def by_vol_id(blocks, slist=None):
    vol_blocks = {}
    for i in blocks:
        if slist and i not in slist:
            continue
        elif not blocks[i].is_valid:
            continue
        if blocks[i].vid_hdr.vol_id not in vol_blocks:
            vol_blocks[blocks[i].vid_hdr.vol_id] = []
        vol_blocks[blocks[i].vid_hdr.vol_id].append(blocks[i].peb_num)
    return vol_blocks