Disk write speed much slower than read speed

You are running a VPS server. This means that there are other clients on your physical machine and how they use the disks impacts how you'll see read and write performance.

Typically on RAID10 you'll have about 1/2 the write rate as the read rate. But, since there are a lot of unknown variables, there could be another client doing a lot of writing to the disk and that is why you are seeing worse write speeds.

It can't hurt to open a ticket with them, but with a VPS, this is what you'll typically see. VPSes are for convenience and value, not for performance.

Edit: To be sure, caching is an issue here, but my point still applies.

Be sure to run the dd command with the fdatasync command to ensure it is actually flushing the file data to disk instead of just the memory, which the kernel does by default. ie:

dd bs=1M count=512 if=/dev/zero of=test conv=fdatasync

Yes, it's normal. Your file is only around 2GB and fit's completely into the cache. It's actually never read from the disk, just from the cache. Make the file size at least 10 times bigger to get any meaningful results or even more, depending one your RAM size (2x the RAM is a good starting point).

I'd really like to have a disk with 2.7 GB/s of read speed :)

One problem in your testing technique is the internal system buffering in Linux that is being taken advantage which greatly skews your results.

In general, disk writes are slower than disk reads of course. At a logical file level writes might be much slower since there are (1) disk allocation processes, (2) updating directory information...etc. Thus there are more operations when a file level write happens, it is not a simple atomic operation easily compariable with file level read operations.

In the benchmark, you need to clear out the buffer cache between each dd of your testing....or reboot your machine between each step :-). BTW, I believe there is a simple way to do this by writing something to an applicable /proc area.

EDIT: The cache clearing process:

 sync && echo 3 > /proc/sys/vm/drop_caches

You should do this BEFORE each dd command.