serverPools := z.getServerPoolsAvailableSpace(ctx)
        total := serverPools.TotalAvailable()
        // choose when we reach this many
        choose := rand.Uint64() % total
        atTotal := uint64(0)
        for _, pool := range serverPools {
                atTotal += pool.Available
                if atTotal > choose && pool.Available > 0 {
                        return pool.Index

| xlHeader       | [4]byte        | `'X', 'L', '2', ' '`
| xlVersionMajor | uint16         | Major xl-meta version.
| xlVersionMinor | uint16         | Minor xl-meta version.
| xlMetaV2       | msgp bin array | Bin array with serialized metadata
| crc            | msgp uint      | Lower 32 bits of 64 bit xxhash of previous array contents (v1.2+ only)