I have a new Dell Mobile Precision M6500 laptop with 8 GB RAM. It has two hard drives - 500 GB @7200 RPM and a 128 GB SSD. The main purpose of these laptop is software development in virtual machines.

The plan is to install the base OS (Windows 7) and all the programs in the 500 GB drive, and let the SSD only contain the virtual machine images. It is my understanding that the we get most performance from the virtual machines if the images are on a separate hard drive than the base OS. Is this the way to go, or should I install the OS on the SSD as well? What are the pros and cons?

The virtual machine images would be between 20 - 30 GB, and I might run 1 or 2 at a time.


One caveat to note: you won't get TRIM support from the virtualized instances, which means you'll lose performance over time on the SSD.

As to your original question, I see no reason to use the 500 GB for anything other than bulk storage (backups and the like); running the OS off of the SSD makes the most sense. The reasoning behind separating the disks hosting a virtual OS and the host OS only really makes sense in a world of non-zero seek times, so since SSDs have no penalty for non-linear access, I'd say combining the two is the way to go. That'd also let you remove the spinning disk entirely, which would save you quite a bit of battery discharge when you're on the road.

  • Going straight SSD will reduce latency, power consumption, heat, noise, and vibration. Those are all big wins IMHO. Unless the price/byte or shorter lifespan are an issue SSDs are almost always the way to go these days. @BMDan has it right.
    – chicks
    Mar 1 '15 at 21:51

@BMDan Trim should be conducted by windows trim service either way on windows because of wear leveling in the service... as the virtualization software attempts to rewrite a page that has been written, windows will change the lba of the page to a different address that has been written less times. Where you will see loss in performance is if the drive remains full for a long period of time while writing small intersections into the remaining free space repeatedly. This is where the wear leveling fails as it will usually not move already written blocks, that don't get rewritten often, to worn down blocks, so that the fresher blocks can be used more aggressively.

With that said there is a disadvantage to having your os on the HDD's because the page file is usually on the disk you installed on. In the case that your machine begins to thrash, you will see a massive degradation in performance. A likely scenario with 8 GB of ram running VM's. I would suggest that, if you decide to install on the hdd's, you at least move the page file to the ssd and increase it's size. That way the spinning media will cache items to the ssd as they are unloaded from ram and be more accessible when needed later.

BMDan is correct in his suggestion though, having the os on the ssd and then only using the ssd will considerably reduce power consumption. Depends on how much space you need.

  • I was wondering why the base Windows wouldn't take care of it, but I don't use Windows any more so I didn't know. Thanks for clearing up that Windows can handle it.
    – chicks
    Mar 1 '15 at 21:52

Certainly it'll be about as fast I can imagine it being (bar the use of FusionIO drives) - what I would say it that the VM's OSs will write as much they would on a regular disk in a non-virtualised environment, meaning write-wear. If that doesn't bother you then off you go.

  • "what I would say it that the VM's OSs will write as much they would on a regular disk in a non-virtualised environment, meaning write-wear." - can you please eloborate on that, is that something I should be concerned with using an SSD? Thanks. Oct 12 '10 at 19:30
  • SSD's have a limited life span. This life span is measured in the amount of times the data is erased. However, because overwriting data is cause for erasing the block, you can't write without erasing. Having a vm will write just as much data to the disk as a normal os would but because it has less ram, this data written to disk can be higher even if the disk is faster. So, if you don't care about the extra wear on the ssd.. go for it. Apr 7 '15 at 0:37

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