Imaging mounted disk volumes under duress

Re:Imaging mounted disk volumes under duress by mappu on Tuesday September 21, 2021 @08:02PM (#28611587)

Oracle, MSSQL and some rarer ones (Actian Zen?) are "VSS-Aware" in that they register a writer driver. This gives them a callback before the snapshot starts - a chance to flush transactions and hold off writing any more, for a second or two until the disk snapshot is created and block tracking has begun. Then they get a second callback saying they can resume.

More interesting is e.g. Hyper-V using this to snapshot the guest VMs.

It just takes you one step beyond being only crash-consistent, so that everything is actually application-consistent.

Implementing a VSS writer is a big 90's-style COM interface, but there is also a lightweight alternative model on top of that that eludes my search right now.

Red Hat has some similar convention for freeze/thaw scripts that can make API calls to the DB to ask it to do the same thing.

[ Parent | on HN ]

Re:Imaging mounted disk volumes under duress by jodrellblank on Wednesday September 22, 2021 @01:12AM (#28613252)

> "More interesting is e.g. Hyper-V using this to snapshot the guest VMs."

It can run through VMware tools into virtual machines, too, and down a layer to SAN storage and trigger a SAN snapshot so that a backup system like Veeam can take data from the SAN while knowing the guests two levels higher flushed data before the snapshot was taken.

or e.g. here Nimble SAN can talk up to servers through VSS to trigger SQL Server quiesce before they take a SAN snapshot: https://infosight.hpe.com/InfoSight/media/cms/active/public/...

[ Parent | on HN ]

Re:Imaging mounted disk volumes under duress by pram on Tuesday September 21, 2021 @09:35PM (#28612202)

Does it just buffer to memory during this, or do the databases block writes?

[ Parent | on HN ]