Dreadful gmirror performance - suggested changes to 'prefer'

[Ivan Voras]

Pete French wrote:
> I am just looking at this again, and am in a bit of a mood
> for writing some patches, so I wanted to run the following idea past people
> as regards the priority system in the 'prefer' balancing method.
> 
> Just to recap, creating a gmirror creates the first device with priority
> zero. Adding extra devices lets you set their priorities, but you cant
> set negative values. The upshot is that the first device in the mirror
> always has the lowest priority - so you cannot (for example) create a mirror
> with a local disc, subsequently add a remote disc, but then set the mirror
> up to prefer the local drive.

Ok.

> I am thinking of a couple of changes - the first being the patch prroposed
> from Andrew Snow which would create the mirror with the priority at something
> other than zero (128 would be my preference) so that extra devices can be
> inserted both above and below it. This solves the problem for newly
> created mirrors as the priority can then be set as appropiate.
> 
> The other change I wanted to make was to add a second 'prefer' mode to gmirror
> though - one which would prefer the *lowest* priority instead of the
> highest priority. I would probably rename the existing mode to 'prefer-high'
> (keeping 'prefer' as a synonym for backward compatibility) and add
> a 'prefer-low' as well. As an existing gmirror can have it's load balance
> algorithm changed on the fly, this lets you change which of the drives
> is preferred in an existing installationg. This is precisely what you need
> when switching between two machines so that the local and remote drives
> become reversed.
> 
> I havent looked at the code in detail, but I can't see that it would be too
> difficult. What do people think ?

Couple of ideas:

- Don't use "128" as the default since it will lead people to think
there's an 8-bit quantity behind the setting (and subsequently develop
weird theories about how the setting works), when it isn't so. Use 100
or 1000.
- Why not go all the way and make another argument or a switch that will
specify exactly which drive to prefer, so you could say "prefer N",
where N is any drive / component, not only the one with lowest/highest
priority? This is slightly more complex and will probably require an
addition to the metadata (which isn't complicated but you have to be
careful) and a workaround so the old semantics of the "plain" setting is
supported.


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