[In recent weeks, there have been a number of posts on this blog looking at the Hyper-V developer workstation proof of concept (booting from a .VHD on a flash drive) that I knocked up for my colleague Garry Martin. This post is slightly different in that Garry did the legwork and provided me with some notes to publish in his name. So, with a little editorial input from yours truly, here are Garry’s notes on drive speeds for ATA, USB Flash, SDHC and CF.]
Card speed is usually specified in “x†ratings. This gives the data rate as a multiple of the data rate of the first CD-ROMs which ran at 150kB/s. Thus a 133x SDHC card is running at 133 * 150kB/s = ~20MB/s.
In contrast, most premium CF cards on the market today run at 300x, or ~45MB/s, roughly equivalent to a modern 80GB laptop hard drive.
The faster USB flash drives currently run at somewhere between 200x and 233x (30MB/s and 35MB/s). Examples are the Corsair Flash Voyager GT 4GB, the OCZ ATV Turbo 4GB and the Lexar JumpDrive Lightning 4GB.
Looking at pricing for the various media types:
| Device | Approximate price to purchase (UK prices, August 2009) |
|---|---|
| SanDisk Extreme III SDHC 4GB (200x) | £20 |
| SanDisk Extreme III SDHC 8GB (200x) | £50 |
| SanDisk Extreme III 4GB CompactFlash (200x) | £30 |
| Corsair Flash Voyager GT 4GB (233x) | £35 |
| OCZ ATV Turbo 4GB (233x) | £35 |
| Lexar JumpDrive Lightning 4GB (200x) | £35 |
[I’m still playing around with flash for my USB boot scenario (especially as an SDHC card will sit nicely in my notebook PC’s card reader slot) but, ironically, for a project that started out looking at booting from a USB flash drive, we will probably settle on the use of external hard disks. This isn’t for reasons of performance but because the internal disks that would have stored the VMs are encrypted – whereas with a USB-bootable hard disk we can store the VHDs for our VM workloads and the parent partition’s bootable VHD. (BitLocker and hibernation are two Windows features that boot from VHD does not support.)]