我在 AWS 上运行了一个邮件服务器 (Ubuntu 16.04)。有时，重新启动时，包含所有邮件的 EBS 卷并不总是挂载。这刚刚发生，我运行df -H，发现该卷未挂载：
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
安装后，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
我所能找到的只有dmesg这些：
[ 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)
和系统日志：
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？