Re: [boost] NuDB: A fast key/value insert-only database for SSD drives in C++11

Subject: Re: [boost] NuDB: A fast key/value insert-only database for SSD drives in C++11
From: Asbjørn (lordcrc_at_[hidden])
Date: 2017-03-29 09:13:31

On 29.03.2017 08:18, Niall Douglas via Boost wrote:
> Whatever is lost is lost, the *key* feature is that
> damaged data doesn't cause further data loss.

I'm struggling to see how you can guarantee that without _any_ guarantees from 
the OS or hardware.

>> If so, why throw it all away? Maybe the user has an OS, a filesystem and
>> some hardware which can guarantee this?
>
> Because a proper implementation of durability should be able to use no
> fsync and no O_SYNC at all. In that case, you get "late durability"
> where minutes of recent writes get lost after power loss. For users
> where that is unacceptable, O_SYNC should be turned on and you now have
> "early durability" where only seconds may be lost. You pay for that
> early durability with much reduced performance.
>

Without O_SYNC and fsync, replace "minutes" with "hours" or "days". This may be 
entirely unacceptable. With O_SYNC you get horrible performance as you note, 
which may be entirely unacceptable.

Also, I'm assuming the hardware may ignore the O_SYNC as much as it can ignore 
the fsync, in which case you're SOL anyway.

Regards
- Asbjørn