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Re: [Qemu-devel] is there a limit on the number of in-flight I/O operati
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From: |
Stefan Hajnoczi |
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Subject: |
Re: [Qemu-devel] is there a limit on the number of in-flight I/O operations? |
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Date: |
Thu, 27 Aug 2015 17:49:52 +0100 |
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User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Mon, Aug 25, 2014 at 03:50:02PM -0600, Chris Friesen wrote:
> The only limit I see in the whole call chain from
> virtio_blk_handle_request() on down is the call to
> bdrv_io_limits_intercept() in bdrv_co_do_writev(). However, that doesn't
> provide any limit on the absolute number of inflight operations, only on
> operations/sec. If the ceph server cluster can't keep up with the aggregate
> load, then the number of inflight operations can still grow indefinitely.
We probably shouldn't rely on QEMU I/O throttling to keep memory usage
reasonable.
Instead rbd should be adjusted to support iovecs as you suggested. That
way no bounce buffers are needed.
Stefan