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【博客606】k8s如何查看pod崩溃前的日志及其原理

k8s如何查看pod崩溃前的日志及其原理

场景

当pod处于crash状态的时候,容器不断重启,此时用kubelet logs可能出现一直捕捉不到日志

解决方法:

kubelet previous参数作用:

If true, print the logs for the previous instance of the container in a pod if it exists. 

单容器pod:

kubectl logs pod-name --previous

多容器pod:

kubectl logs pod-name --previous -c container-name

示例

NAME                              READY       STATUS             RESTARTS   AGE
nginx-7d8b49557c-c2lx9            2/2        Running            5   

kubectl logs nginx-7d8b49557c-c2lx9 --previous
Error: xxxxxxxxxxx

原理

kubelet会保持pod的前几个失败的容器,这个是查看的前提条件

核心原理:

kubelet实现previous的原理:将pod的日志存放在/var/log/pods/podname,并且是链接文件,链接到docker的容器的日志文件,同时kubelet还会保留上一个容器,同时有一个链接文件链接到pod上一个崩溃的容器的日志文件,使用previous就是查看的这个文件

实践:

查看一个pod:

ubuntu@~$ kubelet get pod
NAME                     READY   STATUS    RESTARTS   AGE
busybox                  1/1     Running   2394       99d
nginx-deployment-6wlhd   1/1     Running   0          79d
redis                    1/1     Running   0          49d

到pod所在node查看kubelet放的两个日志文件:

 ls /var/log/pods/default_busybox_f72ab71a-5b3b-4ecf-940d-28a5c3b30683/busybox
2393.log  2394.log

数字的含义:

2393证明是第2393次重启后的日志,2394代表是第2394次重启后的日志

实际这两个日志文件是链接文件,指向了docker的日志文件:

/busybox# stat 2393.log
  File: 2393.log -> /data/kubernetes/docker/containers/68a5b32c9fdb1ad011b32e6252f9cdb759f69d7850e6b7b8591cb4c2bf00bcca/68a5b32c9fdb1ad011b32e6252f9cdb759f69d7850e6b7b8591cb4c2bf00bcca-json.log
  Size: 173           Blocks: 8          IO Block: 4096   symbolic link
Device: fc02h/64514d    Inode: 529958      Links: 1
Access: (0777/lrwxrwxrwx)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2023-01-31 13:32:03.751514283 +0800
Modify: 2023-01-31 13:32:03.039526838 +0800
Change: 2023-01-31 13:32:03.039526838 +0800
 Birth: -
 
 /busybox# stat 2394.log
  File: 2394.log -> /data/kubernetes/docker/containers/2ed9ebf0585215602874b076783e12191dbb010116038b8eb4646273ebfe195c/2ed9ebf0585215602874b076783e12191dbb010116038b8eb4646273ebfe195c-json.log
  Size: 173           Blocks: 8          IO Block: 4096   symbolic link
Device: fc02h/64514d    Inode: 529955      Links: 1
Access: (0777/lrwxrwxrwx)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2023-01-31 14:32:03.991106950 +0800
Modify: 2023-01-31 14:32:03.183119308 +0800
Change: 2023-01-31 14:32:03.183119308 +0800
 Birth: -

看到分别指向了这两个容器的日志文件,一个是当前pod里在跑的容器,一个是pod上次跑的容器,现在已经退出了

docker ps -a
CONTAINER ID        IMAGE                  COMMAND                  CREATED             STATUS                      PORTS               NAMES
2ed9ebf05852        ff4a8eb070e1           "sleep 3600"             24 minutes ago      Up 24 minutes                                   k8s_busybox_busybox_default_f72ab71a-5b3b-4ecf-940d-28a5c3b30683_2394
68a5b32c9fdb        ff4a8eb070e1           "sleep 3600"             About an hour ago   Exited (0) 24 minutes ago                       k8s_busybox_busybox_default_f72ab71a-5b3b-4ecf-940d-28a5c3b30683_2393

原理实际:使用logs的时候读的是当前容器那个文件,使用–previous的时候,读的是上次退出的容器的日志文件,由于kubelet为pod保留了上次退出的容器

验证:我们手动编辑这两个文件的内容,看kubelet是否读的是这两个文件

/busybox# cat 2393.log
{"log":"last crash logs\n","stream":"stderr","time":"2022-11-05T08:11:27.31523845Z"}

/busybox# cat 2394.log
{"log":"now pod log\n","stream":"stderr","time":"2022-11-05T08:11:27.31523845Z"}

ubuntu@10-234-32-51:~$ k logs busybox --previous
last crash logs
ubuntu@10-234-32-51:~$ k logs busybox
now pod log

由于是链接文件,那么可能实际是从别的地方读的,或者说直接读容器目录下的,由于链接文件我们改了后容器目录下的日志文件也跟着改了,我们直接创建两个文件来做验证:

ubuntu@10-234-32-51:~$ k get pod
NAME                     READY   STATUS    RESTARTS   AGE
busybox                  1/1     Running   2395       99d
nginx-deployment-6wlhd   1/1     Running   0          79d
redis                    1/1     Running   0          49d

/busybox# ls
2394.log  2395.log

/busybox# rm 2394.log  2395.log

我们删除,然后自己创建,这时是regular file,而不是链接文件了:
/busybox# ls
2394.log  2395.log

/busybox# stat 2394.log
  File: 2394.log
  Size: 100           Blocks: 8          IO Block: 4096   regular file
Device: fc02h/64514d    Inode: 529965      Links: 1
Access: (0640/-rw-r-----)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2023-01-31 15:42:11.307170422 +0800
Modify: 2023-01-31 15:42:07.711225229 +0800
Change: 2023-01-31 15:42:07.711225229 +0800
 Birth: -

/busybox# stat 2395.log
  File: 2395.log
  Size: 86            Blocks: 8          IO Block: 4096   regular file
Device: fc02h/64514d    Inode: 529967      Links: 1
Access: (0640/-rw-r-----)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2023-01-31 15:41:17.539989934 +0800
Modify: 2023-01-31 15:41:14.348038586 +0800
Change: 2023-01-31 15:41:14.352038525 +0800
 Birth: -
 
/busybox# cat 2394.log
{"log":"previous logs create by myself\n","stream":"stderr","time":"2022-11-05T08:11:27.31523845Z"}
/busybox# cat 2395.log
{"log":"create by myself\n","stream":"stderr","time":"2022-11-05T08:11:27.31523845Z"}

ubuntu@10-234-32-51:~$ k logs busybox
create by myself
ubuntu@10-234-32-51:~$ k logs busybox --previous
previous logs create by myself

得出结论:

kubelet读的是/var/log/pods/下的日志文件,–previous读的也是/var/log/pods/下的日志文件,且专门有个链接文件来指向上一个退出容器的日志文件,以此来获取容器崩溃前的日志


本文转载自: https://blog.csdn.net/qq_43684922/article/details/128881716
版权归原作者 lulu的云原生笔记 所有, 如有侵权,请联系我们删除。

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