July 6, 2011 | Written by: Jon Etkins
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When a virtual machine (or instance) is run in the cloud, it can have two distinct types of storage available:
- Ephemeral storage is directly associated with each particular instance; it is created when the in-stance is first started, and goes away when the instance is deleted. It can be thought of as similar to the internal disk storage in a conventional physical machine.
- Persistent storage is provisioned separately from any particular instance, and can be attached to any instance running under the same account in the same datacenter. It can be likened to an ex-ternal USB drive in the physical world.
With IBM® SmartCloud Enterprise, customers can select from a variety of supplied virtual machine im-ages, and for each image, select from several configurations that provide stepped combinations of CPU count, memory, and ephemeral disk storage. Separately, and before creating an instance, persistent storage may be created in sizes of 256 GB, 512 GB, and 2 TB, which can be attached to the instance in addition to the ephemeral storage.
The purpose of this article is to describe the layout of the virtual disk drives (both ephemeral and persis-tent) available to the Linux and Windows operating systems for each choice of instance size.
(Windows, and the Windows logo are trademarks of Microsoft Corporation in the United States, other countries, or both.)
Ephemeral disk storage for Windows consists of a 60 GB virtual disk that holds the boot partition and C: drive, plus additional storage depending on the size of the offering that is selected. This additional stor-age is split into unformatted volumes of no more than 500 GB each, as shown in the following table. Persistent storage is mounted as a single virtual drive after all ephemeral storage. The customer is free to format and use the extra storage in whatever way the customer wants.
Note that if the “Root only” option is selected at instance creation time, then only the 60 GB Disk 0 will be created and persistent storage (if selected) will appear as Disk 1.
Ephemeral disk storage for Linux instances is divided in a similar fashion to Windows, with a 60 GB vir-tual disk that holds the boot loader, /boot, and / (root) partitions, plus extra storage depending on the size of the offering that is selected, split into unformatted volumes of no more than 500 GB each. The customer is therefore free to format and use the extra storage however the customer wants, although the anticipation is that most customers will use Logical Volume Manager (LVM). In addition, a small separate virtual disk holding the swap partition is allocated after all the ephemeral storage, but before any persistent storage, as shown in the following table.
Note that if the “Root only” option is selected at instance creation time, then only the 60 GB /dev/vda root drive and applicable /dev/vdb swap drive will be created and persistent storage (if selected) will appear as /dev/vdc.
(Linux is a trademark of Linus Torvalds in the United States, other countries, or both.)