Wednesday, October 9, 2013

TOPAS hdisks 100% busy -commands hang

www.unixbabuforum.inI have an IBM 8203-E4A (P6) system that is running AIX 5300-10. The 
hdisks in TOPAS intermittently gets to 100% busy. Sometimes all commands 
hang (can't do anything to gather data, etc) and can't login to a 
duplicate ssh session. Other times all commands work. The paging space 
is 1% used. Has anyone had this problem before and/or have any idea about 
I/O performance issues that I can check? Degreeworks is running on the 
server.
www.unixbabuforum.inFrom system's point of view, if you enable I/O pacing, ordinary commands 
will not hang if a process is using I/O resources greedily. 
This is actually done by setting a high and low water mark per file for each 
process. 
# lsattr -El sys0 -a maxpout -a minpout 
They are 0 by default which means unlimited use of I/O for any process is 
allowed
www.unixbabuforum.inIt does not need system restart. You can set it on the fly even during 
transactions and you will see the change instantly. 
As IBM recommends, start with maxpout=32 and minpout=24 and then gradually 
increase them and monitor the system until experiencing a smooth system. 
One of the side effects of enabling I/O pacing is that I/O-bound 
transactions, specially batch processes, will take longer to finish if you 
set these parameters to low value . That is the reason that you should find 
the best values for these high and low water marks and not just rely on the 
staring point recommended by IBM.

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