Space Maps from Space
By relling on Sep 18, 2007
Jeff Bonwick recently blogged about why ZFS uses space maps for keeping track of allocations. In my recent blog on looking at ZFS I teased you with a comment about the space map floating near the Channel Islands. Now that Jeff has explained how they work, I'll show you what they look like as viewed from space.
This is a view of a space map for a ZFS file system which was created as a recursive copy of the /usr directory followed by a recursive remove of the /usr/share directory. This allows you to see how some space is allocated and some space is free.
I wrote an add-on to NASA's Worldwind to parse zdb output looking for the space map information. Each allocation appears as a green rectangle with a starting offset and length mapped onto a square field floating above the earth. The allocations are green and the frees are yellow. The frees are also floating 100m above the allocations, though it is not easy to see from this view. Each map entry also has an optional user-facing icon which shows up as a shadowed green or yellow square. I snagged these from the StarOffice bullets images. If you hover the mouse over an icon, then a tool tip will appear showing the information about the space. In this example, the tooltip says "Free, txg=611, pass=1, offset=53fe000, size=800"
I can think of about a half dozen cool extensions to make for this, such as showing metaslab boundaries. I also need to trim the shadow field to fit; it extends too far on the right. So much to do, so little time...