Re: booting from virtio-blk

Anthony Liguori
2008-04-01T17:05:40Z

Hollis Blanchard wrote:
> On Tue, 2008-04-01 at 09:46 -0500, Anthony Liguori wrote:
>
> Thanks Anthony, you've saved me a lot of debug time! Rusty, doing 64-bit
> PCI config space accesses with ioread8() definitely violates the
> principle of least surprises, and would have taken me a long time to
> track down. :(
>
> Attached is a boot log of a PowerPC guest booting from virtio-blk root.
>
> "ramdisk_image" is the standard ~4MB image provided with DENX Embedded
> Linux Development Kit. Booting is also *way* faster than NFS root (a few
> seconds to get to a shell :) .
>

That suggests you have vmexit latency issues. A 4MB disk is pretty much
entirely cachable in memory so you probably end up with only a handful
of requests to get the full disk into memory. Conversely, when using
NFS, every single filesystem operation requests in multiple packets
being delivered/received. To complicate matters further, NFS means you
won't be doing any dentry caching so every single filesystem access will
result in requests as opposed to just the first access.

What sort of ping latency do you get with virtio-net?

Regards,

Anthony Liguori