Hello,
From few months, we're experiencing tragic VDP backup performance to our Synology NAS DS1513+ datastore (5x WD Black 2TB in RAID5) via iSCSI. From the time, nothing changed in our environment (except new NAS Synology DSM software updates).
We're backing up vCenter server (115GB) in 42 minutes, Windows Server 2012 (100GB) in 12 minutes, which is acceptable for us. We also have another W2012 server machine (file server) which is 900GB big (100GB system disk, 800GB data disk), which takes 9 to 11 hours! New bytes are around 0.3 to 0.5%... Previously it took about half this time to backup this VM.
We've checked our network, and from network side, everything it's fine. Our server has aggregated 8x 1Gbps links, Synology NAS has 2x 1Gbps links. The VDP appliance (6.1.2.19 version) - has 4 vCPUs (E5-2430) & 12 gigs of RAM, and it's not using max 30% CPU when backing up, and average 45% or RAM.
I've also tested the RAID5 performance (with DD) on our synology, and write performance is about 278MB/s, mdadm showed 291,7MB/s (disk read). I tried to do storage performance check via VDP (in vdp-configure panel), but it fails every time (exceeded allowed time to complete).
Probably I've exhausted all my ideas for now. Maybe it's VDP appliance fault (low performance)?