The performance of disks inside the Proxmox host virtual Windows machine has dropped sharply, why?

Question

linux

Hetzner has a Proxmox host, 2*4GB Sata disks, ZFS, it has several Windows virtual machines. The speed of disks inside virtual machines has dipped a lot. Shows normal speed on host
# dd bs=10M count=1024 if=/dev/zero of=test oflag=dsync
1024+0 records in
1024+0 records out
10737418240 bytes (11 GB) copied, 46.5154 s, 231 MB/s
smart drives and zfs utilities do not show anything suspicious or I do not understand.
In virtual machines (2 pcs.), The speed sank equally strongly, turned them off one by one. Both brake independently from each other. I tested the disks with the HDTunePRO utility - it shows about 15-20 MB (normally 150-200 MB).
Windows itself takes noticeably longer to load.
Host reboots at night, the reason is not understood yet.
Give advice on how to diagnose.

Answer 1

check smart disks, one ssd dies on my last problem server and if you have a mirror then wow miracle

Answer 2

How much RAM does the hypervisor have? Or rather, how much is left free? ZFS needs it badly. There was also a spontaneous reload when ZFS ran out of memory for its operations. Also Proxmox 4