Re: [EXT] mclock scheduler kills clients IOs

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Hi Denis,

We have had the same issue with MClock, and now switch back to WPQ when draining nodes.

I couldn’t identify the cause of the slow-ops.  Even with custom mclock tuning and
1 backfill per osd, it was still causing client io issues.

From: Denis Polom <denispolom@xxxxxxxxx>
Date: Tuesday, 17 September 2024 at 8:16 pm
To: ceph-users@xxxxxxx <ceph-users@xxxxxxx>
Subject: [EXT]  mclock scheduler kills clients IOs
External email: Please exercise caution

Hi guys,

we have Ceph cluster with Quincy 17.2.7 where we are draining 3 hosts
(each one is from one failure domain). Each host has 13 HDDs and there
are another 38 hosts with same size in each failure domain. There is a
lot of free space.


I've set up

primary-affinity on drained OSDs to 0

and set OSDs out

mclock profile is:

  osd_mclock_profile = high_client_ops

but soon after that I got slow ops on some of the OSDs and clients
getting high iowaits until I stop recovering.

It's strange to me that client is still trying to use OSDs where
primary-affinity is set to 0

Here is the case of osd.15 with primary-affinity 0:

REQUESTS 4 homeless 0
8337634 osd15   16.35734e3c     16.e3cs0
[1100,1444,1509,429,540,1106]/1100 [15,1444,403,429,540,1106]/15
e1562924 100087a5278.00000004    0x400024        1       write
8358040 osd15   16.ace50e3c     16.e3cs0
[1100,1444,1509,429,540,1106]/1100 [15,1444,403,429,540,1106]/15
e1562924 100087a5a4e.00000007    0x400024        1       write
8438861 osd15   16.a7b80e3c     16.e3cs0
[1100,1444,1509,429,540,1106]/1100 [15,1444,403,429,540,1106]/15
e1562924 100087a7c6a.0000001f    0x400024        1       write
8443242 osd15   16.de504e3c     16.e3cs0
[1100,1444,1509,429,540,1106]/1100 [15,1444,403,429,540,1106]/15
e1562924 100087a7d76.00000014    0x400024        1       write
LINGER REQUESTS
BACKOFFS

Recovery speed is also not high (about 4GiB) and network is not saturated.

I've tried also following custom profile with other custom settings:

osd advanced osd_mclock_override_recovery_settings true

osd             advanced osd_async_recovery_min_cost 100
osd             advanced osd_max_backfills 1
osd             advanced osd_mclock_profile custom
osd             advanced osd_mclock_scheduler_background_recovery_res
0.100000
osd             advanced osd_mclock_scheduler_client_res 0.900000
osd             advanced osd_mclock_scheduler_client_wgt 6
osd             advanced osd_recovery_max_active 1
osd             advanced osd_recovery_max_chunk 8388608
osd             advanced osd_recovery_op_priority                        1

but result is same. Immediately clients get high iowaits and can't do
any IOs.

This didn't happen on this cluster before when we were using wpq scheduler.

My question is - are there some other options I need to set?

With wpq I was able to set osd_recovery_sleep but this option looks like
not possble to change with mclock.

Any ideas, please?

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