Performance problems ? Read this first

I’m currently doing a time machine backup from a single Mac to the MBLD.

a top is showing the load average is above 3 and the CPU is only used at 20% maximum (by the afpd daemon responsible for Apple network file system):

top - 00:03:25 up 4 days, 5:10, 1 user, load average: 3.27, 3.38, 3.25
Tasks: 96 total, 1 running, 95 sleeping, 0 stopped, 0 zombie
Cpu(s): 2.6%us, 10.3%sy, 0.0%ni, 0.0%id, 86.4%wa, 0.0%hi, 0.7%si, 0.0%st
Mem: 253632k total, 243712k used, 9920k free, 54080k buffers
Swap: 500544k total, 203520k used, 297024k free, 110464k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
10448 nobody 5 -15 14336 5696 1408 D 10.3 2.2 144:49.97 afpd
8708 root 20 0 28672 12m 11m S 1.7 5.1 18:59.79 cnid_dbd
1933 root 20 0 5056 3456 2368 R 0.7 1.4 0:00.13 top
1 root 20 0 4352 1024 704 S 0.0 0.4 0:03.07 init
2 root 20 0 0 0 0 S 0.0 0.0 0:00.00 kthreadd
3 root 20 0 0 0 0 S 0.0 0.0 0:01.15 ksoftirqd/0
4 root RT 0 0 0 0 S 0.0 0.0 0:00.00 watchdog/0
5 root 20 0 0 0 0 S 0.0 0.0 0:00.81 events/0
6 root 20 0 0 0 0 S 0.0 0.0 0:00.01 khelper
9 root 20 0 0 0 0 S 0.0 0.0 0:00.00 async/mgr
89 root 20 0 0 0 0 S 0.0 0.0 0:00.01 sync_supers
91 root 20 0 0 0 0 S 0.0 0.0 0:00.02 bdi-default
93 root 20 0 0 0 0 S 0.0 0.0 9:50.36 kblockd/0
98 root 20 0 0 0 0 S 0.0 0.0 0:00.00 ata/0
99 root 20 0 0 0 0 S 0.0 0.0 0:00.00 ata_aux
101 root 20 0 0 0 0 S 0.0 0.0 0:00.00 kseriod
121 root 20 0 0 0 0 S 0.0 0.0 0:00.00 rpciod/0
152 root 20 0 0 0 0 S 0.0 0.0 0:00.01 khungtaskd
153 root 20 0 0 0 0 D 0.0 0.0 12:05.53 kswapd0
154 root 20 0 0 0 0 S 0.0 0.0 0:00.00 aio/0
155 root 20 0 0 0 0 S 0.0 0.0 0:00.00 nfsiod

So this means the CPU isn’t the bottleneck.

The load average is the average number of processes in ready state waiting for a resource (usually the CPU but in this case it isn’t). So some processes are waiting on some ressources. They could be waiting on some network IO. It could also be software interrupts.

Any thought ?