--On 21 September 2017 15:49 +0100 Karl Pielorz <kpielorz_...@tdx.co.uk> wrote:


Are these timeouts coming from Dom0 or from a VM in a DomU?

domU - as above, dom0 grumbles, but generally seems OK about things. dom0
doesn't do anything silly like invalidate the VM's disks or anything.

I've chased this down in the code - having briefly looked at blkfront/blkback - I can see all the mechanisms in place for performing I/O - but I cannot see there's any timeouts set anywhere (in that code).

I can see the callback that fires when the I/O fails.

It looks like for the purposes of xbd I/O requests are just gathered up, processed - and then fired off to XenServer (i.e. upstream). If they fail, callbacks are fired - and action taken.

But nowhere can I see where there are any timeouts either specified, or specifiable by FreeBSD - nor can I see (certainly at that level) that there are any I/O retries in that code.

So,

 - Timeouts may be set by Xen (i.e. outside of FreeBSD's scope)
- I/O may be retried by 'higher levels' than blkfront/blkback - but I can't see where.

It may simply be that I/O from FreeBSD through XenServer is a 'fire and forget' process, where FreeBSD has no control over timeouts, and currently has no code (at that level) to perform retries.

I'd need to figure out what sits above 'blkfront/blkback' - and whether that's likely to do any retries.

It's certainly not CAM running the storage - so those timeout/retry sysctl values are completely irrelevant.

More study, and maybe a quick post to -hackers to see what lies above blkfront/back etc.

-Kp
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