Device names on Amazon EC2 instances

The ending portion of device names that you use shouldn't overlap as it can cause issues when you start your instance. For example, avoid using combinations such as /dev/xvdf and xvdf for volumes attached to the same instance.

Although you can attach your EBS volumes using the device names used to attach instance store volumes, we strongly recommend that you don't because the behavior can be unpredictable.

The number of NVMe instance store volumes for an instance depends on the size of the instance. NVMe instance store volumes are automatically enumerated and assigned an NVMe device name (Linux instances) or a Windows drive letter (Windows instances).

(Windows instances) Amazon Windows AMIs come with additional software that prepares an instance when it first boots up. This is either the EC2Config service (Windows AMIs prior to Windows Server 2016) or EC2Launch (Windows Server 2016 and later). After the devices have been mapped to drives, they are initialized and mounted. The root drive is initialized and mounted as C:\. By default, when an EBS volume is attached to a Windows instance, it can show up as any drive letter on the instance. You can change the settings to set the drive letters of the volumes per your specifications. For instance store volumes, the default depends on the driver. Amazon PV drivers and Citrix PV drivers assign instance store volumes drive letters going from Z: to A:. Red Hat drivers assign instance store volumes drive letters going from D: to Z:. For more information, see Configure launch settings for Amazon EC2 Windows instances, and Map disks to volumes on your Windows instance.

(Linux instances) Depending on the block device driver of the kernel, the device could be attached with a different name than you specified. For example, if you specify a device name of /dev/sdh, your device could be renamed /dev/xvdh or /dev/hdh. In most cases, the trailing letter remains the same. In some versions of Red Hat Enterprise Linux (and its variants, such as CentOS), the trailing letter could change (/dev/sda could become /dev/xvde). In these cases, the trailing letter of each device name is incremented the same number of times. For example, if /dev/sdb is renamed /dev/xvdf, then /dev/sdc is renamed /dev/xvdg. Amazon Linux creates a symbolic link for the name you specified to the renamed device. Other operating systems could behave differently.

(Linux instances) HVM AMIs do not support the use of trailing numbers on device names, except for /dev/sda1, which is reserved for the root device, and /dev/sda2. While using /dev/sda2 is possible, we do not recommend using this device mapping with HVM instances.

(Linux instances) When using PV AMIs, you cannot attach volumes that share the same device letters both with and without trailing digits. For example, if you attach a volume as /dev/sdc and another volume as /dev/sdc1, only /dev/sdc is visible to the instance. To use trailing digits in device names, you must use trailing digits on all device names that share the same base letters (such as /dev/sdc1, /dev/sdc2, /dev/sdc3).

(Linux instances) Some custom kernels might have restrictions that limit use to /dev/sd[f-p] or /dev/sd[f-p][1-6]. If you're having trouble using /dev/sd[q-z] or /dev/sd[q-z][1-6], try switching to /dev/sd[f-p] or /dev/sd[f-p][1-6].