8.2 ->8.3 regression on disk writes
Michael Ross
gmx at ross.cx
Tue Jul 17 12:38:51 UTC 2012
Am 16.07.2012, 15:34 Uhr, schrieb Steven Hartland
<killing at multiplay.co.uk>:
>
> ----- 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
The machine has 2GB of RAM installed -- at least that's what the hoster
says and what the Debian system it comes with reports. Not a VM.
<expletive deleted>
I was mentally set for "disk trouble" and didn't even spot that,
neither did I spot the little gem above:
"WARNING: This architecture revision has known SMP hardware bugs which may
cause random instability"
Kernel source has this to say:
/*
* Opteron Rev E shows a bug as in very rare occasions a read memory
* barrier is not performed as expected if it is followed by a
* non-atomic read-modify-write instruction.
* As long as that bug pops up very rarely (intensive machine usage
* on other operating systems generally generates one unexplainable
* crash any 2 months) and as long as a model specific fix would be
* impratical at this stage, print out a warning string if the broken
* model and family are identified.
*/
"Very rare" "random instability", read: Crappy hardware.
Plus, after about 1 week of running tests with 8.2, 8.3, 9.0 and 9-STABLE
kernels I managed to provoke the problem on 8.2 as well, so I'm
embarrassed now for calling regression where there is none.
Short, I'm giving up on this machine;
thanks for the help, sorry for the noise,
Michael
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