If your Elastic Compute Service (ECS) instance repeatedly goes down on startup and the "VFS: Unable to mount root fs on unknown-block" message appears in a log, you can use the solution described in this topic to fix the issue.

Problem description

An ECS instance repeatedly goes down on startup and cannot be used normally, and a call stack similar to the following one is displayed:
[    1.573197] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
[    1.574179] CPU: 4 PID: 1 Comm: swapper/0 Not tainted 3.10.0-1160.6.1.el7.x86_64 #1
[    1.575045] Hardware name: Alibaba Cloud Alibaba Cloud ECS, BIOS 8f19b21 04/01/2014
[    1.575900] Call Trace:
[    1.576246]  [<ffffffff8f381400>] dump_stack+0x19/0x1b
[    1.576845]  [<ffffffff8f37a958>] panic+0xe8/0x21f
[    1.577433]  [<ffffffff8f98b794>] mount_block_root+0x291/0x2a0
[    1.578122]  [<ffffffff8f98b7f6>] mount_root+0x53/0x56
[    1.578719]  [<ffffffff8f98b935>] prepare_namespace+0x13c/0x174
[    1.579425]  [<ffffffff8f98b412>] kernel_init_freeable+0x222/0x249
[    1.580150]  [<ffffffff8f98ab28>] ? initcall_blacklist+0xb0/0xb0
[    1.580838]  [<ffffffff8f36fa90>] ? rest_init+0x80/0x80
[    1.581462]  [<ffffffff8f36fa9e>] kernel_init+0xe/0x100
[    1.582073]  [<ffffffff8f394df7>] ret_from_fork_nospec_begin+0x21/0x21
[    1.582814]  [<ffffffff8f36fa90>] ? rest_init+0x80/0x80

Cause

The instance cannot find the file system of the root partition. This issue may be because the kernel update is unexpectedly stopped or fails and thus rootfs is destroyed.

Solution

Notice Before you perform the operations, we recommend that you create snapshots for the ECS instances to back up data to prevent data loss due to accidental operations. For more information about snapshots, see Snapshot overview.

We recommend that you change the system disk of the instance or roll back disks by using snapshots that you have created for the disks. For more information, see Replace the operating system of an instance by using a public image or Roll back a disk by using a snapshot.

If you have requests or feedback, you can submit a ticket to contact Alibaba Cloud.