I found an issue with HDD disk write speed on Hyper-V guest servers when this option is unchecked.

I know the advantage is the speed when this option is checked, but what is the disadvantages?

The HDD is only used for virtual servers, and the server is connected to a UPS.

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It is PRETTY safe - note how it says in case of power failure? A UPS generally avoids power failures. If the server is connected to the UPS properly, it will see that battery power is getting used and start a shutdown in time (as per your configuration) so it never runs out of power (and yes, a shutdown, properly, is flushing those caches).

Generally this is meant to avoid non-ups scenarios where the power suddenly goes "ups" and the cache data is lost. In this case there is no way to know what data was lost and the discs may be in an inconsistent state. RAID controller caches avoid that by having - guess - a battery or similar setup (condensator+flash disc also works for some), and you do that by UPSing the whole server. The main point is that power should NEVER fail - and in your case there is a good enough (make sure of that) battery that it never FAILS - the computer has ample time to properly shut down.

OBVIOUSLY - if you get a crash for some reason (BSOD, driver update freezing the mobo - yes, seen that) - there is no way those flush happens properly in a GUARANTEED fashion. Not sure but IIRC the discs should write the stuff down without intervention - but it may take some time (seconds, me thinks).

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