> But how to avoid system hangs due to running out of memory?
> Is there a safe guide line? Generally slow is tolerable, but
> crash is not.
If the system runs out of memory, it may try to flush pages that are
queued to your NBD device. That will try to allocate more memory for
sending packets, which will fail, meaning the VM can never make progress
freeing pages. Now your box is dead.
The only way to deal with this is to have a scheme for per-socket memory
reservations in the network layer and have NBD reserve memory for sending
and acknowledging packets. NFS and iSCSI also need this, though it's a
bit harder to tickle for NFS. SCSI has DMA reserved memory for analogous
reasons.
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