Nfs突然停了下来！甚至无法重启服务：
# /etc/init.d/nfs-kernel-server restart
* Stopping NFS kernel daemon [ OK ]
* Unexporting directories for NFS kernel daemon... [ OK ]
* Exporting directories for NFS kernel daemon... [ OK ]
* Starting NFS kernel daemon ^C
Syslogs表示nfs被阻止超过120秒。如何理解日志？这里出了什么问题？
# cat /var/log/syslog | grep nfs
Sep 4 10:02:11 InksedgeDev kernel: [2427120.248354] INFO: task nfsd:2318 blocked for more than 120 seconds.
Sep 4 10:02:11 InksedgeDev kernel: [2427120.248363] nfsd D 0000000000000000 0 2318 2 0x00000000
Sep 4 10:02:11 InksedgeDev kernel: [2427120.248406] [<ffffffffa02a28c0>] ? nfsd_proc_getattr+0xa0/0xa0 [nfsd]
Sep 4 10:02:11 InksedgeDev kernel: [2427120.248418] [<ffffffffa02a3c11>] nfsd_vfs_write.isra.13+0x81/0x3b0 [nfsd]
Sep 4 10:02:11 InksedgeDev kernel: [2427120.248422] [<ffffffffa02bf691>] ? lookup_clientid+0x51/0x80 [nfsd]
Sep 4 10:02:11 InksedgeDev kernel: [2427120.248426] [<ffffffffa02bf766>] ? nfsd4_lookup_stateid+0xa6/0xc0 [nfsd]
Sep 4 10:02:11 InksedgeDev kernel: [2427120.248429] [<ffffffffa02a7773>] nfsd_write+0x93/0x110 [nfsd]
Sep 4 10:02:11 InksedgeDev kernel: [2427120.248433] [<ffffffffa02b2281>] nfsd4_write+0x1b1/0x250 [nfsd]
Sep 4 10:02:11 InksedgeDev kernel: [2427120.248437] [<ffffffffa02b35ca>] nfsd4_proc_compound+0x56a/0x7c0 [nfsd]
Sep 4 10:02:11 InksedgeDev kernel: [2427120.248440] [<ffffffffa029fd2b>] nfsd_dispatch+0xbb/0x200 [nfsd]
Sep 4 10:02:11 InksedgeDev kernel: [2427120.248455] [<ffffffffa029f71f>] nfsd+0xbf/0x130 [nfsd]
Sep 4 10:02:11 InksedgeDev kernel: [2427120.248458] [<ffffffffa029f660>] ? nfsd_destroy+0x80/0x80 [nfsd]
Iostat的输出：
# iostat 5 5
Linux 3.13.0-29-generic (InksedgeDev) 09/04/2015 _x86_64_ (2 CPU)
avg-cpu: %user %nice %system %iowait %steal %idle
5.68 0.00 0.48 0.25 0.01 93.58
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
xvdap1 3.63 15.71 82.80 38138709 201066916
xvdb 0.01 0.05 0.01 119271 15231
xvdc 0.01 0.05 0.01 112329 14600
xvdf 4.25 155.14 175.05 376748353 425087092
md0 0.02 0.10 0.01 231288 29831
avg-cpu: %user %nice %system %iowait %steal %idle
0.30 0.00 0.20 49.75 0.00 49.75
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
xvdap1 0.00 0.00 0.00 0 0
xvdb 0.00 0.00 0.00 0 0
xvdc 0.00 0.00 0.00 0 0
xvdf 0.00 0.00 0.00 0 0
md0 0.00 0.00 0.00 0 0
可能是什么原因？如何排除故障？