[Pkg-uml-pkgs] CONFIG_BLK_DEV_UBD_SYNC

[Nicolas Boullis]

Hi,

I've been using for months uml on Debian Etch to virtualize a few
production servers. Everything was fine until I changed my host's
storage solution and see the disk performance drop significantly.

My storage is now on a FiberChannel-based SAN, and its latency seems to
be related to the performance drop.

After some experiments, I figured out that Etch's user-mode-linux was
compiled with CONFIG_BLK_DEV_UBD_SYNC, and that removing that option
*much* improved the performance.

But I thought that it probably was not such a good idea if the guest
kernel never knows when things are really written to disk, as I think it
may lead to bad FS corruption if the host ever rebooted unexpectedly...
And then I realized that the latest user-mode-linux you uploaded does
not use this option either...

My opinion is that it would be better if the disk I/O confirmed only
when the data is actually written to disk (as with
CONFIG_BLK_DEV_UBD_SYNC) but not serialized (for better performance).

Do you have an opinion about all this?


Cheers,

Nicolas Boullis