ZFS Background Destruction

I am running omnios r151008 on a recent box with 3TB SAS disks arranged in a raidz2 pool.

I have this 7TB filesystem where I have my zimbra server store its backups. The backup-sets are highly redundant, so for a time I had deduplication turned on to great effect, but as the size of the backup-sets grew, I ran out of ram and performance on the system became realy bad. So I turned deduplication of. Performance for newly written data is fine again.

The other day, I destroyed a bunch of old snapshots on this filesysem. Thanks to background destruction this did not realy take a long time. But now as background destruction is progressing, the system remains very slugish when doing io on the pool (slow) where the destruction is taking place.

Questions

1. would it be faster to send/receive the content of the deduplicated filesystem to a new non deduplicated and then destroy the entire filesystem (not the pool).
2. is there a way to monitor progress on the background destruction?
3. shouldn't the smarter write throttle change have helped with this by makeing zfs do its internal things with a lower priority.

Charts showing iostat -x data

Some Raw Data from Running iostat -x 10

slow   2185.4    0.0 1218.1    0.0 87.3 54.9   65.1 100 100 
slow   2228.3    0.0 1240.8    0.0 79.6 56.1   60.9 100 100 
slow   2204.9    0.0 1232.5    0.0 83.2 56.2   63.2 100 100 
slow   2229.5    0.0 1242.5    0.0 89.6 56.4   65.5 100 100 
slow   2253.0    0.0 1257.1    0.0 95.0 56.5   67.3 100 100 
slow   2279.7    0.0 1267.2    0.0 100.4 58.7   69.8 100 100 
slow   2228.8    0.0 1245.0    0.0 81.9 56.5   62.1 100 100 
slow   2213.3    0.0 1232.8    0.0 86.7 57.9   65.3 100 100 
slow   2175.9    0.0 1211.6    0.0 84.9 54.3   63.9 100 100 
slow   2260.3    0.0 1257.7    0.0 93.9 57.7   67.1 100 100 
slow   2199.3    0.0 1224.3    0.0 84.3 56.3   63.9 100 100 
slow   2227.1    0.0 1241.2    0.0 91.6 57.5   66.9 100 100 
slow   2185.0    0.0 1215.1    0.0 88.0 54.8   65.4 100 100 
slow   2251.4    0.0 1254.8    0.0 92.9 57.9   67.0 100 100 
slow   2281.2    0.0 1272.4    0.0 96.2 59.6   68.3 100 100 
slow   2203.4    0.0 1223.8    0.0 90.8 55.9   66.6 100 100 
slow   2211.9    0.0 1232.0    0.0 91.0 56.0   66.5 100 100 
slow   1189.8  855.3  663.8 3304.7 43341.7 31.9 21207.7  63  66 
slow      0.0 6802.0    0.0 35629.0 4088757.0 11.8 601114.5 100 100 
slow      0.0 7077.3    0.0 44870.2 4431679.0 11.8 626184.3 100 100 
slow      0.0 6367.8    0.0 30357.8 3891257.8 11.9 611084.9 100 100 
slow      0.0 5952.8    0.0 20642.0 3450759.9 11.9 579689.1 100 100 
slow      0.0 6064.2    0.0 25400.6 3143297.5 11.9 518338.6 100 100 
slow      0.0 6635.5    0.0 53655.0 2431519.5 11.8 366443.0 100 100 
slow      0.0 6233.4    0.0 42255.9 1748349.4 11.8 280482.7 100 100 
slow      0.0 6023.6    0.0 28501.1 1124384.1 10.9 186664.9 100 100 
slow      0.0 3398.3    0.0 9049.5 861359.8  6.2 253469.3 100 100 
slow      0.0 3048.8    0.0 7107.1 734775.7  6.0 241007.3 100 100 
slow      0.0 3183.0    0.0 7140.8 623284.5  6.0 195817.4 100 100 
slow      0.0 3171.4    0.0 7063.1 516293.5  6.0 162799.4 100 100 
slow      0.0 3208.0    0.0 7287.1 403837.9  6.0 125886.4 100 100 
slow      0.0 3219.2    0.0 6600.9 292808.2  6.0 90958.8 100 100 
slow      0.0 2552.2    0.0 14747.0 134342.3  4.7 52639.5 100 100 
slow    149.5 1334.6   83.2 6264.1 4403.3  7.1 2971.8  44  46 
slow   2216.2    0.0 2862.5    0.0 88.4 56.6   65.4 100 100 
slow   2207.1    0.0 1230.1    0.0 83.5 54.9   62.7 100 100 
slow   2134.6    0.0 1185.1    0.0 87.5 51.7   65.2 100 100 
slow   2240.1    0.0 1248.5    0.0 88.6 59.0   65.9 100 100 
slow   2281.7    0.0 1270.1    0.0 99.5 58.8   69.4 100 100 
slow   2261.0    0.0 1258.9    0.0 89.3 57.3   64.9 100 100 
slow    351.2 5137.9  194.8 19608.2 691972.9 16.8 126066.3  85  88 
slow      0.0 6077.0    0.0 22531.0 752606.9 11.9 123847.1 100 100 
slow      0.0 4440.1    0.0 21622.6 362138.5  8.5 81562.7 100 100 
slow      0.0 2280.8    0.0 16759.0 85254.3  3.9 37381.0  99  98 
slow   1156.5  152.1  642.4  693.8 64.1 31.5   73.1  56  56 
slow   2209.8    0.0 1226.5    0.0 88.2 54.4   64.6 100 100 
slow   2182.1    0.0 1212.2    0.0 79.2 54.0   61.0 100 100 
slow   2224.2    0.0 1233.8    0.0 87.9 56.5   64.9 100 100 
slow   2247.8    0.0 1251.2    0.0 94.4 58.3   67.9 100 100 
slow   2211.9    0.0 1230.5    0.0 95.7 57.3   69.2 100 100