Zoltan,
* Boszormenyi Zoltan (zb@cybertec.at) wrote:
> When the virtual disk is a disk file on the host machine, we've measured
> 20% or lower. The host used Fedora 19/x86_64 with IIRC a 3.10.x Linux kernel
> with EXT4 filesystem (this latter is sure, not IIRC). The effect was observed
> both under Qemu/KVM and Xen.
Interesting- that's far worse than I would have expected. Was this test
done with paravirtualized drivers? If not, I can certainly understand
the terrible performance.
Independently of that, I'll add my own 2c that DB people tend to be
pretty paranoid and the current round of VM technologies out there have
caused more than one person to lose data because fsync wasn't honored
all the way down to the disk. This is especially true of 'home-grown'
setups, imv, but I'm sure you could configure the commercial offerings
to lie to the guest OS too. Of course, there are similar concerns about
a SAN or even local RAID cards, but there's a lot more general
familiarity and history around those which reduces the risk there (or at
least, that's the thought).
Thanks,
Stephen
