up vote 5 down vote favorite
share [g+] share [fb]

I have virtual machines running off an external 7200rpm hard drive connected to a laptop with eSATA. I'd like to upgrade the drive to an SSD but can't afford one at the size I need.

Would using a hybrid SSD for this purpose give a significant speed increase?

link|improve this question
89% accept rate
How did this work out? I'm in a similar situation now. – George Mauer Jun 29 '11 at 20:22
@GeorgeMauer Because I need to run multiple VMs and don't want to make a mistake with my company's money, I decided not to go ahead. However you'll note that neither answer thus far contains any measurements so it could still be a good approach. – Alex Angas Jun 30 '11 at 1:02
feedback

2 Answers

up vote 2 down vote accepted

I tend to agree with Billy. Performance metrics from http://www.hardwaregeeks.com/index.php/site/comments/seagate_momentus_xt_500gb_hybrid_ssd/ suggest that for a single VM, the performance increase is definitely there. Hybrid drives attain their speed boosts by caching frequently-used sectors of the hard drive into flash memory, speeding up reads. Multiple VM's, if started up and shut down frequently, might spread out the reads and writes too much to allow the hybrid drive to make much of a difference.

However, if you have machines that are suspended and resumed frequently, those suspension files might stay in cache longer (as they would be the most commonly-accessed files), giving you the boost you're looking for. I'm not sure how intelligent or aggressive the caching mechanism is on these drives, but as far as I understand it, the longer you use a drive, the more accurate the caching algorithm would be at keeping the right files in flash cache.

link|improve this answer
+1. Also remember that the drive doesn't know anything about files, only blocks. So if there's only a portion of a file that's accessed the drive will have no problem using that as the portion cached in flash. – Billy ONeal Oct 22 '10 at 16:02
As Seagate says their algorithm excludes large files from being cached on SSD. I guess this rules out VMs them, but test should be conducted to prove this. I can also see why is that? Well simply because hybrid drive should increase overall performance (like multiple apps not just a huge one) and should perform fair usage policy when choosing files to cache. – Robert Koritnik Jan 17 at 23:53
feedback
up vote 4 down vote

Depends. Hybrid drives provide increased performance when there's a segment of the drive which is commonly accessed, and therefore which can be cached in flash.

I would suspect that for a single vm, that would be faster, but for multiple VMs, each time a VM got started it would flush out most of the flash cache back to disk anyway.

link|improve this answer
I've never tried this but I concur wholeheartedly. – Shinrai Oct 22 '10 at 15:17
It should be noted that this frequently cached LBAs only include those with not too big files. I'm pretty sure VM files would be ruled out of this equation. But tests should either prove or reject this thesis. – Robert Koritnik Jan 17 at 23:50
@Robert: The drive doesn't know about files, it only knows about disk blocks. I don't think the size of a file ever enters the picture. (After all, the drive has to work with whatever filesystem you might put there) – Billy ONeal Jan 18 at 1:04
feedback

Your Answer

 
or
required, but never shown
discard

Not the answer you're looking for? Browse other questions tagged or ask your own question.