Re: BBU Cache vs. spindles

On Fri, Oct 22, 2010 at 12:05 PM, Kevin Grittner
<Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
> Rob Wultsch <wultsch(at)gmail(dot)com> wrote:
>
>> I would think full_page_writes=off + double write buffer should be
>> far superior, particularly given that the WAL is shipped over the
>> network to slaves.
>
> For a reasonably brief description of InnoDB double write buffers, I
> found this:
>
> http://www.mysqlperformanceblog.com/2006/08/04/innodb-double-write/
>
> One big question before even considering this would by how to
> determine whether a potentially torn page "is inconsistent".
> Without a page CRC or some such mechanism, I don't see how this
> technique is possible.
>
> Even if it's possible, it's far from clear to me that it would be an
> improvement.  The author estimates (apparently somewhat loosely)
> that it's a 5% to 10% performance hit in InnoDB; I'm far from
> certain that full_page_writes cost us that much.  Does anyone have
> benchmark numbers handy?
>
> -Kevin
>

Ignoring (briefly) the cost in terms of performance of the different
system, not needing full_page_writes would make geographically
dispersed replication possible for certain cases where it is not
currently (or at least rather painful).

--
Rob Wultsch
wultsch(at)gmail(dot)com