tgtd --bstype aio

Please create option (selector) for use --bstype aio for tgtadm

rdwr : Use normal file I/O. This is the default for disk devices

aio : Use Asynchronous I/O

https://linux.die.net/man/8/tgtadm

On my Servers used Bcache and if  full trasfer or backup use standard type rdwr i view in some transfers broke data. After i change target mode to  aio  all be work good

aio more safe for data