Re: ceph cluster having blocke requests very frequently

Nick Fisk <nick@xxxxxxxxxx> · Thu, 17 Nov 2016 10:01:53 -0000

Hi Thomas,

Do you have the OSD logs from around the time of that slow request (13:12 to 13:29 period)?

Do you also see anything about OSD’s going down in the Mon ceph.log file around that time?

480 seconds is probably far too long for a disk to be busy for, I’m wondering if the OSD is either dying and respawning or if you are running out of some type of system resource….eg TCP connections or something like that, which means the OSD’s can’t communicate with each other.

From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf Of Thomas Danan
Sent: 17 November 2016 08:59
To: nick@xxxxxxxxxx; 'Peter Maloney' <peter.maloney@xxxxxxxxxxxxxxxxxxxx>
Cc: ceph-users@xxxxxxxxxxxxxx
Subject: Re:  ceph cluster having blocke requests very frequently

Hi,

I have recheck the pattern when slow request are detected.

I have an example with following (primary: 411, secondary: 176, 594)
On primary slow requests detected: waiting for subops (176, 594)  during 16 minutes !!!!

2016-11-17 13:29:27.209754 7f001d414700 0 log_channel(cluster) log [WRN] : 7 slow requests, 7 included below; oldest blocked for > 480.477315 secs
2016-11-17 13:29:27.209777 7f001d414700 0 log_channel(cluster) log [WRN] : slow request 480.477315 seconds old, received at 2016-11-17 13:21:26.732303: osd_op(client.2407558.1:206455044 rbd_data.66ea12ae8944a.00000000001acbbc [set-alloc-hint object_size 4194304 write_size 4194304,write 1257472~368640] 0.61fe279f snapc 3fd=[3fd,3de] ondisk+write e210553) currently waiting for subops from 176,594

So the primary OSD is waiting for subops since 13:21 (13:29 - 480 seconds)

2016-11-17 13:36:33.039691 7efffd8ee700 0 -- 192.168.228.23:6800/694486 >> 192.168.228.7:6819/3611836 pipe(0x13ffd000 sd=33 :17791 s=2 pgs=131 cs=7 l=0 c=0x13251de0).fault, initiating reconnect
2016-11-17 13:36:39.570692 7efff6784700 0 -- 192.168.228.23:6800/694486 >> 192.168.228.13:6858/2033854 pipe(0x17009000 sd=60 :52188 s=2 pgs=147 cs=7 l=0 c=0x141159c0).fault, initiating reconnect

After this log, it seems the ops are unblocked and I do not have any more the “currently waiting for subops from 176,594”

So primary OSD was blocked during 15 minutes in total !!!!

On the secondary OSD, I can see the following messages during the same period (but also after and before)

secondary:
2016-11-17 13:34:58.125076 7fbcc7517700 0 -- 192.168.228.7:6819/3611836 >> 192.168.228.42:6832/2873873 pipe(0x12d2e000 sd=127 :6819 s=2 pgs=86 cs=5 l=0 c=0xef18c00).fault with nothing to send, going to sandby

In some other example and with some DEBUG messages activated  I was also able to see the many of the following messages on secondary OSDs.
2016-11-15 03:53:04.298502 7ff9c434f700  1 heartbeat_map is_healthy 'OSD::osd_op_tp thread 0x7ff9bdb42700' had timed out after 15

Thomas

From: Nick Fisk [mailto:nick@xxxxxxxxxx] 
Sent: mercredi 16 novembre 2016 22:13
To: Thomas Danan; nick@xxxxxxxxxx; 'Peter Maloney'
Cc: ceph-users@xxxxxxxxxxxxxx
Subject: RE:  ceph cluster having blocke requests very frequently

Hi,

Yes, I can’t think of anything else at this stage. Could you maybe repost some dump historic op dumps  now that you have turned off snapshots. I wonder if they might reveal anything.

From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf Of Thomas Danan
Sent: 16 November 2016 17:38
To: nick@xxxxxxxxxx; 'Peter Maloney' <peter.maloney@xxxxxxxxxxxxxxxxxxxx>
Cc: ceph-users@xxxxxxxxxxxxxx
Subject: Re:  ceph cluster having blocke requests very frequently

Hi Nick,

We have deleted all Snapshots and observed the system for several hours.
From what I see this did not help to reduce the blocked ops and IO freeze on Client ceph side.

We have also tried to increase a little bit the PGs (by 8 than 128) because this is something we should do and we wanted to see how the cluster was behaving.
During recovery, the number of blocked ops and associated duration increased significantly. Also the number of impacted OSDs was much more important. 

Don’t really know what to conclude from all of this …

Again we have checked Disk / network / and everything seems fine …

Thomas
From: Nick Fisk [mailto:nick@xxxxxxxxxx] 
Sent: mercredi 16 novembre 2016 14:01
To: Thomas Danan; 'Peter Maloney'
Cc: ceph-users@xxxxxxxxxxxxxx
Subject: RE: [ceph-users] ceph cluster having blocke requests very frequently

The snapshot works by using Copy On Write. If you dirty even a 4kb section of a 4MB object in the primary RBD, that entire 4MB object then needs to be read and then written into the snapshot RBD.

From: Thomas Danan [mailto:Thomas.Danan@xxxxxxxxxxxxx] 
Sent: 16 November 2016 12:58
To: Thomas Danan <Thomas.Danan@xxxxxxxxxxxxx>; nick@xxxxxxxxxx; 'Peter Maloney' <peter.maloney@xxxxxxxxxxxxxxxxxxxx>
Cc: ceph-users@xxxxxxxxxxxxxx
Subject: RE:  ceph cluster having blocke requests very frequently

Hi Nick,

Actually I was wondering, is there any difference between Snapshot or simple RBD image ?
With simple RBD image when doing a random IO, we are asking Ceph cluster to update one or several 4MB objects no ?
So Snapshotting is multiplying the load by 2 but not more, Am I wrong ?

Thomas

From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf Of Thomas Danan
Sent: mercredi 16 novembre 2016 13:52
To: nick@xxxxxxxxxx; 'Peter Maloney'
Cc: ceph-users@xxxxxxxxxxxxxx
Subject: Re:  ceph cluster having blocke requests very frequently

Hi Nick,

Yes our application is doing small Random IO and I did not realize that the snapshotting feature could so much degrade performances in that case.

We have just deactivated it and deleted all snapshots. Will notify you if it drastically reduce the blocked ops and consequently the IO freeze on client side.

Thanks

Thomas

From: Nick Fisk [mailto:nick@xxxxxxxxxx] 
Sent: mercredi 16 novembre 2016 13:25
To: Thomas Danan; 'Peter Maloney'
Cc: ceph-users@xxxxxxxxxxxxxx
Subject: RE:  ceph cluster having blocke requests very frequently

From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf Of Thomas Danan
Sent: 15 November 2016 21:14
To: Peter Maloney <peter.maloney@xxxxxxxxxxxxxxxxxxxx>
Cc: ceph-users@xxxxxxxxxxxxxx
Subject: Re: [ceph-users] ceph cluster having blocke requests very frequently

Very interesting ...

Any idea why optimal tunable would help here ?  on our cluster we have 500TB of data, I am a bit concerned about changing it without taking lot of precautions . ...
I am curious to know how much time it takes you to change tunable, size of your cluster and observed impacts on client IO ...

Yes We do have daily rbd snapshot from 16 different ceph RBD clients. Snapshoting the RBD image is quite immediate while we are seing the issue continuously during the day...

Just to point out that when you take a snapshot any writes to the original RBD will mean that the full 4MB object is copied into the snapshot. If you have a lot of small random IO going on the original RBD this can lead to massive write amplification across the cluster and may cause issues such as what you describe.

Also be aware that deleting large snapshots also puts significant strain on the OSD’s as they try and delete hundreds of thousands of objects.

Will check all of this tomorrow . ..

Thanks again

Thomas

Sent from my Samsung device

-------- Original message --------
From: Peter Maloney <peter.maloney@xxxxxxxxxxxxxxxxxxxx> 
Date: 11/15/16 21:27 (GMT+01:00) 
To: Thomas Danan <Thomas.Danan@xxxxxxxxxxxxx> 
Cc: ceph-users@xxxxxxxxxxxxxx 
Subject: Re:  ceph cluster having blocke requests very frequently 
On 11/15/16 14:05, Thomas Danan wrote:
> Hi Peter,
>
> Ceph cluster version is 0.94.5 and we are running with Firefly tunables and also we have 10KPGs instead of the 30K / 40K we should have.
> The linux kernel version is 3.10.0-327.36.1.el7.x86_64 with RHEL 7.2
>
> On our side we havethe following settings:
> mon_osd_adjust_heartbeat_grace = false
> mon_osd_adjust_down_out_interval = false
> mon_osd_min_down_reporters = 5
> mon_osd_min_down_reports = 10
>
> explaining why the OSDs are not flapping but still they are behaving wrongly and generate the slow requests I am describing.
>
> The osd_op_complaint_time is with the default value (30 sec), not sure I want to change it base on your experience
I wasn't saying you should set the complaint time to 5, just saying
that's why I have complaints logged with such low block times.
> Thomas

And now I'm testing this:
        osd recovery sleep = 0.5
        osd snap trim sleep = 0.5

(or fiddling with it as low as 0.1 to make it rebalance faster)

While also changing tunables to optimal (which will rebalance 75% of the
objects)
Which has very good results so far (a few <14s blocks right at the
start, and none since, over an hour ago).

And I'm somehow hoping that will fix my rbd export-diff issue too... but
it at least appears to fix the rebalance causing blocks.

Do you use rbd snapshots? I think that may be causing my issues, based
on things like:

>             "description": "osd_op(client.692201.0:20455419 4.1b5a5bc1
> rbd_data.94a08238e1f29.000000000000617b [] snapc 918d=[918d]
> ack+ondisk+write+known_if_redirected e40036)",
>             "initiated_at": "2016-11-15 20:57:48.313432",
>             "age": 409.634862,
>             "duration": 3.377347,
>             ...
>                     {
>                         "time": "2016-11-15 20:57:48.313767",
>                         "event": "waiting for subops from 0,1,8,22"
>                     },
>             ...
>                     {
>                         "time": "2016-11-15 20:57:51.688530",
>                         "event": "sub_op_applied_rec from 22"
>                     },

Which says "snapc" in there (CoW?), and I think shows that just one osd
is delayed a few seconds and the rest are really fast, like you said.
(and not sure why I see 4 osds here when I have size 3... node1 osd 0
and 1, and node3 osd 8 and 22)

or some (shorter I think) have description like:
> osd_repop(client.426591.0:203051290 4.1f9
> 4:9fe4c001:::rbd_data.4cf92238e1f29.00000000000014ef:head v 40047'2531604)

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