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Please help me understand the output of "sar -b"
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Please help me understand the output of "sar -b"

AmitzAmitz Member

Dear all,

I have installed the sysstat package on my server to get a better impression of my real world disk IO needs. I was always under the impression that I NEED SSD drives to have a smooth ride and stand corrected in the meantime. With an average transaction per second (IOPS) rate of around 23, SATA drives are obviously more than enough for me.

But there is something I do not understand at all: By looking at the output of "sar -b" (see below), it seems to me as if there is no read activity at all, but lots of write activity instead. How is that possible with a server that is mainly running a busy website? Shouldn't every served file produce read activity rather than write activity? Or is there a misconception on my side? I should add that the server has 2 disks in a hardware raid-1 array.

I would be really thankful if you could kindly enlighten me...

Thanks a lot in advance and with kind regards,
-A

10:10:01 PM       tps      rtps      wtps   bread/s   bwrtn/s
10:15:01 PM     44.61      0.00     44.61      0.00   1007.89
10:20:01 PM     40.19      0.00     40.19      0.00    752.55
10:25:01 PM     44.07      0.00     44.07      0.00    804.98
10:30:01 PM     44.97      0.00     44.97      0.00    777.63
10:35:01 PM     39.36      0.00     39.36      0.00    734.69
10:40:01 PM     47.45      0.00     47.45      0.00    810.87
10:45:01 PM     48.95      0.00     48.95      0.00    832.54
10:50:01 PM     42.35      0.00     42.35      0.00    725.63
10:55:01 PM     42.30      0.00     42.30      0.00    731.17
11:00:01 PM     43.26      0.00     43.26      0.00    772.68
11:05:01 PM     35.13      0.00     35.13      0.00    775.72
11:10:01 PM     57.23      0.00     57.23      0.00   1061.18
11:15:01 PM     34.38      0.00     34.38      0.00    680.28
11:20:01 PM     25.49      0.00     25.49      0.00    543.96
11:25:01 PM     26.68      0.00     26.68      0.00    590.71
11:30:01 PM     25.74      0.00     25.74      0.00    548.49
11:35:01 PM     29.00      0.00     29.00      0.00    669.05
11:40:01 PM     27.84      0.00     27.84      0.00    593.63
11:45:01 PM     30.59      0.00     30.59      0.00    690.91
11:50:01 PM     27.49      0.00     27.49      0.00    584.10
11:55:01 PM     38.42      0.00     38.41      0.03   7700.17
11:59:01 PM     32.88      0.00     32.88      0.00    837.92
Average:        23.21      0.00     23.20      0.02    517.93

Comments

  • @Amitz said:

    There are numerous reasons why you might not be reading as often as you should:

    Linux Kernel Host-Page/Buffer Cache

    Apache HTTPd with the mmap or sendfile on

    Various other things that cache things

  • AmitzAmitz Member

    Thank you. Indeed, I thought about caching too, but there is no read activity at all since days (I have just posted an excerpt). Is that realistic?

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