 192.168.X.6.64626 > cassandra-test.mysql: Flags [.], ack 1, win 4390, length 0
14:49:44.169088 IP cassandra-test.mysql > 192.168.X.6.64626: Flags [P.], seq 1:89, ack 1, win 115, length 88
14:49:44.169672 IP 192.168.X.6.64625 > cassandra-test.mysql: Flags [P.], seq 145:171, ack 293, win 4317, length 26
14:49:44.169726 IP cassandra-test.mysql > 192.168.X.6.64625: Flags [P.], seq 293:419, ack 171, win 115, length 126
14:49:44.275111 IP 192.168.X.6.64626 > cassandra-test.mysql: Flags [P.], seq 1:74, ack 89, win 4368, length 73
14:49:44.275131 IP cassandra-test.mysql > 192.168.X.6.64626: Flags [.], ack 74, win 115, length 0
14:49:44.275149 IP 192.168.X.6.64625 > cassandra-test.mysql: Flags [P.], seq 171:180, ack 419, win 4286, length 9
14:49:44.275189 IP cassandra-test.mysql > 192.168.X.6.64626: Flags [P.], seq 89:100, ack 74, win 115, length 11
14:49:44.275264 IP 192.168.X.6.64625 > cassandra-test.mysql: Flags [P.], seq 180:185, ack 419, win 4286, length 5
14:49:44.275281 IP cassandra-test.mysql > 192.168.X.6.64625: Flags [.], ack 185, win 115, length 0
14:49:44.275295 IP cassandra-test.mysql > 192.168.X.6.64625: Flags [F.], seq 419, ack 185, win 115, length 0
14:49:44.275650 IP 192.168.X.6.64625 > cassandra-test.mysql: Flags [F.], seq 185, ack 419, win 4286, length 0
14:49:44.275660 IP cassandra-test.mysql > 192.168.X.6.64625: Flags [.], ack 186, win 115, length 0
14:49:44.275910 IP 192.168.X.6.64627 > cassandra-test.mysql: Flags [S], seq 2336421549, win 8192, options [mss 1351,nop,wscale 2,nop,nop,sackOK], length 0
14:49:44.275921 IP cassandra-test.mysql > 192.168.X.6.64627: Flags [S.], seq 3289359778, ack 2336421550, win 14600, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0
答案1
在 tcpdump 图片上，可以看到连接池和/或频繁重启的连接。检查是否未达到最大连接限制。