Finally I put 5GB of RAM in my server but the same problem happen so I delete my iscsi target , deleted my iscsi volume , recreate every thing from scratch , used internal Dell Raid on 280GB target in Basic and GPT , used 2TB volume size on Xraid in Basic Disk and GPT, used 2.5TB in GPT and I could not reproduce the problem so the only options that I did'nt test was the initialize when you create an ISCSI volume, as soon as I recreated a volume with initialize fonction TADA problem come back , I created a 8GB volume with initialize function turn on and from this point my target become sluggish and irregular when I used it , if I create a volume without initialize everything is working like is suppose to so now I don't know where to look at