Metering – How Amazon EFS Reports File System and Object Sizes
This section explains how Amazon EFS reports file system sizes and sizes of objects within a file system.
Metering Amazon EFS File System Objects
Customer-visible objects in an Amazon EFS system can be regular files, directories, symbolic links, and special files (FIFOs and sockets). Each of these objects is metered for 2 kibibytes (KiB) of metadata (for its inode) and one or more increments of 4 KiB of data. The following list explains the metered data size for different types of file system objects.
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Regular files – The metered data size of a regular file is the logical size of the file rounded to the next 4-KiB increment, except that it may be less for sparse files.
A sparse file is a file to which data is not written to all positions of the file before its logical size is reached. For a sparse file, if the actual storage used is less than the logical size rounded to the next 4-KiB increment, Amazon EFS reports actual storage used as the metered data size.
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Directories – The metered data size of a directory is the actual storage used for the directory entries and the data structure that holds them, rounded to the next 4 KiB increment. The metered data size doesn't include the actual storage used by the file data.
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Symbolic links and special files – The metered data size for these objects is always 4 KiB.
When Amazon EFS reports the space used for an object, through the NFSv4.1
space_used attribute, it includes the object's current metered data size, but
not its metadata size. There are two utilities available for measuring the disk usage
of a
file, the du and stat utilities. Here's an example of how to use
the du utility, on an empty file, with the -k option to return the
output in kilobytes:
$ du -k file 4 file
Here's an example of how to use the stat utility on an empty file to return
the file's disk usage:
$ /usr/bin/stat --format="%b*%B" file | bc 4096
To measure the size of a directory, use the stat utility, find the
Blocks value, and then multiply that value by the block size. Here's an
example of how to use the stat utility on an empty directory:
$ /usr/bin/stat --format="%b*%B" . | bc 4096
Metering an Amazon EFS File System
The metered size of an entire Amazon EFS file system is the sum of the sizes (including metadata) of all of its current objects. The size of each object is calculated from a representative sampling that represents the size of the object during the metered hour, for example the hour from 8:00 am to 9:00 am.
For example, an empty file contributes 6 KiB (2 KiB metadata + 4 KiB data) to the metered size of its file system. Upon creation, a file system has a single empty root directory and therefore has a metered size of 6 KiB.
The metered sizes of a particular file system define the usage for which the owner account is billed for that file system for that hour.
Note
The computed metered size doesn't represent a consistent snapshot of the file system at any particular time during that hour. Instead, it represents the sizes of the objects that existed in the file system at varying times within each hour, or possibly the hour before it. These sizes are summed to determine the file system's metered size for the hour. The metered size of a file system is thus eventually consistent with the metered sizes of the objects stored when there are no writes to the file system.
This metered size for an Amazon EFS file system can be seen in the following ways:
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DescribeFileSystems API – Used in SDKs, HTTP, and the AWS CLI.
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File Systems table – For each file system listed in the AWS Management Console.
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DF command – In Linux, the
dfcommand can be run at the terminal prompt of an EC2 instance. Use thedfcommand and not theducommand. Don't use theducommand on the root of the file system for storage metering purposes. The results don't provide full data.
Note
The metered size is also used to determine your I/O throughput baseline and burst rates. For more information, see Throughput Scaling in Amazon EFS.
