8.2 ->8.3 regression on disk writes

[Steven Hartland]

----- Original Message ----- 
From: "Michael Ross" <gmx at ross.cx>
To: <freebsd-stable at freebsd.org>
Sent: Monday, July 16, 2012 2:23 PM
Subject: 8.2 ->8.3 regression on disk writes


> Hello,
> 
> using 8.2 the machine runs fine,
> using 8.3 or higher, not so much.
> 
> In laymans terms,
> if I do "too many" writes/time just once, the machine can't do any disk  
> access for a couple of hours.
> 
> As in: What's already running stays running, no crashes or anything,
> but as soon as I need to read from disk (login, start program not cached  
> in memory from previous run),
> I'm all out of luck.
> I killed the testing ftp-transfer about 15 seconds after the transfer  
> speed dropped,
> now I'm waiting for 10+ minutes for ``top'' to start.
> 
> 
> I can install ports and kernels and world fine,
> but "ezjail-admin install" or transferring a few GB of files from another  
> machine sends it to limbo.
> 
> The next step would likely be to go through the kernel changes between 8.2  
> and 8.3 to narrow it down,
> I'd appreciate pointers as to what kernel changes to look out for,
> or other suggestions on what to do.
> 
> Verbose dmesg: http://gurder.ross.cx/misc/dmesg.txt

You have some strangeness there:

I see:

"real memory  = 17179869184 (16384 MB)"
"avail memory = 2050920448 (1955 MB)"

So even though you have 16GB ram your only using 2GB of it which
will likely cause slowness under ZFS including disabling prefetch

"ZFS NOTICE: Prefetch is disabled by default if less than 4GB of
RAM is present; to enable, add "vfs.zfs.prefetch_disable=0" to
/boot/loader.conf."

Is this a VM or something?

    Regards
    Steve


================================================
This e.mail is private and confidential between Multiplay (UK) Ltd. and the person or entity to whom it is addressed. In the event of misdirection, the recipient is prohibited from using, copying, printing or otherwise disseminating it or any information contained in it. 

In the event of misdirection, illegible or incomplete transmission please telephone +44 845 868 1337
or return the E.mail to postmaster at multiplay.co.uk.