As shown in Figure 5, the four kinds of blocks are
tracked in four different zones: Free Zone, Available Zone,
Unavailable Zone, Checkpointed Zone. Conventional FTLs
already distinguish between the free, available, and
other (i.e., unavailable + checkpointed) blocks. LightTx
requires the differentiation of the unavailable and checkpointed
blocks, in addition. To enable this differentiation,
the addresses of flash blocks in the Unavailable Zone are
added to a zone metadata page at each checkpoint. The
checkpoint operation is periodically performed to move
each flash block to its appropriate zone (if the block’s
zone has changed). We call this zone sliding.
Zone Sliding. Zone sliding moves those flash blocks
that have their states changed to new appropriate zones.
Among the four zones, only the Available and Unavailable
Zones have blocks whose states are potentially modified
during the interval between two checkpoints. This is
because new pages can only be updated in the Available
Zone and only the Available and Unavailable Zones have
active transactions that can be committed or aborted.
Thus, only the states of transactions in these two zones
need to be checked during zone sliding.
Zone sliding is triggered when the Available Zone runs
short of free space. When it is triggered, the flash blocks
in the Available and Unavailable Zones are checked and
moved if their states have been changed. Then, free
flash blocks are allocated to the Available Zone. To reduce
the frequency of zone sliding, LightTx preallocates a
dedicated number of free blocks from the Free Zone
to the Available Zone. Movement of flash blocks from
the Checkpointed Zone to the Free Zone is performed
during garbage collection of the FTL, using conventional
garbage collection strategy. As such, with both zone sliding
and garbage collection, the flash blocks are moved
between the four zones.
Example: Figure 5 illustrates a snapshot view of an SSD with
20 flash blocks, where each block has four pages. Tag denotes the TxID i and TxCnt j (TxVer is not illustrated
in this figure). Using the zone-based tracking scheme, the