First of all, uptime of 95% means 18 and a quarter entire days of downtime per year. That's horrendous. I wouldn't host my dog's website on a server with that kind of SLA - and I don't even have a dog.
Secondly, although Twitter got away with large helpings of downtime, that doesn't mean that every business type can. Twitter is not (or at least was not, for most of its existence) business-critical to anyone. If Twitter goes down, oh well. Shucks.
If you're running recently-featured-on-HN Stripe, however, where thousands or more other businesses depend on you keeping your servers up to make their money, I'd say even 10 minutes of downtime is unacceptable.
Finally, this doesn't have to cost a lot. Just find a host that offers the SLA you're looking for, and have a reasonably fast failover to another similar host somewhere else.
The definition of "uptime", the host SLAs only covers network uptime and environment uptime, but clients consider "uptime" to mean application level uptime, which includes downtimes for server maintenance, steady-state backups, backup restores, deploying new releases, etc ... anything other than service is not 100% fully functional = downtime in their minds.
Also for costs, "reasonably fast failover to another similar host" implies live redundant equipment at another host which is double the hosting costs, that's a big pill to swallow, so big that most orgs would rather suffer the downtime when they see the real cost of full redundancy.
> First of all, uptime of 95% means 18 and a quarter entire days of downtime per year. That's horrendous.
It may be acceptable to some clients depending on what other provisions are part of the SLA (though probably not with as little as 95%).
I've seen a 98% SLA which was applied both annually (~7-ana-third days) and daily (2% of a day being about half an hour) with significant remuneration if the daily SLA was not kept as well as the annual one. If I remember rightly, maintenance windows counted against the SLA except in certain (specified in the contract) circumstances.
Of course for many applications this would still be completely unacceptable, but for others it might be fine depending on the costs and the comeback if the SLA is broken.
Secondly, although Twitter got away with large helpings of downtime, that doesn't mean that every business type can. Twitter is not (or at least was not, for most of its existence) business-critical to anyone. If Twitter goes down, oh well. Shucks.
If you're running recently-featured-on-HN Stripe, however, where thousands or more other businesses depend on you keeping your servers up to make their money, I'd say even 10 minutes of downtime is unacceptable.
Finally, this doesn't have to cost a lot. Just find a host that offers the SLA you're looking for, and have a reasonably fast failover to another similar host somewhere else.