Re: [Qemu-devel] block migration and MAX_IN_FLIGHT_IO

[Stefan Hajnoczi]

On Thu, Feb 22, 2018 at 12:13:50PM +0100, Peter Lieven wrote:
> I stumbled across the MAX_INFLIGHT_IO field that was introduced in 2015 and 
> was curious what was the reason
> to choose 512MB as readahead? The question is that I found that the source VM 
> gets very unresponsive I/O wise
> while the initial 512MB are read and furthermore seems to stay unreasponsive 
> if we choose a high migration speed
> and have a fast storage on the destination VM.
> 
> In our environment I modified this value to 16MB which seems to work much 
> smoother. I wonder if we should make
> this a user configurable value or define a different rate limit for the block 
> transfer in bulk stage at least?

I don't know if benchmarks were run when choosing the value.  From the
commit description it sounds like the main purpose was to limit the
amount of memory that can be consumed.

16 MB also fulfills that criteria :), but why is the source VM more
responsive with a lower value?

Perhaps the issue is queue depth on the storage device - the block
migration code enqueues up to 512 MB worth of reads, and guest I/O has
to wait?

Stefan

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