Performance issues with 8.0 ZFS and sendfile/lighttpd

[Ivan Voras]

2009/11/6 Miroslav Lachman <000.fbsd at quip.cz>:

> I do not understand why there are 10MB/s read from disks when network
> traffic dropped to around 1MB/s (8Mbps)
>
> root at cage ~/# iostat -w 20
>      tty             ad4              ad6             cpu
>  tin tout  KB/t tps  MB/s   KB/t tps  MB/s  us ni sy in id
>   0   14 41.66  53  2.17  41.82  53  2.18   0  0  2  0 97
>   0   18 50.92  96  4.77  54.82 114  6.12   0  0  3  1 96
>   0    6 53.52 101  5.29  54.98 108  5.81   1  0  4  1 94
>   0    6 54.82  98  5.26  55.89 108  5.89   0  0  3  1 96

Yes, this could limit your IO if the requests are random enough.
Unfortunately I don't know how would you track down what is really
going on. Maybe some tracing with DTrace?

I'd tell you to use "top -m io" to see if there is a process
responsible, but apparently these statistics are not updated for ZFS,
which in itself may be a bug (which is why I'm crossposting to
freebsd-fs).