Dne 16. 08. 22 v 12:08 Heming Zhao napsal(a):
Ooh, very sorry, the subject is wrong, not IO performance but cpu high load
is triggered by pvmove.
On Tue, Aug 16, 2022 at 11:38:52AM +0200, Zdenek Kabelac wrote:
Dne 16. 08. 22 v 11:28 Heming Zhao napsal(a):
Hello maintainers & list,
I bring a story:
One SUSE customer suffered lvmpolld issue, which cause IO performance dramatic
decrease.
How to trigger:
When machine connects large number of LUNs (eg 80~200), pvmove (eg, move a
single
disk to a new one, cmd like: pvmove disk1 disk2), the system will suffer high
cpu load. But when system connects ~10 LUNs, the performance is fine.
We found two work arounds:
1. set lvm.conf 'activation/polling_interval=120'.
2. write a speical udev rule, which make udev ignore the event for mpath
devices.
echo 'ENV{DM_UUID}=="mpath-*", OPTIONS+="nowatch"' >\
/etc/udev/rules.d/90-dm-watch.rules
Run above any one of two can make the performance issue disappear.
** the root cause **
lvmpolld will do interval requeset info job for updating the pvmove status
On every polling_interval time, lvm2 will update vg metadata. The update job
will
call sys_close, which will trigger systemd-udevd IN_CLOSE_WRITE event, eg:
2022--xxx systemd-udevd[pid]: dm-179: Inotify event: 8 for
/dev/dm-179
(8 is IN_CLOSE_WRITE.)
These VGs underlying devices are multipath devices. So when lvm2 update
metatdata,
even if pvmove write a few data, the sys_close action trigger udev's "watch"
mechanism to gets notified frequently about a process that has written to the
device and closed it. This causes frequent, pointless re-evaluation of the udev
rules for these devices.
My question: Does LVM2 maintainers have any idea to fix this bug?
In my view, does lvm2 could drop VGs devices fds until pvmove finish?
Hi
Please provide more info about lvm2 metadata and also some 'lvs -av'
trace so we can get better picture about the layout - also version of
lvm2,systemd,kernel in use.
pvmove is progressing by mirroring each segment of an LV - so if there would
be a lot of segments - then each such update may trigger udev watch rule
event.
But ATM I could hardly imagine how this could cause some 'dramatic'
performance decrease - maybe there is something wrong with udev rules on
the system ?
What is the actual impact ?
Note - pvmove was never designed as a high performance operation (in fact it
tries to not eat all the disk bandwidth as such)
Regards
Zdenek
My mistake, I write here again:
The subject is wrong, not IO performance but cpu high load is triggered by
pvmove.
There is no IO performance issue.
When system is connecting 80~200, the cpu load increase by 15~20, the
cpu usage by ~20%, which corresponds to about ~5,6 cores and led at
times to the cores fully utilized.
In another word: a single pvmove process cost 5-6 (sometime 10) cores
utilization. It's abnormal & unaccepted.
The lvm2 is 2.03.05, kernel is 5.3. systemd is v246.
BTW:
I change this mail subject from: lvmpolld causes IO performance issue
to: lvmpolld causes high cpu load issue
Please use this mail for later discussing.
Hi
Could you please retest with recent version of lvm2. There have been certainly
some improvements in scanning - which might have caused in the older releases
some higher CPU usage with longer set of devices.
Regards
Zdenek
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