Re: [PATCH] Avoid fragment virtio-blk transfers by copying

[Avi Kivity <avi@qumranet.com>]

Anthony Liguori wrote:
> A major source of performance loss for virtio-blk has been the fact that we
> split transfers into multiple requests.  This is particularly harmful if you
> have striped storage beneath your virtual machine.
>
> This patch copies the request data into a single contiguous buffer to ensure
> that we don't split requests.  This improves performance from about 80 MB/sec
> to about 155 MB/sec with my fibre channel link.  185 MB/sec is what we get on
> native so this gets us pretty darn close.
>
>   

If the guest issues a request for a terabyte of memory, the host will 
try to allocate it and drop to swap/oom.  So we need to either fragment 
beyond some size, or to avoid copying and thus the need for allocation.


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