Why remove the memtable from imm_memtables in LsmStorgae.sync()?

[qgaye]

mini-lsm/mini-lsm/src/lsm_storage.rs

Lines 165 to 177 in 130b47b

	// Add the flushed L0 table to the list.
	{
	let mut guard = self.inner.write();
	let mut snapshot = guard.as_ref().clone();
	// Remove the memtable from the immutable memtables.
	snapshot.imm_memtables.pop();
	// Add L0 table
	snapshot.l0_sstables.push(sst);
	// Update SST ID
	snapshot.next_sst_id += 1;
	// Update the snapshot.
	*guard = Arc::new(snapshot);
	}

In sync() we push the memtable to imm_memtables first, but remove it during flush to ssTable. What's the reason for removing memtable from imm_memtables? If we do not remove it, we can get key-value in imm_memtables which is located in memory.

And what is the role of imm_memtables in LsmStorage? It's unclear to me.

[skyzh]

imm_memtables are immutable memtables. Memory is not unlimited, and it will eventually become full. We will need to flush those memtables to disk as SST files. After flushing to disk, you can read them from block cache, and we can also run background compaction so consolidate SSTs.

[qgaye]

Soga. I found that I overlooked block cache in SsTable.
There is another point I don't really understand. The variable imm_memtable seems to be only used for temporarily store memtable. Is this all its functions? or are other aspects ignored by me?

[skyzh]

In the future, we will have flush thread in the background, which flushes imm_memtable to disk. sync will be very light-weight. It will only do a fsync on WAL file.

[qgaye]

very much looking forward to the future chapter