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

O. Hartmann ohartman at zedat.fu-berlin.de
Tue Oct 6 09:29:18 UTC 2009

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 

Maybe this is of interest:

Watch the threaded I/O impact ...


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