dm-era sample setup

#!/bin/sh

# the big device holding data
DATA_DEV=/dev/sdc

# the smaller device which will hold metadata
# (apparently has to be a different _physical_ device than the one holding data!)
ERA_DEV=/dev/sdb


LOGICAL_BLOCKSIZE_DATA=`blockdev --getbsz ${DATA_DEV}`
PHYS_BLOCKSIZE_DATA=`blockdev --getpbsz ${DATA_DEV}`  # sector size, usually 512
PHYS_BLOCKSIZE_ERA=`blockdev --getpbsz ${ERA_DEV}`    # sector size, usually 512

NUM_SECTORS_DATA=`blockdev --getsz ${DATA_DEV}`

# from the offical dm-era doc:
#   memory_usage for era = (4 * nr_blocks) bytes + buffers
#
# (we assume that 'nr_blocks' references to logical blocks here)
# ('blockdev --getsz' however returns number of sectors, i.e. 512 byte blocks)
# -> we need to calculate the number of logical blocks from the given values
#
# FULL_SIZE_DATA = NUM_SECTORS_DATA * PHYS_BLOCKSIZE_DATA
FULL_SIZE_DATA=`echo "scale=0;$NUM_SECTORS_DATA * $PHYS_BLOCKSIZE_DATA" | bc -l`
# LOGICAL_BLOCKS_DATA = FULL_SIZE_DATA / LOGICAL_BLOCKSIZE_DATA
LOGICAL_BLOCKS_DATA=`echo "scale=0;$FULL_SIZE_DATA / $LOGICAL_BLOCKSIZE_DATA" | bc -l`
#
# now calculate formula like in the doc: memory_usage = (4 * nr_blocks) bytes + buffers
BUF_SIZE=5  # amount of 4k buffers included in the metadata device
NUM_SECTORS_ERA=`echo "scale=0;( (4 * $LOGICAL_BLOCKS_DATA) + (4096 * $BUF_SIZE) ) / $PHYS_BLOCKSIZE_ERA" | bc -l`

# part of /dev/sdb for (era) metadata
dmsetup create metadev-era --table "0 ${NUM_SECTORS_ERA} linear ${ERA_DEV} 0"

# whole /dev/sdc for data (linear mapping)
dmsetup create metadev-data --table "0 ${NUM_SECTORS_DATA} linear ${DATA_DEV} 0"

# era device layer above the linear /dev/sdc mapping to track changed blocks within /dev/sdc
dmsetup create era --table "0 ${NUM_SECTORS_ERA} era /dev/mapper/metadev-era /dev/mapper/metadev-data ${LOGICAL_BLOCKSIZE_DATA}"

# snapshot device for the metadata (you can't access /dev/mapper/era-dev-sdc directly!)
dmsetup create era-access --table "0 ${NUM_SECTORS_ERA} linear /dev/mapper/metadev-era 0"

# write layer for the data mapping (since the era device locks down our initial linear mapping)
dmsetup create metadev-data-access --table "0 ${NUM_SECTORS_DATA} linear /dev/mapper/metadev-data 0"

# mount
mkdir -p /media/sdc-data
mount /dev/mapper/metadev-data-access /media/sdc-data


## PLAYING AROUND

# taking a metadata snapshot (before accessing it)
#   dmsetup message era 0 take_metadata_snap

# reading the metadata
#   era_dump /dev/mapper/era-access

# get changed blocks since specific era <ERA>
#   era_invalidate --written-since <ERA> /dev/mapper/era-access