ESXi is VMware's stripped-down version of ESX -- that is, ESXi does not include the management service console -- and comes in the two editions: Embedded and Installable. The Embedded version is pre-installed on a Universal Serial Bus (USB) or Secure Digital drive for certain server models; the ESXi Installable edition can be placed on any supported storage device.
The ESXi Embedded edition is available only for servers from manufacturers that support the option. An ESXi image is typically provided on a vendor USB flash drive and plugs into an internal port on a server's motherboard. Most users don't order this option and want to install ESXi on USB flash drives themselves.
Fortunately, loading ESXi Installable on a USB flash drive is fairly simple, and you can use this version with any server that supports booting from removable USB drives. But before we review this process, let's cover why installing ESXi from a USB drive is a good idea.
Why you should use ESXi Installable from a USB thumb drive
The primary reason for booting ESXi Installable from USB flash drive is so you don't need to install hard drives in your host servers. Adding hard drives increases a server's price by hundreds of dollars. Also, in some cases, it's not desirable to have internal hard drives (i.e., in blade servers).
Most ESX and ESXi hosts use shared storage because certain features, such as VMotion, Fault Tolerance, Distributed Resource Scheduler and High Availability, require it. As a result, local storage goes mostly unused in host servers and serves mainly as a location from which to install ESX/ESXi.
ESXi on USB flash drives also works well for "headless computing," where nothing is stored directly on a server. If there is a failure, a server can be easily replaced without having to reinstall operating systems or applications. Therefore, when a server is replaced, a virtualization administrator just has to remove the USB thumb drive from the old server and plug it into the new one.
A final reason, which may not apply for many, is that you can dual-boot with ESXi Installable on a USB flash drive. Simply remove the thumb drive, and you can boot from a partition on your local hard disk.
Disadvantages of installing ESXi on a USB flash drive
Despite the numerous advantages of booting from an ESXi-equipped flash drive, there are some reasons for not choosing this method.
First, having local disks on a host can be very handy for creating local Virtual Machine File System volumes for both noncritical and redundant VMs, such as domain name servers that could keep running were your shared storage unavailable. Additionally, local storage is good for making backup copies of your VMs to and for using Storage VMotion to copy VM's to it when doing maintenance on shared storage.
The second disadvantage of bootable ESXi thumb drives is reliability. You cannot use RAID with flash drives. Therefore, if it fails or has problems, an ESXi host will suffer downtime until you replace it with a new flash drive and reconfigure it. Using a local disk in a RAID configuration, however, ensures that your server can continue running if a drive fails.
The final reason not to install ESXi from a USB drive is performance. USB flash drives are much slower than SCSI or serial-attached SCSI (SAS) hard drives, which typically transfer data at 3 Gbps. In contrast, USB 2.0 -- which is common in most servers -- has a maximum data speed of 480 Mbps, or 60 MB per second (MBps), but the transfer rates are generally much slower than that. In real-world usage, for example, USB flash drive read speeds average 30 MBps and an average write speed of 5 MBps.
The slower speeds, however, will not have much effect on an ESXi host and should not be a deterrent from using USB. The boot time of a host will besomewhat slower, but only by a minute or two. Additionally, some of the subsequent reads and writes to configuration and log files will be slightly slower, but this will not affect the performance of any VMs running on the host.
ESXi Installable requirements for a flash drive
The requirements for installing ESXi on a USB flash drive are quite simple: All you need is an ISO image of ESXi Installable and a flash drive with at least 1 GB of storage.
Despite being 60 MB, the true size of ESXi is closer to 1 GB after installation because of the additional files and partitions that are created. They are not directly part of ESXi but are used for the installation of VMware Tools onto guest operating systems and storing a backup image of ESXi whenever it's updated, among other things.
Before you begin the installation process, get a USB flash drive that is at least 1 GB. (But grabbing a larger thumb drive is OK.) Because this device is going to be the heart of your server, I would recommend buying a quality, name-brand USB flash drive. Furthermore, avoid novelty and lower-cost, generic flash drives that may have high failure rates. Also, be aware that all flash drives are not equal, and performance varies greatly between brands, models and sizes.
You can test the speed of your flash drives using the freeware utility HD_Speed. When comparing two different model/size flash drives, for instance, I had one drive average 31 MBps for reads and 6 MBps for writes, while the other averaged 23 MBps for reads and 8 MBps for writes.
Another requirement is that your server supports booting from a removable USB device. Most modern servers have this feature, but sometimes you need to enable it in the server's BIOS.
You can also specify the load order for boot devices. USB is usually above hard drives and network Preboot Execution Environment (or PXE) boots but below CD-ROM and floppy drives.
Additionally, a server's BIOS may allow you to choose USB 1.1 or 2.0 for your ports. Be sure this is set to 2.0, because it's much faster than 1.1.
You should use an internal USB slot for your ESXi flash drive. Most modern servers have at least one internal USB slot. To check, open the cover and look around. Here are pictures of a Hewlett-Packard Co. server empty and with a flash drive.
Also ensure that the flash drive fits inside your server with the cover closed. While you can install it externally, it's not recommended; the drive can be easily removed, bumped or broken.
In part two of this series on installing ESXi on a USB flash drive, we cover the methods for installation and some additional information on configuring and maintaining flash drives.
Eric Siebert is a 25-year IT veteran with experience in programming, networking, telecom and systems administration. He is a guru-status moderator on the VMware community VMTN forums and maintains VMware-land.com, a VI3 information site.