ZFS Re: A specific example of a disk i/o problem

[Thomas Backman]

On Oct 6, 2009, at 10:57 AM, O. Hartmann wrote:

> Thomas Backman wrote:
>> On Oct 5, 2009, at 6:05 PM, Dieter wrote:
>>> In message <E316139E-FFCF-432F-8DCE-62B120C38E55 at exscape.org>,  
>>> Thomas Backman writes:
>>>
>>>> I run 8.0-RC1/amd64 with ZFS on an A64 3200+ with 2GB RAM and an  
>>>> old
>>>> 80GB 7200rpm disk.
>>>>
>>>> My problem is that I get completely unacceptable latency on  
>>>> console IO
>>>> (both via SSH and serial console) when the system is performing  
>>>> disk
>>>> IO. The worst case I've noticed yet was when I tried copying a core
>>>> dump from a lzjb compressed ZFS file system to a gzip-9 compressed
>>>> one, to compare the file size/compression ratio. screen(1) took at
>>>> LEAST ten seconds - probably a bit more - I'm not exaggerating  
>>>> here -
>>>> to switch to another window, and an "ls" in an empty directory also
>>>> about 5-10 seconds.
>>>
>>> You might find the "RELENG_7 heavy disk = system crawls" thread  
>>> interesting:
>>>
>>> } I didn't actually solve it or do anything.
>>> } I just upgraded to RELENG_8.
>>> }
>>> } Now it's behaving more like FreeBSD should.
>>> } I can do sequential reads/writes and still
>>> } use kbd/mouse/X11/buildworld and so on.
>> Doesn't really help much I'm afraid, since 8.0-RC1 = RELENG_8 =  
>> stable/8. Been running current/stable-8 since May.
>> Regards,
>> Thomas
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>> "
>
> Is this really ZFS-bound? I also saw (and still see!) massive  
> performance impacts when a lot of disk activities, especially when  
> compiling large packages (done on UFS2 disks). Copying data from one  
> ZFS drive to (on different harddrive) another ZFS drive (which is  
> compressed) doesn't impact performance that much.
>
> When the box hit this performance issue, console, X11 and other  
> activities tend to be stuck for minutes! This happens on an UP box  
> (Athlon64 3500+, 2 GB ECC RAM, 2x 250 GB WD SATA-II disks and 1 TB  
> WD SATA-II disk (1x 250 GB UFS2 for system, 1x 250 GB ZFS compressed  
> for backup, 1x 1 TB ZFS holding /home).
>
> But this impact is also noticable on my lab's machine: Quad core  
> Intel Q6600 CPU at 3,0 GHZ, 8 GB RAM, 1x 320 GB SATA-II UFS2 disk  
> for system, 2x SATA-II 500 ZFS and 1x 750 GB disks ZFS and even on a  
> 8 core, two socket Dell PowerEdge 1950-III box with two XEON CPUs  
> running at 2,5 GHz, 16 GB RAM and two SATA-II harddrives attached to  
> the built in SAS controller.
>
> Maybe this is of interest:
> http://www.phoronix.com/scan.php?page=article&item=freebsd8_ubuntu910&num=1
>
> Watch the threaded I/O impact ...
>
> Regards,
> Oliver
It doesn't appear to be an ZFS issue, no. (Note that I wasn't the one  
who added it to the subject line :)
I just tried it to my one UFS filesystem, and while screen(1) DID  
remain 100% responsive, everything didn't. vim worked OK most of the  
time, but other FS ops on the same disk... oh no. I aborted the "file / 
etc/*" after 57 seconds. Tried it again after stopping the disk IO  
(cat /dev/zero > /bootdir/filename) - 1.52 seconds.

This raises the question: is this some kind of hardware specific  
issue? If so, what hardware? I can't think of anything my computer  
would have in common with the ones you've listed!
I mean, come on, FreeBSD's IO performance can't be this abysmal for  
everybody or nobody would use it for serious applications. Something  
must be wrong for a few of us, but where and why?

Regards,
Thomas