I was reading http://www.h2database.com/html/advanced.html#durability_problems and i found
Some databases claim they can guarantee durability, but such claims are wrong. A durability test was run against H2, HSQLDB, PostgreSQL, and Derby. All of those databases sometimes lose committed transactions. The test is included in the H2 download, see org.h2.test.poweroff.Test
Also it says
Where losing transactions is not acceptable, a laptop or UPS (uninterruptible power supply) should be used.
So is there any database that is durable. The document says about fsync() command and most hard drives do not obey fsync(). It also talks about no reliable way to flush hard drive buffers
So, is there a time after which a committed transaction becomes durable, so we can buy ups that gives minimum that much backup of power supply.
Also is there a way to know that a transaction committed is durable. Suppose we don't buy ups and after knowing that a transaction is durable we can show success message.
The problem depends on whether or not you can instruct the HDD/SDD to commit transactions to durable media. If the mass storage device does not have the facility to flush to durable media, then no data storage system on top of it can be said to be truely durable.
There are plenty of NAS devices with built in UPS however and these seem to fit the requirement for durable media - if the database on a seperate server commits data to that device and does a checkpoint then the commits are flushed to the media. So long as the media survives a power outage then you can say its durable. The UPS on the NAS should be capable of issuing a controlled shutdown to its associated disk pack, guaranteeing permenance.
Alternatively you could use something like SQL Azure which writes commits to multiple (3) seperate database storage instances on different servers. Although we have no idea if those writes ever reach a permenant storage media, it doesnt actually matter - the measurement of durability is read-repeatability; and this seems to meet that requirement.