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we have mysql-server(5.5.47)that hosted on physical server. It listen external internet interface(with restrict user access), mysql server intensively used from different places(we use different libraries to communicate with mysql). But sometimes whole mysql server(or network) stuck and stop accept connection, and a clients failed with etimedout(connect)/timeout(recv), even direct connection from server to mysql with mysql cli not working(stuck without any response — seems to be try to establish connections). First thought was that it is related to tcp backlog, so mysql backlog was increased — but this not help at all.

Issue not repeatable, so last time when this issue happened we sniff traffic, and what we get:

http://grab.by/STwq — screenshot

*.*.27.65 — it is client
*.*.20.80 — it is mysql server

From session we can assume that tcp connection established, but server retransmit SYN/ACK to client(from dump we see that server receive ACK, why retransmit ?), but in normal case mysql must generate init packet and send to client, after connection was established. It is only screen from 1 session, but all other sessions mostly same, SYN -> SYN/ACK -> ACK -> and server retransmit SYN/ACK up to retries_count.

After restart mysql all get normal immediately after restart. So not sure it is related to network or mysql.

Any thoughts would be appropriate.

Thank you!

user2746626
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    This is not really a question for sw developers because this is clearly an operations issue. Sysadmins or DBAs would be in a lot better position to answer this question. – Shadow Sep 25 '16 at 15:56
  • @Shadow, could you post link where i could ask this type of question, serverfault ? – user2746626 Sep 25 '16 at 19:50

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