Disk performance under CURRENT

[Scott Long]

Kevin Oberman wrote:
> I just ran test of disk writing performance under V4 (STABLE) and V5
> (CURRENT) and was surprised at the difference.
> 
> The test was simple and not at all rigorous. Just a dd bs=256k
> if=/dev/zero of=/dev/ad2. This is about the simplest way of dealing with
> a disk. No file system or anything else. Just raw data to the device.
> 
> Under STABLE, I get an average of 25 MB/sec to the disk. Under CURRENT,
> it drops to 15 MB/sec. I did this because I had noted that it was now
> taking over an hour to backup my system disk (40 GB) when it was only
> taking 40 minutes when I was running V4.6. The STABLE system was built
> yesterday. The CURRENT system last Sunday.
> 
> Any idea why this is so much slower? It looks to me like it must be in
> either geom or the disk driver.

Btw, before we run off and dig into performance work here, can you send
the output of the following from your test machine:

sysctl debug.witness_watch

Thanks,

Scott