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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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Re: How does write-back cache work?

Postby asherxo » Mon Aug 24, 2020 9:45 am

yaroslav (staff) wrote: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/

Thanks for the detailed explanation yaroslav. And thanks for the link to knowledgebase. Helped a lot. thanks again
Last edited by asherxo on Thu Sep 03, 2020 3:32 pm, edited 1 time in total.
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Re: How does write-back cache work?

Postby yaroslav (staff) » Mon Aug 24, 2020 10:15 am

You are always welcome :)
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Re: How does write-back cache work?

Postby richardcharles » Wed Sep 23, 2020 6:22 am

Hi all,

I have a standalone win2018 with a parity accelerated volume.

As I can understand, in the faster ssd storage tiers we have the most frequent accessed files.

But what about the write back cache? What kind of data will go through the WBC? Block or files?

Thanks in advance
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Re: How does write-back cache work?

Postby yaroslav (staff) » Wed Sep 23, 2020 7:06 am

Greetings,

Win2018, there is no such a WS :) Is it 2019?
Blocks are cached. Learn more about Write-Back Cache implementation at https://knowledgebase.starwindsoftware. ... principles.
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