IO utilization stuck at 99%

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Fatlum Bajrami

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Dec 10, 2021, 7:01:05 AM12/10/21
to ClickHouse
Hi people,

I have two clickhouse servers running 1 shard each, they have been running okay for a long time but suddenly one of the servers is showing 98-99% disk I/O, i have tryed to restart the faulty server but with no success, io is constantly showing 99%, i have searched google and found nothing related to this topic other than this suppose being normal.

Below is a iotop log 

99.99 % clickhouse-server --config=~server.pid [BackgrProcPool]
15509 be/4 clickhou    0.00 B/s  859.71 K/s  0.00 % 99.99 % clickhouse-server --config=~use-server.pid [TCPHandler]
15223 be/4 clickhou    0.00 B/s  429.85 K/s  0.00 % 99.94 % clickhouse-server --config=~server.pid [BackgrProcPool]
15218 be/4 clickhou    0.00 B/s  683.86 K/s  0.00 % 99.43 % clickhouse-server --config=~server.pid [BackgrProcPool]
15213 be/4 clickhou    0.00 B/s  371.24 K/s  0.00 % 99.31 % clickhouse-server --config=~server.pid [BackgrProcPool]
15221 be/4 clickhou    0.00 B/s  550.99 K/s  0.00 % 99.28 % clickhouse-server --config=~server.pid [BackgrProcPool]

Alexey Milovidov

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Dec 12, 2021, 2:52:56 PM12/12/21
to Fatlum Bajrami, ClickHouse
Hi. Disk IO is related to background merge operations and this is normal.
What's not normal is that it is processing with a speed of just a few
megabytes per second.
It may indicate one of the following:
- faulty disks that need replacement (you can test them with "fio" or
similar); if the issue is happening on only one server out of many,
this is the case.
- very slow storage subsystem; if you are using small network disks in
the cloud, this may be the case and you need to provision more IO.
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Fatlum Bajrami

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Dec 24, 2021, 4:41:00 AM12/24/21
to ClickHouse
Thank you Milovidov for the replay,

This was a gcp instance ubuntu 16.04 after some reading i had the idea that it probably is a kernel problem and updated to 20.04 and the problem was resolved. 
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