Not 100% positive on all this, but I believe the other suggestion is incorrect.
"Δευτερεύων διακομιστής", or "εφεδρικός" (as it also appears - the latter term being a bit more self-explanatory, perhaps) is the equivalent of "secondary server" - one that's acting as a kind of stand-in when the main server runs the risk of being in need of a breather...
"Sub-server" is just a subsystem under the umbrella of the main server that's been assigned a specific task: there may be a database subserver and an application subserver.
See: http://www.ine.otoe.gr/UplDocs/ekdoseis/leksiko/lexiko_C.pdf (page 13 > clustering: They seem to prefer the longer way of putting - "υποσύστημα διακομιστή" which may not especially suitable for an automated message, such as the one you provided as a context)
See also: http://artemis-new.cslab.ece.ntua.gr:8080/jspui/bitstream/12... (page 51: This guy uses the first term I am suggested - "υπο-εξυπηρετητής")
Personally, I might have preferred "υποδιακομιστής" - but that doesn't really give any search hits, and you may not feel like inventing...
Also worth a glance: https://el.wikipedia.org/wiki/Βικιπαίδεια:Μετάφραση_όρων_πλη...

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Note added at 1 day18 hrs (2016-09-10 11:00:52 GMT) Post-grading
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TYPOS CORRECTED
Not 100% positive on all this, but I believe the other suggestion is incorrect.
"Δευτερεύων διακομιστής", or "εφεδρικός" (as it also appears - the latter term being a bit more self-explanatory, perhaps) is the equivalent of "secondary server" - one that's acting as a kind of stand-in when the main server runs the risk of being in need of a breather...
"Sub-server" is just a subsystem under the umbrella of the main server that's been assigned a specific task: there may be a database subserver and an application subserver.
See: http://www.ine.otoe.gr/UplDocs/ekdoseis/leksiko/lexiko_C.pdf (page 13 > clustering: They seem to prefer the longer way of putting - "υποσύστημα διακομιστή" which may not be especially suitable for an automated message, such as the one you provided as a context)
See also: http://artemis-new.cslab.ece.ntua.gr:8080/jspui/bitstream/12... (page 51: This guy uses the first term I am suggesting - "υπο-εξυπηρετητής")
Personally, I might have preferred "υποδιακομιστής" - but that doesn't really give any search hits, and you may not feel like inventing...
Also worth a glance: https://el.wikipedia.org/wiki/Βικιπαίδεια:Μετάφραση_όρων_πλη...