Open-iscsi: How to enable writeback / increase write speed on mdadm RAID-5/6?

Question

linux

A long time ago it was discovered that if the server that holds the iscsi target writes to the mdadm raid-5/6 array, the performance will be limited to the speed of 1 disk, because. after each write, the parity disk will need to be updated, and the iscsi server waits for the write to complete.
In the old iscsitarget - you could set IOMode=wb (enable writeback on the server) - and the recording went at the maximum write speed to all disks in parallel. In Ubuntu 16.10, iscsitarget was nailed, and I had to migrate to open-iscsi, but there I did not find how to enable writeback on the server.
How to turn it on? Enabling writeback on the client does not solve this problem, after the write cache is full, the speed drops to one disk.

Answer 1

performance will be limited to the speed of 1 disk, tk. after each entry will need to be updated the parity disk

This is raid4 with a separate disk for redundancy storage.
In 5 and 6, redundant blocks are spread evenly across all disks in the array, rather than a separate disk is allocated to store the redundant block.
/sys/block/md?/md/stripe_cache_size fixed? Bitmap internal/external/disabled? With internal recording performance, too, sags well.