Search for high I/O latency issues on Linux servers

Search for high I/O latency issues on Linux servers

0. First, check the system status on top.

Two parameters are found to be abnormal. One is high average load, and the other is that cpu % wa is always above 50%.

Check the meaning of the % wa parameter:

 
 

  
  
wa -- iowait  
  
  
 Amount of time the CPU has been waiting for I/O to complete. 
 
 

1. view disk read/write data

View disk status with iostat

 
 

  
  
$ iostat -x 2 5  
  
  
 avg-cpu: %user %nice %system %iowait %steal %idle  
  
  
  3.66 0.00 47.64 48.69 0.00 0.00  
  
  
 
  
  
 Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await r_await w_await svctm %util  
  
  
 sda 44.50 39.27 117.28 29.32 11220.94 13126.70 332.17 65.77 462.79 9.80 2274.71 7.60 111.41  
  
  
 dm-0 0.00 0.00 83.25 9.95 10515.18 4295.29 317.84 57.01 648.54 16.73 5935.79 11.48 107.02  
  
  
 dm-1 0.00 0.00 57.07 40.84 228.27 163.35 8.00 93.84 979.61 13.94 2329.08 10.93 107.02 
 
 

2. view the process status

Process status table

 
 

  
  
PROCESS STATE CODES  
  
  
 D uninterruptible sleep (usually IO)  
  
  
 R running or runnable (on run queue)  
  
  
 S interruptible sleep (waiting for an event to complete)  
  
  
 T stopped, either by a job control signal or because it is being traced.  
  
  
 W paging (not valid since the 2.6.xx kernel)  
  
  
 X dead (should never be seen)  
  
  
 Z defunct ("zombie") process, terminated but not reaped by its parent. 
 
 

View the Ddisk sleep process in the process.

 
 

  
  
# for x in `seq 1 1 10`; do ps -eo state,pid,cmd | grep "^D"; echo "----"; sleep 5; done 
 
 

Check that the kjournald process exists.

Check what this process does?

Kjournald is an ext3 file system logging process.

View Process status information

 
 

  
  
# cat /proc/487/io  
  
  
 rchar: 48752567  
  
  
 wchar: 549961789  
  
  
 syscr: 5967  
  
  
 syscw: 67138  
  
  
 read_bytes: 49020928  
  
  
 write_bytes: 549961728  
  
  
 cancelled_write_bytes: 0 
 
 

Check where everything is written.

 
 

  
  
# lsof -p 487 
 
 

The cause of kjournald status D is still being checked ....