diff --git a/app/models/ci/job_trace_chunk.rb b/app/models/ci/job_trace_chunk.rb
index aa92bd7e3c1..baa48a602f1 100644
--- a/app/models/ci/job_trace_chunk.rb
+++ b/app/models/ci/job_trace_chunk.rb
@@ -39,7 +39,7 @@ module Ci
         raise 'Unsupported data store'
       end
 
-      save if changed?
+      save! if changed?
       schedule_to_db if fullfilled?
     end
 
@@ -49,7 +49,7 @@ module Ci
 
     def append(new_data, offset)
       current_data = self.data || ""
-      raise 'Offset is out of bound' if offset > current_data.bytesize
+      raise 'Offset is out of bound' if offset > current_data.bytesize || offset < 0
       raise 'Outside of chunk size' if CHUNK_SIZE < offset + new_data.bytesize
 
       self.set_data(current_data.byteslice(0, offset) + new_data)
@@ -73,6 +73,7 @@ module Ci
 
     def use_database!
       return if db?
+      return unless size > 0
 
       self.update!(raw_data: data, data_store: :db)
       redis_delete_data
diff --git a/spec/factories/ci/job_trace_chunks.rb b/spec/factories/ci/job_trace_chunks.rb
index f24e015f186..e2cc2e77dda 100644
--- a/spec/factories/ci/job_trace_chunks.rb
+++ b/spec/factories/ci/job_trace_chunks.rb
@@ -1,5 +1,7 @@
 FactoryBot.define do
   factory :ci_job_trace_chunk, class: Ci::JobTraceChunk do
     job factory: :ci_build
+    chunk_index 0
+    data_store :redis
   end
 end
diff --git a/spec/models/ci/job_trace_chunk_spec.rb b/spec/models/ci/job_trace_chunk_spec.rb
new file mode 100644
index 00000000000..fa316159e1a
--- /dev/null
+++ b/spec/models/ci/job_trace_chunk_spec.rb
@@ -0,0 +1,312 @@
+require 'spec_helper'
+
+describe Ci::JobTraceChunk, :clean_gitlab_redis_shared_state do
+  set(:job) { create(:ci_build, :running) }
+  let(:chunk_index) { 0 }
+  let(:data_store) { :redis }
+  let(:raw_data) { nil }
+  let(:job_trace_chunk) do
+    described_class.new(job: job, chunk_index: chunk_index, data_store: data_store, raw_data: raw_data)
+  end
+
+  describe '#data' do
+    subject { job_trace_chunk.data }
+
+    context 'when data_store is redis' do
+      let(:data_store) { :redis }
+
+      before do
+        job_trace_chunk.send(:redis_set_data, 'Sample data in redis')
+      end
+
+      it { is_expected.to eq('Sample data in redis') }
+    end
+
+    context 'when data_store is database' do
+      let(:data_store) { :db }
+      let(:raw_data) { 'Sample data in db' }
+
+      it { is_expected.to eq('Sample data in db') }
+    end
+
+    context 'when data_store is others' do
+      before do
+        job_trace_chunk.send(:write_attribute, :data_store, -1)
+      end
+
+      it { expect { subject }.to raise_error('Unsupported data store') }
+    end
+  end
+
+  describe '#set_data' do
+    subject { job_trace_chunk.set_data(value) }
+
+    let(:value) { 'Sample data' }
+
+    context 'when value bytesize is bigger than CHUNK_SIZE' do
+      let(:value) { 'a' * (described_class::CHUNK_SIZE + 1) }
+
+      it { expect { subject }.to raise_error('too much data') }
+    end
+
+    context 'when data_store is redis' do
+      let(:data_store) { :redis }
+
+      it do
+        expect(job_trace_chunk.send(:redis_data)).to be_nil
+
+        subject
+
+        expect(job_trace_chunk.send(:redis_data)).to eq(value)
+      end
+
+      context 'when fullfilled chunk size' do
+        let(:value) { 'a' * described_class::CHUNK_SIZE }
+
+        it 'schedules stashing data' do
+          expect(StashTraceChunkWorker).to receive(:perform_async).once
+
+          subject
+        end
+      end
+    end
+
+    context 'when data_store is database' do
+      let(:data_store) { :db }
+
+      it 'sets data' do
+        expect(job_trace_chunk.raw_data).to be_nil
+
+        subject
+
+        expect(job_trace_chunk.raw_data).to eq(value)
+        expect(job_trace_chunk.persisted?).to be_truthy
+      end
+
+      context 'when raw_data is not changed' do
+        it 'does not execute UPDATE' do
+          expect(job_trace_chunk.raw_data).to be_nil
+          job_trace_chunk.save!
+
+          # First set
+          expect(ActiveRecord::QueryRecorder.new { subject }.count).to be > 0
+          expect(job_trace_chunk.raw_data).to eq(value)
+          expect(job_trace_chunk.persisted?).to be_truthy
+
+          # Second set
+          job_trace_chunk.reload
+          expect(ActiveRecord::QueryRecorder.new { subject }.count).to be(0)
+        end
+      end
+
+      context 'when fullfilled chunk size' do
+        it 'does not schedule stashing data' do
+          expect(StashTraceChunkWorker).not_to receive(:perform_async)
+
+          subject
+        end
+      end
+    end
+
+    context 'when data_store is others' do
+      before do
+        job_trace_chunk.send(:write_attribute, :data_store, -1)
+      end
+
+      it { expect { subject }.to raise_error('Unsupported data store') }
+    end
+  end
+
+  describe '#truncate' do
+    subject { job_trace_chunk.truncate(offset) }
+
+    shared_examples_for 'truncates' do
+      context 'when offset is negative' do
+        let(:offset) { -1 }
+
+        it { expect { subject }.to raise_error('Offset is out of bound') }
+      end
+
+      context 'when offset is bigger than data size' do
+        let(:offset) { data.bytesize + 1 }
+
+        it { expect { subject }.to raise_error('Offset is out of bound') }
+      end
+
+      context 'when offset is 10' do
+        let(:offset) { 10 }
+
+        it 'truncates' do
+          subject
+
+          expect(job_trace_chunk.data).to eq(data.byteslice(0, offset))
+        end
+      end
+    end
+
+    context 'when data_store is redis' do
+      let(:data_store) { :redis }
+      let(:data) { 'Sample data in redis' }
+
+      before do
+        job_trace_chunk.send(:redis_set_data, data)
+      end
+
+      it_behaves_like 'truncates'
+    end
+
+    context 'when data_store is database' do
+      let(:data_store) { :db }
+      let(:raw_data) { 'Sample data in db' }
+      let(:data) { raw_data }
+
+      it_behaves_like 'truncates'
+    end
+  end
+
+  describe '#append' do
+    subject { job_trace_chunk.append(new_data, offset) }
+
+    let(:new_data) { 'Sample new data' }
+    let(:offset) { 0 }
+    let(:total_data) { data + new_data }
+
+    shared_examples_for 'appends' do
+      context 'when offset is negative' do
+        let(:offset) { -1 }
+
+        it { expect { subject }.to raise_error('Offset is out of bound') }
+      end
+
+      context 'when offset is bigger than data size' do
+        let(:offset) { data.bytesize + 1 }
+
+        it { expect { subject }.to raise_error('Offset is out of bound') }
+      end
+
+      context 'when offset is bigger than data size' do
+        let(:new_data) { 'a' * (described_class::CHUNK_SIZE + 1) }
+
+        it { expect { subject }.to raise_error('Outside of chunk size') }
+      end
+
+      context 'when offset is EOF' do
+        let(:offset) { data.bytesize }
+
+        it 'appends' do
+          subject
+
+          expect(job_trace_chunk.data).to eq(total_data)
+        end
+      end
+
+      context 'when offset is 10' do
+        let(:offset) { 10 }
+
+        it 'appends' do
+          subject
+
+          expect(job_trace_chunk.data).to eq(data.byteslice(0, offset) + new_data)
+        end
+      end
+    end
+
+    context 'when data_store is redis' do
+      let(:data_store) { :redis }
+      let(:data) { 'Sample data in redis' }
+
+      before do
+        job_trace_chunk.send(:redis_set_data, data)
+      end
+
+      it_behaves_like 'appends'
+    end
+
+    context 'when data_store is database' do
+      let(:data_store) { :db }
+      let(:raw_data) { 'Sample data in db' }
+      let(:data) { raw_data }
+
+      it_behaves_like 'appends'
+    end
+  end
+
+  describe '#size' do
+    subject { job_trace_chunk.size }
+
+    context 'when data_store is redis' do
+      let(:data_store) { :redis }
+
+      context 'when data exists' do
+        let(:data) { 'Sample data in redis' }
+
+        before do
+          job_trace_chunk.send(:redis_set_data, data)
+        end
+
+        it { is_expected.to eq(data.bytesize) }
+      end
+
+      context 'when data exists' do
+        it { is_expected.to eq(0) }
+      end
+    end
+
+    context 'when data_store is database' do
+      let(:data_store) { :db }
+
+      context 'when data exists' do
+        let(:raw_data) { 'Sample data in db' }
+        let(:data) { raw_data }
+
+        it { is_expected.to eq(data.bytesize) }
+      end
+
+      context 'when data does not exist' do
+        it { is_expected.to eq(0) }
+      end
+    end
+  end
+
+  describe '#use_database!' do
+    subject { job_trace_chunk.use_database! }
+
+    context 'when data_store is redis' do
+      let(:data_store) { :redis }
+
+      context 'when data exists' do
+        let(:data) { 'Sample data in redis' }
+
+        before do
+          job_trace_chunk.send(:redis_set_data, data)
+        end
+
+        it 'stashes the data' do
+          expect(job_trace_chunk.data_store).to eq('redis')
+          expect(job_trace_chunk.send(:redis_data)).to eq(data)
+          expect(job_trace_chunk.raw_data).to be_nil
+
+          subject
+
+          expect(job_trace_chunk.data_store).to eq('db')
+          expect(job_trace_chunk.send(:redis_data)).to be_nil
+          expect(job_trace_chunk.raw_data).to eq(data)
+        end
+      end
+
+      context 'when data does not exist' do
+        it 'does not call UPDATE' do
+          expect(ActiveRecord::QueryRecorder.new { subject }.count).to eq(0)
+        end
+      end
+    end
+
+    context 'when data_store is database' do
+      let(:data_store) { :db }
+
+      it 'does not call UPDATE' do
+        expect(ActiveRecord::QueryRecorder.new { subject }.count).to eq(0)
+      end
+    end
+  end
+end