[Gluster-users] Gluster High CPU/Clients Hanging on Heavy Writes

[Yuhao Zhang]

Hi Xavi,

Thank you for the suggestions, these are extremely helpful. I haven't thought it could be ZFS problem. I went back and checked a longer monitoring window and now I can see a pattern. Please see this attached Grafana screenshot (also available here: https://cl.ly/070J2y3n1u0F <https://cl.ly/070J2y3n1u0F> . Note that the data gaps were when I took down the server for rebooting):



Between 8/4 - 8/6, I tried two transfer tests, and experienced 2 the gluster hanging problems. One during the first transfer, and another one happened shortly after the second transfer. I blocked both in pink lines. 

Looks like during my transfer tests, free memory was almost exhausted. The system has a very high cached memory, which I think was due to ZFS ARC. However, I am under the impression that ZFS will release space from ARC if it observes low system available memory. I am not sure why it didn't do that. 

I did't tweak related ZFS parameters. zfs_arc_max was set to 0 (default value). According to doc, it is "Max arc size of ARC in bytes. If set to 0 then it will consume 1/2  of  system RAM." So it appeared that this setting didn't work.

When the server was under heavy IO, the used memory was instead decreased, which I can't explain.

May I ask if you, or anyone else in this group, has recommendation on ZFS settings for my setup? My server has 64GB physical memory and 150GB SSD space reserved for L2_ARC.The zpool has 6 vdevs and each has 12TB * 10 hard drives on raidz2. Total usable space in the zpool is 482TB.

Thank you,
Yuhao

> On Aug 7, 2018, at 01:36, Xavi Hernandez <jahernan at redhat.com> wrote:
> 
> Hi Yuhao, 
> 
> On Mon, 6 Aug 2018, 15:26 Yuhao Zhang, <zzyzxd at gmail.com <mailto:zzyzxd at gmail.com>> wrote:
> Hello,
> 
> I just experienced another hanging one hour ago and the server was not even under heavy IO.
> 
> Atin, I attached the process monitoring results and another statedump.
> 
> Xavi, ZFS was fine, during the hanging, I can still write directly to the ZFS volume. My ZFS version: ZFS: Loaded module v0.6.5.6-0ubuntu16, ZFS pool version 5000, ZFS filesystem version 5
> 
> I highly recommend you to upgrade to version 0.6.5.8 at least. It fixes a kernel panic that can happen when used with gluster. However this is not your current problem.
> 
> Top statistics show low available memory and high CPU utilization of kswapd process (along with one of the gluster processes). I've seen frequent memory management problems with ZFS. Have you configured any ZFS parameters? It's highly recommendable to tweak some memory limits.
> 
> If that were the problem, there's one thing that should alleviate it (and see if it could be related):
> 
> echo 3 >/proc/sys/vm/drop_caches
> 
> This should be done on all bricks from time to time. You can wait until the problem appears, but in this case the recovery time can be larger. 
> 
> I think this should fix the high CPU usage of kswapd. If so, we'll need to tweak some ZFS parameters.
> 
> I'm not sure if the high CPU usage of gluster could be related to this or not.
> 
> Xavi
> 
> Thank you,
> Yuhao

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