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I have a 3 servers setup, one frontend HAproxy sending requests to 2 backend servers.
The backend servers use identical hardware and also identical software (Debian 6.0.6). They are similarly configured also receive roughly the same number of requests to process (HAproxy takes care of this).
Despite of this one has constantly a higher load than the other and it seems to be related to hard disks as far as I can tell by using iotop
The processes that seem to read most are php-fpm processes.

Any advices on where I could look would be hopefully helpful.

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(1) I am fully aware that the question clearly indicated the hardware are identical. But I still have to ask this question: Are all individual hard disk the same model? (2) Are the php-fpm configured (pool/socket/port) same? (3) How is DB configured? –  John Siu Jan 15 '13 at 5:14
    
HDD: yes, they seem to be exactly the same model, they're in SW RAID1. –  Alex Flo Jan 15 '13 at 8:07
    
PHP-FPM: yes, I looked at this and the config files are similar. –  Alex Flo Jan 15 '13 at 8:08
    
DB (MySQL) are configured similarly on both servers and they are in a master-slave replication (a server is both slave and master) –  Alex Flo Jan 15 '13 at 8:12
    
Same bios? Same layout (slot wise)? –  TomTom Jan 15 '13 at 13:32

1 Answer 1

up vote 2 down vote accepted

After much digging I figured out the main reason why one server was working much better than the other:
one had 1024 for /sys/block/sda/queue/nr_requests while the other one had the default value which was 128.

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Ok, WHY? Did someone change that on one server? –  TomTom Jan 15 '13 at 13:33

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