high i/o usage

[Maxim Dounin]

Hello!

On Mon, Oct 05, 2009 at 09:02:48AM +0200, Stefanita Rares Dumitrescu wrote:

> hi guys,
> 
> i am using centos 5, nginx version: nginx/0.8.16
> , and php-fpm and i have switched to nginx on one of my very busy servers.
> 
> however, i see very high i/o usage, and the site works pretty sloppy.
> 
> below is my iostat and df output. as you can see i get around 25
> mb/s written at any time.
> 
> mysql is on a second drive, and there is pretty much nothing else.
> however i cannot understand why the i/o is so high. i expect that
> maybe because of log writing, but i checked the log folder, and it's
> not.
> 
> I have setup in the beginnging the worker processes to 8, but for
> some unknown reason after 1 2 minutes, nginx stops, so i had to
> increase it to 64, and now it stays up. but i have left the i/o
> thing, which i cannot make anything out of it.
> 
> 
> Device:            tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
> sda             123.88         0.00        25.94          0         52
> sda1              0.00         0.00         0.00          0          0
> sda2              0.00         0.00         0.00          0          0
> sda3              1.00         0.00         0.02          0          0
> sda4            122.89         0.00        25.92          0         52

Isn't fastcgi_temp_path points to this drive?  If yes, this IO 
probably caused by fastcgi buffering application responses to 
disk.  Solution is to tune fastcgi_buffers.

You may also consider to returning big responses directly by nginx 
(e.g. via X-Accel-Redirect).

Maxim Dounin

> sdb              15.42         0.07         0.00          0          0
> sdb1             15.42         0.07         0.00          0          0
> sdc              69.15         0.58         1.27          1          2
> sdc1             69.15         0.58         1.27          1          2
> 
> 
> root at rum [/usr/local/nginx/logs]# df -h
> Filesystem            Size  Used Avail Use% Mounted on
> /dev/sda4             331G   50G  265G  16% /
> /dev/sda3              15G  1.2G   13G   9% /tmp
> /dev/sda1              99M   18M   77M  19% /boot
> tmpfs                 7.9G     0  7.9G   0% /dev/shm
> /dev/sdb1             367G   60G  289G  18% /home2
> /dev/sdc1              58G   13G   43G  23% /home3
>