我有一个在AWS上运行的邮件服务器（Ubuntu 16.04）。 有时，在重新启动时，包含所有邮件的EBS卷并不总是安装。 这刚刚发生，我跑了df -H ，注意到卷没有安装：
Filesystem Size Used Avail Use% Mounted on udev 246M 0 246M 0% /dev tmpfs 52M 2.0M 50M 4% /run /dev/xvda1 8.4G 2.6G 5.4G 33% / tmpfs 256M 4.1k 256M 1% /dev/shm tmpfs 5.3M 0 5.3M 0% /run/lock tmpfs 256M 0 256M 0% /sys/fs/cgroup tmpfs 52M 0 52M 0% /run/user/1002
挂载之后，现在显示（ /var/vmail ）：
Filesystem Size Used Avail Use% Mounted on udev 246M 0 246M 0% /dev tmpfs 52M 2.0M 50M 4% /run /dev/xvda1 8.4G 2.6G 5.4G 33% / tmpfs 256M 4.1k 256M 1% /dev/shm tmpfs 5.3M 0 5.3M 0% /run/lock tmpfs 256M 0 256M 0% /sys/fs/cgroup /dev/xvdf 8.4G 5.5G 2.5G 70% /var/vmail tmpfs 52M 0 52M 0% /run/user/1002
这是我的/etc/fstab ：
LABEL=cloudimg-rootfs / ext4 defaults,discard 0 0 /dev/xvdf /var/vmail ext4 defaults,nofail 0 2
我能从dmesgfind的是：
[ 1.704193] blkfront: xvdf: barrier or flush: disabled; persistent grants: disabled; indirect descriptors: enabled; [ 6.636754] EXT4-fs (xvdf): mounted filesystem with ordered data mode. Opts: (null)
和syslog：
Feb 23 20:57:37 mail2 systemd[1]: Found device /dev/xvdf. Feb 23 20:57:37 mail2 systemd[1]: Starting File System Check on /dev/xvdf... Feb 23 20:57:37 mail2 systemd-fsck[687]: /dev/xvdf: clean, 49282/524288 files, 1401891/2097152 blocks Feb 23 20:57:37 mail2 systemd[1]: Started File System Check on /dev/xvdf. Feb 23 20:57:37 mail2 kernel: [ 1.367621] blkfront: xvdf: barrier or flush: disabled; persistent grants: disabled; indirect descriptors: enabled; Feb 23 20:57:37 mail2 kernel: [ 6.412505] EXT4-fs (xvdf): mounted filesystem with ordered data mode. Opts: (null) Feb 23 21:00:36 mail2 kernel: [ 1.388299] blkfront: xvdf: barrier or flush: disabled; persistent grants: disabled; indirect descriptors: enabled;
为什么会发生这种情况，我该如何解决？