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Comparing Filesystem Performance in Virtual Machines

mitchellh.com

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Re: Comparing Filesystem Performance in Virtual Machines

#41

Post by jamesaguilar »

mitchellh wrote: post #38
wila wrote: post #35

Earlier quoted context omitted.

The problem is that the tests are flawed and the native speed being slower as virtual is a pretty big red herring for that. That's OK because benchmarking that tests what you are after is actually very difficult. Small assumptions can create big differences. If you can't guarantee that the data has actually been written to the disk then you're testing caching mechanisms, something you already point out in your articl…

While the article uses only 64 KB/MB files for the analysis, the full Excel workbook contains data that had up to 512 MB files. The VMs only had 256 MB of RAM, so I did indeed test the RAM-spillover cases. The results were very similar, though unsurprisingly the VMs didn't perform quite as well (though they did still beat over native). I never tested going over the native's RAM.

The amount of memory allocated to the vm does not include the host fs cache. So it is still easily possible for a 500M file to fit in that.

Re: Comparing Filesystem Performance in Virtual Machines

#42

Post by arde »

orthecreedence wrote: post #18
icebraining wrote: post #8

Earlier quoted context omitted.

He's probably talking about the Shared Folders performance.

That's the conclusion I drew, but it's really unclear. He said at the end that VMWare blows Virtualbox out of the water. The graphs show that for shared folders but as far as disk access in general, they look fairly evenly matched (at least, that's what the graphs I saw depicted).

Yeah, and who cares about shared folder performance anyway?

Re: Comparing Filesystem Performance in Virtual Machines

#43

Post by kika »

stefanha wrote: post #33

This benchmark is bogus because the iozone -I flag is missing. -I uses O_DIRECT to avoid the page cache. Due to page cache usage it's hard to say what this benchmark is comparing. The I/O pattern seen by the actual disk, shared folder, or NFS may be different between benchmark runs. It all depends on amount of RAM available, state of cache, readahead, write-behind, etc. Please rerun the benchmark with -I to get an ap…

It will avoid page cache in the VM, but will not avoid cache on the host, right?

Re: Comparing Filesystem Performance in Virtual Machines

#45

Post by vl »

kika wrote: post #43
stefanha wrote: post #33

This benchmark is bogus because the iozone -I flag is missing. -I uses O_DIRECT to avoid the page cache. Due to page cache usage it's hard to say what this benchmark is comparing. The I/O pattern seen by the actual disk, shared folder, or NFS may be different between benchmark runs. It all depends on amount of RAM available, state of cache, readahead, write-behind, etc. Please rerun the benchmark with -I to get an ap…

It will avoid page cache in the VM, but will not avoid cache on the host, right?

Depends on configuration, but most setups propagate O_DIRECT all the way through - otherwise it would be impossible to run many apps, such as DBs in VM.

Re: Comparing Filesystem Performance in Virtual Machines

#46

Post by gcv »

arde wrote: post #42
orthecreedence wrote: post #18

Earlier quoted context omitted.

That's the conclusion I drew, but it's really unclear. He said at the end that VMWare blows Virtualbox out of the water. The graphs show that for shared folders but as far as disk access in general, they look fairly evenly matched (at least, that's what the graphs I saw depicted).

Yeah, and who cares about shared folder performance anyway?

It's the nicest way to keep and edit your work on the host and run it in the isolated environment of the guest. VirtualBox shared folders have completely unacceptable performance here [1], and VMware handles itself much better.

[1] 10-15 second delay on returning a response in a Rails app, in my experience.

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