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How does write-back cache work?

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How does write-back cache work?

Postby danswartz » Mon Nov 25, 2019 4:57 pm

I'm curious about this. If a write comes in from an iSCSI client, is it acked right away? If so, what happens if the synch link goes down before it gets written, and this is the node that is demoted (e.g. no long visible to vsphere?) Wouldn't that expose you to lost write(s)? Or is the write not acked until it has been sent over the synch link to the other node, and has been acknowledged as received there, even if the other node hasn't written it yet?
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Re: How does write-back cache work?

Postby yaroslav (staff) » Wed Nov 27, 2019 11:37 am

Data is acked to be written only if written to the underlying storage.
Should the Sync link go down, both servers are to decide which one is to stop serving client writes; writing to the node that has lower synchronization priority will be stopped afterward. That's how VSAN helps to avoid split-brain with Heartbeat.
If there's no sync link, no acknowledgment comes from the partner node. Small note: If both nodes are active and synchonized, clients cannot get an acknowledge package until data is confirmed to be safely written to the partner host.
If you demote the node from vCenter (without shutting it down) everything you have in server's RAM should stay there. Once you unplug or reboot that host, data is lost. That's how non-persistent memory works.
Here’s how L1 cache works in StarWind VSAN: https://knowledgebase.starwindsoftware. ... rinciples/
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