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: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2017-03-27 13:42:29

>> The issue I am taking here is the claim of durability
> 
> This is the first I'm hearing that the "Durability" in ACID implies
> protection from data corruption after the fact - please provide a
> source.

It doesn't imply that.

Durability means that corruption to the database will not cause further
data loss during subsequent use. For example, if you use a single bit to
indicate that a record is deleted, and corruption flips that bit to
deleted, and your implementation has no way of noticing the corruption,
you have lost data after the corruption. Ideally when a user next
accesses that record, they should see an error like "Record corrupt".

There is a fun history of corruption and SQLite at
https://www.sqlite.org/howtocorrupt.html. Last time I looked, there was
a popular fork of SQLite which implements per-row checksumming, but the
default build does not (the canonical advice is: "use a proper filing
system like ZFS if you don't want bit errors"). But SQLite is very
carefully written to check consistency during modifications, and where
it can it will refuse to modify data when the metadata doesn't match up.

So, a database can be durable and not detect arbitrary damage to user
data. It just cannot lose further user data due to corruption of its own
structures.

Niall

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