If you’re not aware of Amazon Elastic Compute Cloud Service, I’d suggest you to read What is Amazon EC2? before going through the FAQ.

  • Starting July 2018, all newly created EC2 resources will receive longer format IDs. The new format will be -<17 characters>, e.g. “vpc-1234567890abcdef0” for VPCs or “subnet-1234567890abcdef0” for subnets.

  • Impact analysis: Some failure modes could include:

    • If your systems use regular expressions to validate the ID format, you might error if a longer format is encountered.
    • If there are expectations about the ID length in your database schemas, you might be unable to store a longer ID.

  • Any resource created with the old ID format will always retain its shorter ID. Any resource created with the new format will retain its longer ID, even if you opt back out.

  • Amazon Elastic Compute Cloud (Amazon EC2) is a web service that provides resizable compute capacity in the cloud. It is designed to make web-scale computing easier for developers.

  • Amazon EC2 reduces the time required to obtain and boot new server instances to minutes, allowing you to quickly scale capacity, both up and down, as your computing requirements change.

  • When you launch your Amazon EC2 instances you have the ability to store your root device data on Amazon EBS or the local instance store. By using Amazon EBS, data on the root device will persist independently from the lifetime of the instance. This enables you to stop and restart the instance at a subsequent time, which is similar to shutting down your laptop and restarting it when you need it again. Alternatively, the local instance store only persists during the life of the instance. This is an inexpensive way to launch instances where data is not stored to the root device. For example, some customers use this option to run large web sites where each instance is a clone to handle web traffic.

  • Once you create a custom AMI, you will need to bundle it. If you are bundling an image with a root device backed by Amazon EBS, you can simply use the bundle command in the AWS Management Console. If you are bundling an image with a boot partition on the instance store, then you will need to use the AMI Tools to upload it to Amazon S3. Amazon EC2 uses Amazon EBS and Amazon S3 to provide reliable, scalable storage of your AMIs so that we can boot them when you ask us to do so.

  • Amazon EC2 is used jointly with Amazon S3 for instances with root devices backed by local instance storage.

  • You are limited to running up to a total of 20 On-Demand instances across the instance family, purchasing 20 Reserved Instances, and requesting Spot Instances per your dynamic Spot limit per region.

  • In order to maintain the quality of Amazon EC2 addresses for sending email, we enforce default limits on the amount of email that can be sent from EC2 accounts.

  • In our experience, ECC memory is necessary for server infrastructure, and all the hardware underlying Amazon EC2 uses ECC memory.

  • Our SLA guarantees a Monthly Uptime Percentage of at least 99.99% for Amazon EC2 and Amazon EBS within a Region.

  • Accelerated Computing instance family is a family of instances which use hardware accelerators, or co-processors, to perform some functions, such as floating-point number calculation and graphics processing, more efficiently than is possible in software running on CPUs. Amazon EC2 provides three types of Accelerated Computing instances – GPU compute instances for general-purpose computing, GPU graphics instances for graphics intensive applications, and FPGA programmable hardware compute instances for advanced scientific workloads.

  • GPU instances work best for applications with massive parallelism such as workloads using thousands of threads. Graphics processing is an example with huge computational requirements, where each of the tasks is relatively small, the set of operations performed form a pipeline, and the throughput of this pipeline is more important than the latency of the individual operations. To be able build applications that exploit this level of parallelism, one needs GPU device specific knowledge by understanding how to program against various graphics APIs (DirectX, OpenGL) or GPU compute programming models (CUDA, OpenCL)

  • P3 instances are the next-generation of EC2 general-purpose GPU computing instances, powered by up to 8 of the latest-generation NVIDIA Tesla V100 GPUs. These new instances significantly improve performance and scalability, and add many new features, including new Streaming Multiprocessor (SM) architecture for machine learning (ML)/deep learning (DL) performance optimization, second-generation NVIDIA NVLink high-speed GPU interconnect, and highly tuned HBM2 memory for higher-efficiency.

  • G3 instances use NVIDIA Tesla M60 GPUs and provide a high-performance platform for graphics applications using DirectX or OpenGL.

  • P3 instances with their high computational performance will benefit users in artificial intelligence (AI), machine learning (ML), deep learning (DL) and high performance computing (HPC) applications.

  • P3 instances will support VPC only.

  • F1 is an AWS instance with programmable hardware for application acceleration. With F1, you have access to FPGA hardware in a few simple clicks, reducing the time and cost of full-cycle FPGA development and scale deployment from months or years to days.

  • The design that you create to program your FPGA is called an Amazon FPGA Image (AFI). AWS provides a service to register, manage, copy, query, and delete AFIs. After an AFI is created, it can be loaded on a running F1 instance. You can load multiple AFIs to the same F1 instance, and can switch between AFIs in runtime without reboot.

  • Can I add an FPGA to any EC2 instance type? Ans: No. F1 instances comes in two instance sizes f1.2xlarge and f1.16 xlarge.

  • Compute Optimized instances are designed for applications that benefit from high compute power. These applications include compute-intensive applications like high-performance web servers, high-performance computing (HPC), scientific modelling, distributed analytics and machine learning inference.

  • Each C4 instance type is EBS-optimized by default.

  • The c4.8xlarge instance type provides the ability for an operating system to control processor C-states and P-states. This feature is currently available only on Linux instances. You may want to change C-state or P-state settings to increase processor performance consistency, reduce latency, or tune your instance for a specific workload.

  • C4 and C5 are Compute Optimized instances

  • The generational improvement in CPU performance and lower price of C5 instances, which combined result in a 25% price/performance improvement relative to C4 instances, benefit a broad spectrum of workloads that currently run on C3 or C4 instances.

  • C5 instances use EBS volumes for storage, are EBS-optimized by default, and offer up to 9 Gbps throughput to both encrypted and unencrypted EBS volumes.

  • C5 instances use the Elastic Network Adapter (ENA) for networking and enable Enhanced Networking by default. With ENA, C5 instances can utilize up to 25 Gbps of network bandwidth.

  • M5 instances offer a good choice for running development and test environments, web, mobile and gaming applications, analytics applications, and business critical applications including ERP, HR, CRM, and collaboration apps. Customers who are interested in running their data intensive workloads (e.g. HPC, or SOLR clusters) on instances with a higher memory footprint will also find M5 to be a good fit.

  • Compared with EC2 M4 Instances, the new EC2 M5 Instances deliver customers greater compute and storage performance, larger instance sizes for less cost, consistency and security. The biggest benefit of EC2 M5 Instances is based on its usage of the latest generation of Intel Xeon Scalable processors (aka Skylake), which deliver up to 14% improvement in price/performance compared to M4.

  • M5 instances leverage EBS volumes for storage. There is currently no local storage option for M5 instances.

  • M5 instances support only ENA based Enhanced Networking. M5 instances will not support netback.

  • M5 instances use EBS volumes for storage, are EBS-optimized by default, and offer up to 10 Gbps throughput to both encrypted and unencrypted EBS volumes.

  • Amazon EC2 allows you to choose between Fixed Performance Instances (e.g. C, M and R instance families) and Burstable Performance Instances (e.g. T2). Burstable Performance Instances provide a baseline level of CPU performance with the ability to burst above the baseline.

  • T2 instances provide a cost-effective platform for a broad range of general purpose production workloads. T2 Unlimited instances can sustain high CPU performance for as long as required. If your workloads consistently require CPU usage much higher than the baseline, consider a dedicated CPU instance family such as the M or C.

  • You can see the CPU Credit balance for each T2 instance in EC2 per-Instance metrics in Amazon CloudWatch. T2 instances have four metrics, CPUCreditUsage, CPUCreditBalance, CPUSurplusCreditBalance and CPUSurplusCreditsCharged. CPUCreditUsage indicates the amount of CPU Credits used. CPUCreditBalance indicates the balance of CPU Credits. CPUSurplusCredit Balance indicates credits used for bursting in the absence of earned credits. CPUSurplusCreditsCharged indicates credits that are charged when average usage exceeds the baseline.

  • T2 instances can be purchased as On-Demand Instances, Reserved Instances or Spot Instances.

  • No, a stopped instance does not retain its previously earned credit balance.

  • Amazon EC2 High Memory instances offer 6 TB, 9 TB, or 12 TB of memory in a single instance. These instances are designed to run large in-memory databases,

  • High Memory instances support Amazon EBS volumes for storage. High Memory instances are EBS-optimized by default, and offer up to 14 Gbps of storage bandwidth to both encrypted and unencrypted EBS volumes.

  • Once a Dedicated Host is allocated within your account, it will be standing by for your use. You can then launch an instance with a tenancy of “host” using the RunInstances API, and can also stop/start/terminate the instance through the API.

  • X1 instances are ideal for running in-memory databases like SAP HANA, big data processing engines like Apache Spark or Presto, and high performance computing (HPC) applications.

  • Dense-storage instances are designed for workloads that require high sequential read and write access to very large data sets, such as Hadoop distributed computing, massively parallel processing data warehousing, and log processing applications. The Dense-storage instances offer the best price/GB-storage and price/disk-throughput across other EC2 instances.

  • High I/O instances (I2) are targeted at workloads that demand low latency and high random I/O in addition to moderate storage density and provide the best price/IOPS across other EC2 instance types. Dense-storage instances (D2) and HDD-storage instances (H1) are optimized for applications that require high sequential read/write access and low cost storage for very large data sets and provide the best price/GB-storage and price/disk-throughput across other EC2 instances.

  • The current generation of Dense-storage instances (D2 instances) can be launched in both EC2-Classic and Amazon VPC.

  • High I/O instances are ideal for applications that require access to millions of low latency IOPS, and can leverage data stores and architectures that manage data redundancy and availability.

  • The data stored on a local instance store will persist only as long as that instance is alive. However, data that is stored on an Amazon EBS volume will persist independently of the life of the instance.

  • Amazon EBS provides four current generation volume types and are divided into two major categories: SSD-backed storage for transactional workloads and HDD-backed storage for throughput intensive workloads.

  • ST1 volumes are backed by hard disk drives (HDDs) and are ideal for frequently accessed, throughput intensive workloads with large datasets and large I/O sizes, such as MapReduce, Kafka, log processing, data warehouse, and ETL workloads. SC1 volumes are backed by hard disk drives (HDDs) and provides the lowest cost per GB of all EBS volume types. It is ideal for less frequently accessed workloads with large, cold datasets. Similar to st1, sc1 provides a burst model.

  • SSD-backed volumes include Provisioned IOPS SSD (io1) and General Purpose SSD (gp2). HDD-backed volumes are designed for throughput-intensive and big-data workloads, large I/O sizes, and sequential I/O patterns.

  • While you are able to attach multiple volumes to a single instance, attaching multiple instances to one volume is not supported at this time.

  • EBS snapshots are only available through the Amazon EC2 APIs.

  • Snapshots can be done in real time while the volume is attached and in use. However, snapshots only capture data that has been written to your Amazon EBS volume, which might exclude any data that has been locally cached by your application or OS. In order to ensure consistent snapshots on volumes attached to an instance, we recommend cleanly detaching the volume, issuing the snapshot command, and then reattaching the volume. For Amazon EBS volumes that serve as root devices, we recommend shutting down the machine to take a clean snapshot.

  • If you share a snapshot, you won’t be charged when other users make a copy of your snapshot. If you make a copy of another user’s shared volume, you will be charged normal EBS rates.

  • To access your file system, you mount the file system on an Amazon EC2 Linux-based instance using the standard Linux mount command and the file system’s DNS name. Once you’ve mounted, you can work with the files and directories in your file system just like you would with a local file system. Amazon EFS uses the NFSv4.1 protocol.

  • Amazon EFS file systems can be mounted on an Amazon EC2 instance, so any data that is accessible to an Amazon EC2 instance can also be read and written to Amazon EFS. To load data that is not currently stored on the Amazon cloud, you can use the same methods you use to transfer files to Amazon EC2 today, such as Secure Copy (SCP).

  • Amazon EFS file systems can also be mounted on an on-premises server, so any data that is accessible to an on-premises server can be read and written to Amazon EFS using standard Linux tools.

  • Amazon EC2 instances within your VPC can access your file system directly, and Amazon EC2 Classic instances outside your VPC can mount a file system via ClassicLink. On-premises servers can mount your file systems via an AWS Direct Connect connection to your VPC.

  • Amazon EFS supports one to thousands of Amazon EC2 instances connecting to a file system concurrently.

  • Today, I3, C5d, M5d and F1 instances offer NVMe instance storage. Amazon EC2 NVMe instance storage is encrypted using an XTS-AES-256 block cipher. All keys are irrecoverably destroyed on any de-allocation of the storage, including instance stop and instance terminate actions.

  • NVMe instance storage encryption is always on, and cannot be disabled. No, disk encryption on NVMe instance storage does not support integration with AWS KMS system. Customers cannot bring in their own keys to use with NVMe instance storage.

  • Elastic Fabric Adapter(EFA) brings the scalability, flexibility, and elasticity of cloud to tightly-coupled HPC applications. With EFA, tightly-coupled HPC applications have access to lower and more consistent latency and higher throughput than traditional TCP channels, enabling them to scale better. EFA support can be enabled dynamically, on-demand on any supported EC2 instance without pre-reservation, giving you the flexibility to respond to changing business/workload priorities.

  • An ENA ENI provides traditional IP networking features necessary to support VPC networking. An EFA ENI provides all the functionality of an ENA ENI, plus hardware support for applications to communicate directly with the EFA ENI without involving the instance kernel (OS-bypass communication) using an extended programming interface. Due to the advanced capabilities of the EFA ENI, EFA ENIs can only be attached at launch or to stopped instances.

  • EFA support can be enabled either at the launch of the instance or added to a stopped instance. EFA devices cannot be attached to a running instance.

  • By default, all accounts are limited to 5 Elastic IP addresses per region. If you need more the 5 Elastic IP addresses, we ask that you apply for your limit to be raised.

  • In order to help ensure our customers are efficiently using the Elastic IP addresses, we impose a small hourly charge for each address when it is not associated to a running instance.

  • Elastic Load Balancing offers two types of load balancers that both feature high availability, automatic scaling, and robust security. These include the Classic Load Balancer that routes traffic based on either application or network level information, and the Application Load Balancer that routes traffic based on advanced application level information that includes the content of the request.

  • The Classic Load Balancer is ideal for simple load balancing of traffic across multiple EC2 instances, while the Application Load Balancer is ideal for applications needing advanced routing capabilities, microservices, and container-based architectures.

  • To receive a history of all EC2 API calls (including VPC and EBS) made on your account, you simply turn on CloudTrail in the AWS Management Console.

  • Yes. Amazon EC2 Auto Scaling is a fully managed service designed to launch or terminate Amazon EC2 instances automatically to help ensure you have the correct number of Amazon EC2 instances available to handle the load for your application. EC2 Auto Scaling helps you maintain application availability through fleet management for EC2 instances, which detects and replaces unhealthy instances, and by scaling your Amazon EC2 capacity up or down automatically according to conditions you define.

  • In the case of hibernate, your instance gets hibernated and the RAM data persisted. In the case of Stop, your instance gets shutdown and RAM is cleared.

  • Hibernating instances are charged at standard EBS rates for storage.

  • Yes, RAM data is always encrypted when it is moved to the EBS root volume. Encryption on the EBS root volume is enforced at instance launch time. This is to ensure protection for any sensitive content that is in memory at the time of hibernation.

  • To use hibernation, the root volume must be an encrypted EBS volume.

  • You can import your VM images using the Amazon EC2 API tools:

    • Import the VMDK, VHD or RAW file via the ec2-import-instance API. The import instance task captures the parameters necessary to properly configure the Amazon EC2 instance properties (instance size, Availability Zone, and security groups) and uploads the disk image into Amazon S3.
    • If ec2-import-instance is interrupted or terminates without completing the upload, use ec2-resume-import to resume the upload. The import task will resume where it left off.
    • Use the ec2-describe-conversion-tasks command to monitor the import progress and obtain the resulting Amazon EC2 instance ID.
    • Once your import task is completed, you can boot the Amazon EC2 instance by specifying its instance ID to the ec2-run-instances API.
    • Finally, use the ec2-delete-disk-image command line tool to delete your disk image from Amazon S3 as it is no longer needed.

  • You can export your Amazon EC2 instance using the Amazon EC2 CLI tools:

    • Export the instance using the ec2-create-instance-export-task command. The export command captures the parameters necessary (instance ID, S3 bucket to hold the exported image, name of the exported image, VMDK, OVA or VHD format) to properly export the instance to your chosen format. The exported file is saved in an S3 bucket that you previously created
    • Use ec2-describe-export-tasks to monitor the export progress
    • Use ec2-cancel-export-task to cancel an export task prior to completion.

  • Instance usages are billed for any time your instances are in a “running” state. If you no longer wish to be charged for your instance, you must “stop” or “terminate” the instance to avoid being billed for additional instance usage. Billing starts when an instance transitions into the running state.

  • Each instance is charged for its data in and data out at corresponding Data Transfer rates. Therefore, if data is transferred between these two instances, it is charged at “Data Transfer Out from EC2 to Another AWS Region” for the first instance and at “Data Transfer In from Another AWS Region” for the second instance. Same case in different regions.

  • The Convertible RI is useful for customers who can commit to using EC2 instances for a three-year term in exchange for a significant discount on their EC2 usage, are uncertain about their instance needs in the future, or want to benefit from changes in price. Like Standard RIs, Convertible RIs are available for purchase for a one-year or three-year term.

  • No, a RI is associated with a specific region, which is fixed for the duration of the reservation’s term. You cannot convert your Convertible RI for Convertible RI(s) of a lesser total value.

  • With a single API call, EC2 Fleet lets you provision compute capacity across different instance types, Availability Zones and across On-Demand, Reserved Instances (RI) and Spot Instances purchase models to help optimize scale, performance and cost.

  • You’ll effectively pay for the full value of the reservation as long as the Capacity Reservation remains active.

  • You cannot modify a Capacity Reservation that has ended or has been deleted.

  • A Reserved Instance (RI) is an EC2 offering that provides you with a significant discount on EC2 usage when you commit to a one-year or three-year term.

  • Zonal RI: If you want to take advantage of the capacity reservation, then you should buy an RI in a specific Availability Zone.

  • If you do not require the capacity reservation, then you should buy a regional RI. Regional RIs provide AZ and instance size flexibility, which offers broader applicability of the RI’s discounted rate.

  • Availability Zone and instance size flexibility make it easier for you to take advantage of your regional RI’s discounted rate. Availability Zone flexibility applies your RI’s discounted rate to usage in any Availability Zone in a region, while instance size flexibility applies your RI’s discounted rate to usage of any size within an instance family.

  • Instance size flexibility is not available on RIs of other platforms such as Windows, Windows with SQL Standard, Windows with SQL Server Enterprise, Windows with SQL Server Web, RHEL, and SLES.

  • No, RIs do not apply to Spot instances or instances running on Dedicated Hosts. To lower the cost of using Dedicated Hosts, purchase Dedicated Host Reservations.

  • You can list any Reserved Instances that have been active for at least 30 days, and for which we have received payment. Typically, this means that you can list your reservations once they are in the active state. It is important to note that if you are an invoice customer, your Reserved Instance can be in the active state prior to AWS receiving payment. In this case, your Reserved Instance will not be listed until we have received your payment.

  • Yes, if you are a Premium Support customer, you will be charged for Premium Support when you purchase a Reserved Instance through the Reserved Instance Marketplace.

  • While running, Spot instances are exactly the same as On-Demand or Reserved instances. The main differences are that Spot instances typically offer a significant discount off the On-Demand prices, your instances can be interrupted by Amazon EC2 for capacity requirements with a 2-minute notification, and Spot prices adjust gradually based on long term supply and demand for spare EC2 capacity.

  • Over the last 3 months, 92% of Spot instance interruptions were from a customer manually terminating the instance because the application had completed its work. In the circumstance EC2 needs to reclaim your Spot instance it can be for two possible reasons, with the primary one being Amazon EC2 capacity requirements (e.g. On Demand or Reserved Instance usage). Secondarily, if you have chosen to set a “maximum Spot price” and the Spot price rises above this, your instance will be reclaimed with a two-minute notification. This parameter determines the maximum price you would be willing to pay for a Spot instance hour, and by default, is set at the On-Demand price. As before, you continue to pay the Spot market price, not your maximum price, at the time your instance was running, charged in per-second increments.

  • You can choose to have your Spot instances terminated, stopped or hibernated upon interruption. Stop and hibernate options are available for persistent Spot requests and Spot Fleets with the “maintain” option enabled. By default, your instances are terminated.

  • Spot blocks (Fixed Duration Spot instances) are designed not to be interrupted and will run continuously for the duration you select, independent of Spot market price. In rare situations, Spot blocks may be interrupted due to AWS capacity needs. In these cases, we will provide a two-minute warning before we terminate your instance (termination notice), and you will not be charged for the affected instance(s).

  • A Spot Fleet allows you to automatically request and manage multiple Spot instances that provide the lowest price per unit of capacity for your cluster or application, like a batch processing job, a Hadoop workflow, or an HPC grid computing job.

  • No, we do not support multi-region Fleet requests. Only multi-Availability Zone Spot Fleet.

  • The Amazon Time Sync Service provides a time reference that can be securely accessed from an instance without requiring VPC configuration changes and updates. It is built on Amazon’s proven network infrastructure and uses redundant reference time sources to ensure high accuracy and availability. All instances running in a VPC can access the service.

  • Regional Data Transfer rates apply if at least one of the following is true, but is only charged once for a given instance even if both are true:

    • The other instance is in a different Availability Zone, regardless of which type of address is used.
    • Public or Elastic IP addresses are used, regardless of which Availability Zone the other instance is in.

  • Cluster Compute Instances combine high compute resources with a high performance networking for High Performance Compute (HPC) applications and other demanding network-bound applications. Cluster Compute Instances provide similar functionality to other Amazon EC2 instances but have been specifically engineered to provide high performance networking.

  • Cluster Compute and Cluster GPU Instances use differs from other Amazon EC2 instance types in two ways.

    • First, Cluster Compute and Cluster GPU Instances use Hardware Virtual Machine (HVM) based virtualization and run only Amazon Machine Images (AMIs) based on HVM virtualization. Paravirtual Machine (PVM) based AMIs used with other Amazon EC2 instance types cannot be used with Cluster Compute or Cluster GPU Instances.
    • Second, in order to fully benefit from the available low latency, full bisection bandwidth between instances, Cluster Compute and Cluster GPU Instances must be launched into a cluster placement group through the Amazon EC2 API or AWS Management Console.

  • We recommend that you launch the minimum number of instances required to participate in a cluster in a single launch. For very large clusters, you should launch multiple placement groups, e.g. two placement groups of 128 instances, and combine them to create a larger, 256 instance cluster. While it may be possible to launch different cluster instance types into a single placement group, at this time we only support homogenous placement groups.

  • The EC2 Compute Unit (ECU) provides the relative measure of the integer processing power of an Amazon EC2 instance.

  • Micro instances provide a small amount of consistent CPU resources and allow you to burst CPU capacity up to 2 ECUs when additional cycles are available. They are well suited for lower throughput applications and web sites that consume significant compute cycles periodically but very little CPU at other times for background processes, daemons, etc.

  • The CloudWatch metric for CPU utilization will report 100% utilization if the instance bursts so much that it exceeds its available CPU resources during that CloudWatch monitored minute. CloudWatch reporting 100% CPU utilization is your signal that you should consider scaling – manually or via Auto Scaling – up to a larger instance type or scale out to multiple Micro instances. Currently Amazon DevPay is not available for Micro instances.

  • The launch of C5 instances introduced a new hypervisor for Amazon EC2, the Nitro Hypervisor. As a component of the Nitro system, the Nitro Hypervisor primarily provides CPU and memory isolation for EC2 instances.

  • For example, instances running under the Nitro Hypervisor boot from EBS volumes using an NVMe interface. Instances running under Xen boot from an emulated IDE hard drive, and switch to the Xen paravirtualized block device drivers.

  • Optimize CPUs gives you greater control of your EC2 instances on two fronts. First, you can specify a custom number of vCPUs when launching new instances to save on vCPU-based licensing costs. Second, you can disable Intel Hyper-Threading Technology (Intel HT Technology) for workloads that perform well with single-threaded CPUs, such as certain high-performance computing (HPC) applications.

  • You should use Optimize CPUs if:

    • You are running EC2 workloads that are not compute bound and are incurring vCPU-based licensing costs. By launching instances with custom number of vCPUs you may be able to optimize your licensing spend.
    • You are running workloads that will benefit from disabling hyper-threading on EC2 instances.

  • CPU optimized instances will be priced the same as equivalent full-sized instance. You can use Optimize CPUs with only virtualized EC2 instances.

  • There is no Data Transfer charge between two Amazon Web Services within the same region (i.e. between Amazon EC2 US West and another AWS service in the US West). Data transferred between AWS services in different regions will be charged as Internet Data Transfer on both sides of the transfer.

Reference: Amazon EC2 FAQs