geom_mirror performance issues

[Paul Mather]

Tomas Zvala wrote:

> Hello,
>     I've been playing with geom_mirror for a while now and few issues 
> came up to my mind.
>     a) I have two identical drives (Seagate 120GB SATA 8MB Cache 
> 7.2krpm) that are able to sequentialy read at 58MB/s at the same time 
> (about 115MB/s throughput). But when I have them in geom_mirror I get 
> 30MB/s at best. Thats about 60MB/s for the mirror (about half the 
> potential). The throughput is almost the same for both 'split' and 
> 'load' balancing algorithms altough with load algorithm it seems that 
> all the reading is being done from just one drive.

When I did similar geom_mirror tests (dd'ing large files), I noticed 
with the 'load' balance algorithm that both drives of my mirror were in 
fact used, but not simultaneously.  One drive would be used exclusively 
for a few seconds, and then it would switch over to use the other drive 
for a few seconds, and then back again.  I attributed this behaviour to 
perhaps that the 'load' statistics in GEOM were only being updated 
relatively infrequently (to minimise performance impact), although I 
don't know if this is the case.

The odd thing is that the overall throughput of 'load' was the 
second-highest of all the balancing algorithms I tried, coming just a 
little behind the 'split' balance algorithm.  (Mind you, this was on a 
300 MHz Pentium II system, where the CPU overheads of a transaction are 
more significant than a GHz-level system.)

>     d) When I use round-robin algorithm the performance halves (i get 
> about 20MB/s raw throughput). Why is this? I would expect round-robin 
> algorithm to be the most effective one for reading as every drive gets 
> exactly half the load.

My speculation is that perhaps relative drive head positions might 
deviate more under 'round-robin' leading to more time lost seeking 
(which is the dominant cost in disk I/O).

I also found 'round-robin' to be relatively slow compared to 'load' and 
'split.'

Cheers,

Paul.