Amazon EC2 Instance Store

An Amazon EC2 instance store provides temporary block-level storage for use with an Amazon EC2 instance. The size of an instance store ranges from 150 GiB to up to 48 TiB, and varies by instance type. Larger instance types have larger instance stores. For more information, see Instance Stores Available on Instance Types.

An instance store consists of one or more instance store volumes. Instance store volumes must be mounted before they can be used. By default, instances launched from an Amazon EBS-backed instance have no mounted instance store volumes. Instances launched from an instance store-backed AMI have a mounted instance store volume for the virtual machine's root device volume, and can have other mounted instances store volumes, depending on the instance type. For more information about instance store-backed AMIs and Amazon EBS-backed AMIs, see Storage for the Root Device.

Instance store volumes are usable only from a single instance during its lifetime; they can't be detached and then attached to another instance. If you create an AMI from an instance, the data on its instance store volumes isn't preserved and isn't present on the instance store volumes for the instances that you launch from this AMI. While an instance store is dedicated to a particular instance, the disk subsystem is shared among instances on a host computer, as shown in the following figure.

The data in an instance store persists only during the lifetime of its associated instance. If an instance reboots (intentionally or unintentionally), data in the instance store persists. However, data on instance store volumes is lost under the following circumstances:

Therefore, do not rely on instance store volumes for valuable, long-term data. Instead, keep your data safe by using a replication strategy across multiple instances, storing data in Amazon S3, or using Amazon EBS volumes. For more information, see Amazon Elastic Block Store (Amazon EBS).

When you launch an instance, whether it's launched from an Amazon EBS-backed AMI or an instance store-backed AMI, you can attach instance store volumes to the instance using block device mapping. For more information, see Adding Instance Store Volumes to an AMI.

Amazon EC2 instances are divided into different instance types, which determine the size of the instance store available on the instance by default. When you launch an instance, you can specify an instance type or use the default instance type, which is an m1.small instance.

The instance type also determines the type of hardware for your instance store volumes. A high I/O instance (hi1.4xlarge) uses solid state drives (SSD) to deliver very high random I/O performance. This is a good option when you need storage with very low latency, but you don't need it to persist when the instance terminates, or you can take advantage of fault tolerant architectures. For more information see High I/O Instances.

The following table shows the instance types along with the size of the instance store available with each instance type.

Within an instance store, instance store volumes are exposed as block devices. The virtual devices for instance store volumes are ephemeral[0-3]. Instance types that support one instance store volume have ephemeral0. Instance types that support two instance store volumes have ephemeral0 and ephemeral1. Instance types that support four instance store volumes have ephemeral0, ephemeral1, ephemeral2, and ephemeral3. Each instance store volume is pre-formatted with the ext3 file system. However, you can reformat volumes with the file system of your choice after you launch your instance. A Windows instance uses a built-in tool, EC2Config Service, to reformat the instance store volumes available on an instance with the NTFS file system.

Every instance store-backed AMI and instance has a mapping of the instance store volumes attached to the instance. Each entry in the mapping consists of a device name and the volume that it's mapped to. The instance store volumes are available to the instance, but you can't access them until they are mounted. A Windows instance uses the EC2Config Service to mount the instance store volumes for an instance. On Linux, the instance type determines which instance store volumes are mounted for you and which are available for you to mount yourself. The block device driver for the instance assigns the actual volume name when mounting the volume, and the name assigned can be different than the name that Amazon EC2 recommends.

The following table lists the devices names reserved for instance store volumes by instance type, and the default state of the storage device (formatted, mounted, available) on an instance store-backed instance. For example, an m1.small instance has ephemeral0 (ext3, 15 GiB) and swap (Linux swap, 896 MB).

An instance can have multiple instance store volumes mapped to a device. However, the number and size of these volumes must not exceed the instance store available for the instance type. For more information, see Instance Stores Available on Instance Types.

Instance store volumes are ideal for temporary storage of information that changes frequently, such as buffers, caches, scratch data, and other temporary content, or for data that is replicated across a fleet of instances, such as a load-balanced pool of web servers.

Amazon EBS-backed AMIs don't include instance store by default. However, you might want instances launched from your Amazon EBS-backed AMIs to include instance store volumes. This section describes how to create an AMI that includes instance store volumes.

Because of the way that Amazon EC2 virtualizes disks, the first write to any location on a standard instance store volume performs more slowly than subsequent writes. (The performance of the solid state drives (SSD) used by high I/O instances is not affected this way.) For most applications, amortizing this cost over the lifetime of the instance is acceptable. However, if you require high disk performance, we recommend that you initialize your drives by writing once to every drive location before production use. If you require further improvements in latency or throughput, we recommend using Amazon EBS.

To initialize the stores, use the following Linux/UNIX dd commands, depending on which store you want to initialize (/dev/sdb, etc.).

To initialize the instance store volumes, use the following commands on the m1.large, m1.xlarge, c1.xlarge, m2.xlarge, m2.2xlarge, and m2.4xlarge instance types:

dd if=/dev/zero of=/dev/sdb bs=1M          
dd if=/dev/zero of=/dev/sdc bs=1M          
dd if=/dev/zero of=/dev/sdd bs=1M          
dd if=/dev/zero of=/dev/sde bs=1M 

For information about the instance storage that is available for each instance type, see Instance Stores Available on Instance Types.

To perform initialization on all instance store volumes at the same time, use the following command:

dd if=/dev/zero bs=1M|tee /dev/sdb|tee /dev/sdc|tee /dev/sde > /dev/sdd

Configuring drives for RAID initializes them by writing to every drive location. When configuring software-based RAID, make sure to change the minimum reconstruction speed:

echo $((30*1024)) > /proc/sys/dev/raid/speed_limit_min