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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