Fellow DBAs and MySQL Users

[apologies for eventual duplicates - I've posted this to percona-discussion@googlegroups.com also]

I've been hunting an issue with my database cluster for several months now without much success. Maybe I'm overlooking something here.

I've been observing the database slowing down and lagging behind for thousands of seconds (sometimes over the course of several days) even without any query load besides replication itself.

I am running Percona MySQL 5.1.51 (InnoDB plug-in version 1.12) on Dell R710 (6 x 3.5 inch 15K RPM disks in RAID10; 24GB RAM; 2x Quad-core Intel processors) running Debian Lenny. MySQL data, binary logs, relay logs, innodb log files are on separated partitions from each other, on a RAID system separated from the operating system disks.

Default Storage Engine is InnoDB, and the usual InnoDB memory structures are stable and look healthy.

I have about 500 (read) queries per second on average, and about 10% of this as writes on the master.

I've been observing something that looks like between 6 and 10 pending reads per second uniformly on my cacti graphs.

The issue is characterized by the server suddenly slowing down writes without any previous warning or change, and lagging behind for several thousand seconds (triggering all sorts of alerts on my monitoring system). I don't observe extra CPU activity, just a reduced disk access ratio (from about 5-6MB/s to 500KB/s) and replication lagging. I could correlate it neither InnoDB hashing activity, nor with long-running-queries, nor with background read/write thread activities.

I don't have any clues of what is causing this behavior, and I'm unable to reproduce it under controlled conditions. I've observed the issue both on severs with and without workload (apart from the usual replication load). I am sure no changes were applied to the server or to the cluster.

I'm looking forward for suggestions and theories on the issue - all ideas are welcome.
Thank you for your time and attention,
Kind regards,
--
Luis Motta Campos
is a DBA, Foodie, and Photographer


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