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Transfer of large filesover iSCSI reported as complete by OS way before network activity ceases

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New to iSCSI and trying to set up a Synology RS813 NAS as a backup repository for a 2012.R2 hyper-v host. The iSCSI initiator on the 2012.R2 server connects to the NAS iSCSI target (block based) via 2 MCS connections via different physical network adapters and switches.  The throughput is around 100-140MB/s as reported by 2012.R2 file copy info GUI window when copying very large files (1TB), so is compatible with the expected NAS performance. Rate is confirmed using performance monitor on the logical disk.

However, it is noticeable that the initial 1% of the transfer (and pretty much the whole transfer of a 26GB file) is at vey high reported speed of up to 1GB/sec.  Watching the network activity on both the NAS and 2012.R2 adaptors, the activity starts some seconds after the copy is initiated by the OS and continues for a long time (30 secs plus) after the OS closes the GUI copy info window. 

Write cache is off for the logical disk.  Both network adapters are on an Intel I350-t card with jumbo packets set to 9014 and everything else default (so 512 send buffers).

I'm presuming that there is caching going on at the 2012.R2 end (either OS or offloaded to Intel hardware) but have not been very successful in finding any web-based guidance as I'm not entirely sure what to search for.  Consequently I really appreciate any comments as to whether this is expected behaviour and, given that this is a backup target, whether there are any implications for data integrity during the interval when the OS thinks the file is written but data are still being sent over the networks.

Thanks for helping a complete newbie to this sort of issue.


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