ceph-mds中standby_replay高速热备状态的示例分析

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ceph的MDS是cephFS文件存储服务的元数据服务。

当创建cephfs后便会有ceph-mds服务进行管理。默认情况下ceph会分配一个mds服务管理cephfs,即使已经创建多个mds服务，如下：

[root@ceph-admin my-cluster]# ceph-deploy mds create ceph-node01 ceph-node02.......[root@ceph-admin ~]# ceph -s  cluster:    id:     06dc2b9b-0132-44d2-8a1c-c53d765dca5d    health: HEALTH_OK   services:    mon: 2 daemons, quorum ceph-admin,ceph-node01    mgr: ceph-admin(active)    mds: mytest-fs-1/1/1 up  {0=ceph-admin=up:active}, 2 up:standby    osd: 3 osds: 3 up, 3 in    rgw: 2 daemons active   data:    pools:   8 pools, 64 pgs    objects: 299  objects, 137 MiB    usage:   3.4 GiB used, 297 GiB / 300 GiB avail    pgs:     64 active+clean

此时mds中只有ceph-admin处于active状态，其余处于standby状态。

standby已近是一种灾备状态，但事实上切换速度比较缓慢，对于实际业务系统必定会造成影响。

测试情况如下：

[root@ceph-admin my-cluster]# killall ceph-mds[root@ceph-admin my-cluster]# ceph -s  cluster:    id:     06dc2b9b-0132-44d2-8a1c-c53d765dca5d    health: HEALTH_WARN            1 filesystem is degraded   services:    mon: 2 daemons, quorum ceph-admin,ceph-node01    mgr: ceph-admin(active)    mds: mytest-fs-1/1/1 up  {0=ceph-node02=up:rejoin}, 1 up:standby    osd: 3 osds: 3 up, 3 in    rgw: 2 daemons active   data:    pools:   8 pools, 64 pgs    objects: 299  objects, 137 MiB    usage:   3.4 GiB used, 297 GiB / 300 GiB avail    pgs:     64 active+clean [root@ceph-admin my-cluster]# ceph -s  cluster:    id:     06dc2b9b-0132-44d2-8a1c-c53d765dca5d    health: HEALTH_OK   services:    mon: 2 daemons, quorum ceph-admin,ceph-node01    mgr: ceph-admin(active)    mds: mytest-fs-1/1/1 up  {0=ceph-node02=up:active}, 1 up:standby    osd: 3 osds: 3 up, 3 in    rgw: 2 daemons active   data:    pools:   8 pools, 64 pgs    objects: 299  objects, 137 MiB    usage:   3.4 GiB used, 297 GiB / 300 GiB avail    pgs:     64 active+clean   io:    client:   20 KiB/s rd, 3 op/s rd, 0 op/s wr

系统在active的mds被kill之后，standby的mds在经过rejoin状态后才变成了active，大约经过3-5s。而在生产环境中由于元数据的数据量更庞大，往往会更漫长。

而要让mds更快切换，需要将我们的mds服务切换至standby_replay状态，官方对于此状态的说明如下：

The MDS is following the journal of another up:active MDS. Should the active MDS fail, having a standby MDS in replay mode is desirable as the MDS is replaying the live journal and will more quickly takeover. A downside to having standby replay MDSs is that they are not available to takeover for any other MDS that fails, only the MDS they follow.

事实上就是standby_replay会实时根据active的mds元数据日志进行同步更新，这样就能加快切换的速率，那么如何让mds运行在standby_replay状态？

[root@ceph-node01 ~]# ps aufx|grep mdsroot      700547  0.0  0.0 112704   976 pts/1    S+   13:45   0:00          \_ grep --color=auto mdsceph      690340  0.0  0.5 451944 22988 ?        Ssl  10:09   0:03 /usr/bin/ceph-mds -f --cluster ceph --id ceph-node01 --setuser ceph --setgroup ceph[root@ceph-node01 ~]# killall ceph-mds[root@ceph-node01 ~]# /usr/bin/ceph-mds -f --cluster ceph --id ceph-node01 --setuser ceph --setgroup ceph --hot-standby 0starting mds.ceph-node01 at -

我们手动关闭了ceph-mds服务并且添加了--hot-standby 参数重新启动了mds服务。

接下来看到,ceph-node02的mds已经处于standby-replay状态：

[root@ceph-admin my-cluster]# ceph -s  cluster:    id:     06dc2b9b-0132-44d2-8a1c-c53d765dca5d    health: HEALTH_OK   services:    mon: 2 daemons, quorum ceph-admin,ceph-node01    mgr: ceph-admin(active)    mds: mytest-fs-1/1/1 up  {0=ceph-admin=up:active}, 1 up:standby-replay, 1 up:standby    osd: 3 osds: 3 up, 3 in    rgw: 2 daemons active   data:    pools:   8 pools, 64 pgs    objects: 299  objects, 137 MiB    usage:   3.4 GiB used, 297 GiB / 300 GiB avail    pgs:     64 active+clean   io:    client:   851 B/s rd, 1 op/s rd, 0 op/s wr

当然或者可以ceph-mds的启动脚本来让服务启动时自动处于standby-replay状态

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