vmstorage nodes automatically switch to readonly mode when the directory pointed by -storageDataPath contains less than -storage.minFreeDiskSpaceBytes of free space. vminsert nodes stop sending data to such nodes and start re-routing the data to the remaining vmstorage nodes.
vmstorage sets vm_storage_is_read_only metric at http://vmstorage:8482/metrics to 1 when it enters read-only mode. The metric is set to 0 when the vmstorage isn't in read-only mode.
vmstorage 模块，有两个命令行参数需要关注一下：
-storageDataPath指定存储数据的目录-storage.minFreeDiskSpaceBytes指定目录最小剩余空间
当存储数据的目录，剩余空间小于 -storage.minFreeDiskSpaceBytes 指定的值，这个 vmstorage 节点就会被设置为只读模式，不再接收新数据，vminsert 发现 vmstorage 是只读模式了，就不往这个 node 发数据了，嗯，这个特性挺好。
