Mmap on SSD (directly mapping the device vs. mapping a file)

[Daniel Noack <fsf.deathman@gmail.com>]

Hi, folks!

First of all I have to say sorry that I'm not a subscriber of the list now,
but I will subscribe in the near future (need an extra email account for it
;) ). It would be nice if you could put me on CC for an answer to my
question.

Like you can see on the subject I experimented a little bit with mmap in the
last time. I've written a little B+tree library which uses mmap to store
the tree to a file or the whole device (means it is also possible to map
the hwole device (i.e. /dev/sdb)). I used msync after every successfull
change on the tree. Next thing I did was to use this for a little benchmark
on performance of different storage devices (ramdisk, HDD, and a very fast
flashcard directly atteched to the PCIe bus). I recognized that in allmost
all cases when directly mapping teh device without a filesystem the
file-mapped version was a little bit slower. But when I tried it on an SSD
device the file-mapped version was an order of magnitude faster. I also
tried secure erase and did the benchmarks many times and in many
configurations, but I came to the same results. Can anyone explain me
what's happening here or have I something missed. Is this behavior correct?

Thanks,
Daniel