Volume Migration

[tmart]

This reference: http://doc.3tera.com/AppLogic24/CliVolume.html#AnchorMigrate indicates the usage for volume migration. I am not completely clear on what a volume migration is. Since it bears little resemblance to application migration (no syntax for grid descriptor), I cannot imagine that it helps to migrate volumes between grids. My next guess might have been movement of a volume mirror off of one node and onto another, but again the syntax does not appear to support this meaning.

So what is volume migration and when might I need to do it?

[PeterNic]

Tim,

Sorry - badly named command; especially after we added support for volume import and export, I agree it is confusing.

The volume migrate command allows you to move the mirror streams of the volume between physical servers of the grid. The command is intended to assist you with freeing up servers you want to get removed from the grid -- if you disable a server, say srv5, and then do 'vol migrate --all', AppLogic will move any volume streams that reside on srv5 to other servers, so that you can remove the server later, without affecting the redundancy. (For now, you can't move both streams at the same time, so if you need to remove multiple servers, you may have to disable server/migrate volumes in order.)

The command has also been used to coax volume mirrors to go to particular servers, especially when trying to optimize the performance by placing streams in a way that maximizes I/O throughput for the application (e.g., one high-performance volume per pair of servers); in most cases, this is not needed, but if you really want to squeeze the last ounce of performance, you can do so (also, in that case, don't forget to create the volumes with the --prefill option -- the volume creation will take a long time but all blocks will be fully allocated and likely sequential, maximizing both read and write performance). When I say "coax", that's what I mean -- the isn't a "target=" option in which you can specify where you want the streams placed (since this wasn't really the intent of the command).

I will post this command for review for the next release, and both improve the documentation and see if we can pick a different name for the command.

Regards,
-- Peter