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What could be writing on the disk that iotop does not show?

# iotop -a
Total DISK READ: 8.19 M/s | Total ****DISK WRITE: 3.34 M/s****
  428 be/4 root          0.00 B     84.00 K  0.00 %  0.02 % [kjournald]
 2600 be/3 root          0.00 B      8.00 K  0.00 %  0.01 % auditd -s disable
 2582 be/4 root          0.00 B      4.00 K  0.00 %  0.00 % syslog-ng

(iotop is NOT run using -o)

How do you interpret this?

# iostat
Linux 3.0.51-0.7.9-default (x)   03/10/13        _x86_64_

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
       1.08    0.00    1.33    5.27    0.00   92.32

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
sda               5.10       173.02        14.35  824994560   68417967
sdb              79.08      1028.23      1018.19 4902923384 4855045168
sr0               0.00         0.00         0.00        988          0
dm-0            175.34      1028.23      1018.19 4902916232 4855045168
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UPDATE An important detail that I forgot to say that this is Novell's NSS filesystem. This filesystem has compression features. What we see here may be things getting decompressed on read for example! – Marki Dec 4 '14 at 10:12

1 Answer 1

For performance, reads are critical (someone is waiting for the requested data), writes can be postponed without ill efects (as long as they get to the disk before the system goes down, obviously). If the machine has plenty of memory free, "no much writes" is to be expected.

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I don't think you understood the question. It is: how can it be that at the top of iostat you see writes, while in the process list just below that output it shows no writes? – Marki Mar 24 '13 at 12:09

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