On Fri, Jan 15, 2016 at 5:15 AM, Tim Ward t...@telensa.com
[firebird-support] <firebird-support@yahoogroups.com> wrote:
> I know the folklore around forced writes
>
> - turning forced writes on is safer
>
Not exactly folklore. Firebird doesn't have a journal but instead depends
on careful writes to keep the database constantly consistent. After a
crash - system or database - there's no recovery process. The database is
available instantly.
The basic principle of careful write is writing data before pointers. When
adding data, the page with the data is written before the page that points
to it. When removing data, the pointer is cleared before the data. This
principal applies to internal structures like pointers as well as records
and indexes. If you allow the operating system to write pages in its
preferred order, the database on disk may be inconsistent. As long as
nothing goes wrong, the state of the database on disk isn't an issue. At
one time, Windows didn't flush its page cache until a file was closed,
leading to losing hours or days of work.
What's different now is that machines are both faster and more reliable.
If nothing crashes, careful write doesn't matter.
> - turning forced writes on is slower
> - the safety bit is much more of an issue with Windows than with Linux,
> to the extent that it used not to work on Linux and nobody noticed for
> years.
>
> But I haven't found any actual, y'know, like, data, evidence, so on.
> Things like (with reference to Linux, I'm not interested in Windows):
>
> (1) Is there any data about how much slower it is, eg has anyone done
> any benchmark runs on their systems? - I've found just one blog entry
> somewhere with a number ("up to three times slower") but without any
> (published) data behind it.
>
I wouldn't particularly trust benchmarks that are more than three years
old.
>
> (2) Is there any evidence about how much safer it is? Statistics on
> corruptions with and without? Analysis of individual database corruption
> events showing whether turning on forced writes would or would not have
> prevented the corruption event?
>
The evidence I have is too old to be relevant, so I can offer only theory,
not
practice. In theory, there should be no difference...
Cheers,
Ann
