Hi Tinker,
On Sun, May 10, 2015 at 6:16 AM, Tinker ti...@openmailbox.org wrote:
Hi Neel,
Thank you very much for your response -
That's great!
I guess this should deliver the full capacity for basically any IO system
around, be it a fast SSD or ramdisk. (Since the guest may not need to flush
data immediately to the host, I guess read performance is the more sensitive
point.)
Which disk emulation mode is best for a BSD guest, virtio-blk or
ahci-hd?
You'll need to experiment because there are so many moving pieces: the
guest driver implementation, VM exit overhead, capabilities of a
emulated disk controller. From the narrow perspective of VM-exit
overhead 'virtio-blk' is better because the CPU decodes all the
information needed to handle the I/O exit.
In general, should any other consideration be made for performance (using
the direct or nocache BHyVe configuration options, particular
consideration for proper sector alignment when using a disk image stored on
the host's SSD-based ZFS, mounting the host ZFS filesystem with noatime)?
Using nocache will prevent double-buffering of the virtual disk file
in the host. Using direct will guarantee consistency of the virtual
disk file on the underlying storage device. If you are purely focused
on performance then don't use either option but be aware that you'll
be using buffer cache on the host and also that the guest filesystem
could be corrupted on an unplanned host shutdown.
With respect to alignment I am not sure if you can request a specific
alignment when creating a flat file for the virtual disk. However, if
you use a ZVOL as a backing device then it can be created on top of a
partition which has the correct alignment.
best
Neel
Kind regards,
Tinker
On 2015-05-10 04:03, Neel Natu wrote:
Hi Tinker,
On Sat, May 9, 2015 at 9:49 AM, Tinker ti...@openmailbox.org wrote:
Hi!
For an environment with very heavy parallell IO, should the performance
be
just as good in a BHyVe guest as in the FreeBSD host environment?
What I thought of is that I guess within the host environment, the
storage
subsystem should have all kinds of optimizations like an internal work
queue
that pushes lots of work alinearly/asynchronously to the disk controller
and
this way allows it, in turn, to give all its performance.
Does the virtualized disk interface carry over all that goodness to the
guest?
bhyve creates 8 worker threads for each virtual disk controller (both
ahci and virtio-blk).
All guest I/O is handled asynchronously by these worker threads which
provide parallelism.
(https://wiki.freebsd.org/bhyve seems to say yes, presuming you configure
BHyVe to run the virtual disk in AHCI mode?)
The wiki is out of date.
Since r280037 the virtio-blk emulation also gets the benefits of using
the block_if worker threads.
best
Neel
Thanks!
Tinker
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