Re: RAID performance - new kernel results

[Phil Turmel <philip@turmel.org>]

On 04/16/2013 03:28 PM, Roy Sigurd Karlsbakk wrote:
>>> I suspect that the single ping packets being lost are an
>>> indication of a problem, but this should not impact the users
>>> (TCP should look after the re-transmission, etc). Wether this is
>>> related to the longer 10-50 second outage I'm not sure.
>> 
>> No, single lost pings are *not* a sign of a problem. It is
>> perfectly normal for a network to have random traffic spikes that
>> fill a switch's store-and-forward buffers. ICMP pings are
>> *datagrams*, like UDP, so they aren't retransmitted when dropped.
>> Losing them as infrequently as you say suggests your network isn't
>> heavily loaded.
> 
> Switches (unlike bridges) do not use store-and-forward. They use
> cut-through, meaning they use store-and-forward for the initial
> packet from A to B and then store the path and switch it later,
> sniffing the MAC addresses and just use pass-through.

Nothing you said changes my statement that switches often drop single
packets.  The occasional dropped ping is a red herring.  A cheap switch
that can't ever buffer will simply drop *more* random packets.

> As was said, the traffic on the network was minimal, so I really
> doubt this had an impact. Getting 30 seconds+ of drops must come from
> a bad network stack or a really bad switch, but then again, two
> switches were tested, so I doubt the switches alone could do that.

We seem to violently agree here.  Multiple consecutive drops is a real
problem.

> What may be doing it, is bad (or perhaps incompatible) bonding
> setup.

My point was to not prematurely conclude that the problem is in the network.

Phil