> Can you advise:
> - is the 4MB IO scenario specific to the Elan configuration of our
> first customer who used DDN 9500s
No
> - what is the drawback of 4MB IOs wrt LNET
LNET supports a 1MByte MTU in all network configs. This limitation applies
because larger I/Os require a scatter/gather descriptor larger than 4K (the
lowest common denominator page size). We can't exceed this in general
because it requires multi-page allocations for descriptors and "small"
message buffers. Also, 1MByte is quite enough to allow LNET to hide latency
and get good network bandwidth efficiency.
We have added a _hack_ that allows some LNDs to support a larger payload and
lustre to exploit this, if and only if both the client and server are on the
same network (no routers) and both support a larger page size.
A better solution would be to allow the server to read/write bulk data in
several chunks - e.g. 4 1MByte chunks to support a 4MByte bulk RPC. This is
completely general purpose, it does not cause any network inefficiency and
still allows the server to do large I/O to disk.
However I would still beware of issues regarding disk I/O scatter/gather
descriptor size if the server has a 4K page size.
Cheers,
Eric
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