Mohwa Blog

도커 빌드 캐시

January 09, 2022

이 글을 더 정확히 이해하기위해, 도커 중간 이미지 및 레이어를 먼저 읽어보시는것을 추천드립니다.

테스트 환경

먼저, Dockerfile 생성합니다.

FROM alpine:latest
RUN echo first
COPY ./second.txt .
ADD ./third.txt .
CMD ["echo", "fourth"]

첫 번째 이미지를 생성한 경우

우선 생성된 모든 이미지들이 저장되는 공간을 확인해보면, 모두 비어있는것을 확인할 수 있습니다.

[root@docker-test docker]# ls /var/lib/docker/image/overlay2/imagedb/content/sha256

docker build 명령으로, docker-layers:0.0.1 이미지를 생성합니다.

[root@docker-test docker-layer]# docker build -t docker-layers:0.0.1 .

Sending build context to Docker daemon  4.096kB
Step 1/5 : FROM alpine:latest
latest: Pulling from library/alpine
59bf1c3509f3: Pull complete
Digest: sha256:21a3deaa0d32a8057914f36584b5288d2e5ecc984380bc0118285c70fa8c9300
Status: Downloaded newer image for alpine:latest
 ---> c059bfaa849c
Step 2/5 : RUN echo first
 ---> Running in 24aaa5617f83
first
Removing intermediate container 24aaa5617f83
 ---> 3a18fe3979f0
Step 3/5 : COPY ./second.txt .
 ---> 6da5ddb808b7
Step 4/5 : ADD ./third.txt .
 ---> a537bad7aa2b
Step 5/5 : CMD ["echo", "fourth"]
 ---> Running in da1be8fb3579
Removing intermediate container da1be8fb3579
 ---> 17be1d9b9c5e
Successfully built 17be1d9b9c5e
Successfully tagged docker-layers:0.0.1
Image Chain
Parent
Parent
Parent
Parent
Image1
c059bfaa849c/5.59MB
FROM alpine:latest
Image5
6533a93ed3e4/0B
CMD ['echo', 'fourth']
Image4
6b786cea2e12/6B
ADD ./third.txt .
Image3
13b825a700e2/7B
COPY ./second.txt .
Image2
c1ee277ee0b6/0B
RUN echo first

캐시된 이미지들이 존재하지않는것을, 한번 더 확인할 수 있습니다.

이제 docker-layers:0.0.1 의 이미지들이 생성된것을 확인할 수 있습니다.

[root@docker-test docker]# ls /var/lib/docker/image/overlay2/imagedb/content/sha256

c059bfaa849c4d8e4aecaeb3a10c2d9b3d85f5165c66ad3a4d937758128c4d18
3a18fe3979f0ba52526965cfa3fbfa44e9cb1a01fac9d8df3f8dea9a5b5cc895
6da5ddb808b77a5e7f64bf7c134bdce69e1eb67c1c4afe4f8d0a4a0d1edfb1c2
a537bad7aa2b21ea19666a67cc8ee66ffa151128ed07f20c17f1584e595ff9ca
17be1d9b9c5ea8e703fabf23c45c374e79486f272aa552247b7e9cd6a5e97af5

두번째 이미지를 생성한 경우

먼저 /var/lib/docker/image/overlay2/imagedb/content/sha256 디렉토리를 확인해보면, 이전 docker-layers:0.0.1 이미지로부터 생성된 이미지들을 확인할 수 있습니다.

[root@docker-test docker]# ls /var/lib/docker/image/overlay2/imagedb/content/sha256

c059bfaa849c4d8e4aecaeb3a10c2d9b3d85f5165c66ad3a4d937758128c4d18
3a18fe3979f0ba52526965cfa3fbfa44e9cb1a01fac9d8df3f8dea9a5b5cc895
6da5ddb808b77a5e7f64bf7c134bdce69e1eb67c1c4afe4f8d0a4a0d1edfb1c2
a537bad7aa2b21ea19666a67cc8ee66ffa151128ed07f20c17f1584e595ff9ca
17be1d9b9c5ea8e703fabf23c45c374e79486f272aa552247b7e9cd6a5e97af5

docker build 명령으로, docker-layer:0.0.2 이미지를 생성합니다.

[root@docker-test docker-layer]# docker build -t docker-layers:0.0.2 .

Sending build context to Docker daemon  4.096kB
Step 1/5 : FROM alpine:latest
 ---> c059bfaa849c
Step 2/5 : RUN echo first
 ---> Using cache
 ---> 3a18fe3979f0
Step 3/5 : COPY ./second.txt .
 ---> Using cache
 ---> 6da5ddb808b7
Step 4/5 : ADD ./third.txt .
 ---> Using cache
 ---> a537bad7aa2b
Step 5/5 : CMD ["echo", "fourth"]
 ---> Using cache
 ---> 17be1d9b9c5e
Successfully built 17be1d9b9c5e
Successfully tagged docker-layers:0.0.2
Image Chain
Parent
Parent
Parent
Parent
Image1(Using cache)
c059bfaa849c/5.59MB
FROM alpine:latest
Image5(Using cache)
17be1d9b9c5e/0B
CMD ['echo', 'fourth']
Image4(Using cache)
a537bad7aa2b/6B
ADD ./third.txt .
Image3(Using cache)
6da5ddb808b7/7B
COPY ./second.txt .
Image2(Using cache)
3a18fe3979f0/0B
RUN echo first

docker-layers:0.0.1 로 부터 캐시된 이미지들이, 재 활용된것을 확인할 수 있습니다.

Dockerfile 내용을 변경한 후, 이미지를 생성한 경우

/var/lib/docker/image/overlay2/imagedb/content/sha256 디렉토리를 확인합니다.

[root@docker-test sha256]# ls /var/lib/docker/image/overlay2/imagedb/content/sha256

c059bfaa849c4d8e4aecaeb3a10c2d9b3d85f5165c66ad3a4d937758128c4d18
3a18fe3979f0ba52526965cfa3fbfa44e9cb1a01fac9d8df3f8dea9a5b5cc895
6da5ddb808b77a5e7f64bf7c134bdce69e1eb67c1c4afe4f8d0a4a0d1edfb1c2
a537bad7aa2b21ea19666a67cc8ee66ffa151128ed07f20c17f1584e595ff9ca
17be1d9b9c5ea8e703fabf23c45c374e79486f272aa552247b7e9cd6a5e97af5

기존 Dockerfile 내용을 일부 수정합니다.

기존 이미지 체인에 변화를 주기위해, 컨테이너 파일 시스템에 영향을 주는 새로운 COPY 명령(COPY ./new_second.txt .)을 추가합니다.

FROM alpine:latest
RUN echo first
COPY ./second.txt .
COPY ./new_second.txt .
ADD ./third.txt .
CMD ["echo", "fourth"]

위 Dockerfile 에서 COPY 될 new_second.txt 파일을 생성합니다.

[root@docker-test docker-layer]# echo new_second > new_second.txt && ls

Dockerfile  new_second.txt  second.txt  third.txt

docker build 명령으로, docker-layers:0.0.3 이미지를 생성합니다.

[root@docker-test docker-layer]# docker build -t docker-layers:0.0.3 .

Sending build context to Docker daemon   5.12kB
Step 1/6 : FROM alpine:latest
 ---> c059bfaa849c
Step 2/6 : RUN echo first
 ---> Using cache
 ---> 3a18fe3979f0
Step 3/6 : COPY ./second.txt .
 ---> Using cache
 ---> 6da5ddb808b7
Step 4/6 : COPY ./new_second.txt .
 ---> c3f400b3c3b4
Step 5/6 : ADD ./third.txt .
 ---> b089e6d887fe
Step 6/6 : CMD ["echo", "fourth"]
 ---> Running in 25d40b3f392a
Removing intermediate container 25d40b3f392a
 ---> 2e50e1c93943
Successfully built 2e50e1c93943
Successfully tagged docker-layers:0.0.3
Image Chain
Parent(modified)
Parent(modified)
Parent
Parent
Parent
Image1(Using cache)
c059bfaa849c/5.59MB
FROM alpine:latest
Image6(New Image)
2e50e1c93943/0B
CMD ['echo', 'fourth']
Image5(New Image)
b089e6d887fe/6B
ADD ./third.txt .
Image4(New Image)
c3f400b3c3b4/11B
COPY ./new_second.txt .
Image3(Using cache)
6da5ddb808b7/7B
COPY ./second.txt .
Image2(Using cache)
3a18fe3979f0/0B
RUN echo first

일부 이미지(c059bfaa849c ~ 6da5ddb808b7)들만 재 활용되었고, 추가된 COPY 명령과,

그 뒤의 모든 명령들이 새로운 이미지로 추가되었습니다.

다시 /var/lib/docker/image/overlay2/imagedb/content/sha256 디렉토리를 확인합니다.

[root@docker-test ~]# ls /var/lib/docker/image/overlay2/imagedb/content/sha256
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이전 이미지 목록에,

c059bfaa849c4d8e4aecaeb3a10c2d9b3d85f5165c66ad3a4d937758128c4d18
3a18fe3979f0ba52526965cfa3fbfa44e9cb1a01fac9d8df3f8dea9a5b5cc895
6da5ddb808b77a5e7f64bf7c134bdce69e1eb67c1c4afe4f8d0a4a0d1edfb1c2
a537bad7aa2b21ea19666a67cc8ee66ffa151128ed07f20c17f1584e595ff9ca
17be1d9b9c5ea8e703fabf23c45c374e79486f272aa552247b7e9cd6a5e97af5

새로운 이미지들이 더해진것을 확인할 수 있습니다.

c3f400b3c3b4a725e4bf150ce314a3ba006fb622ea3de26c04a5697dd3184327
b089e6d887fe05bada5545875ec3d97d55f5b41c4cba260a9c64b66fa30cddf8
2e50e1c939430406a23641225d384c1da7b9b7699f3a08b02596d4214ed1cd3d

캐시된 이미지가 무효화된 이유

Image Chain
Parent(modified)
Parent(modified)
Parent
Parent
Parent
Image1(Using cache)
c059bfaa849c/5.59MB
FROM alpine:latest
Image6(New Image)
2e50e1c93943/0B
CMD ['echo', 'fourth']
Image5(New Image)
b089e6d887fe/6B
ADD ./third.txt .
Image4(New Image)
c3f400b3c3b4/11B
COPY ./new_second.txt .
Image3(Using cache)
6da5ddb808b7/7B
COPY ./second.txt .
Image2(Using cache)
3a18fe3979f0/0B
RUN echo first

이미지 레이어 체인의 부모/자식 관계로인해, 추가된 이미지(new_second.txt) 이후의 이미지들이 바라보던 parent 의 변경과함께, 새로운 이미지들이 추가되었고, 그로인해, 이전에 캐시된 이미지들을 사용할 수 없게된 것이다.

즉, 변경된 명령 이후의 모든 명령들은, 캐시된 이미지를 사용하지않고, 새로운 이미지를 만들어냅니다.

변경전 Dockerfile

FROM alpine:latest
RUN echo first
COPY ./second.txt .
ADD ./third.txt .
CMD ["echo", "fourth"]
Image Chain
Parent
Parent
Parent
Parent
Image1
c059bfaa849c/5.59MB
FROM alpine:latest
Image5
17be1d9b9c5e/0B
CMD ['echo', 'fourth']
Image4
a537bad7aa2b/6B
ADD ./third.txt .
Image3
6da5ddb808b7/7B
COPY ./second.txt .
Image2
3a18fe3979f0/0B
RUN echo first

변경 후 Dockerfile

FROM alpine:latest
RUN echo first
COPY ./second.txt .
COPY ./new_second.txt .
ADD ./third.txt .
CMD ["echo", "fourth"]
Image Chain
Parent
Parent
Parent
Parent
Parent
Image1
c059bfaa849c/5.59MB
FROM alpine:latest
Image6
2e50e1c93943/0B
CMD ['echo', 'fourth']
Image5
b089e6d887fe/6B
ADD ./third.txt .
Image4
c3f400b3c3b4/11B
COPY ./new_second.txt .
Image3
6da5ddb808b7/7B
COPY ./second.txt .
Image2
3a18fe3979f0/0B
RUN echo first

/var/lib/docker/image/overlay2/imagedb/metadata/sha256 목록입니다.

[root@docker-test sha256]# ls /var/lib/docker/image/overlay2/imagedb/metadata/sha256

# 기존 CMD ["echo", "fourth"] 명령
17be1d9b9c5ea8e703fabf23c45c374e79486f272aa552247b7e9cd6a5e97af5

# 새로운 CMD ["echo", "fourth"] 명령
2e50e1c939430406a23641225d384c1da7b9b7699f3a08b02596d4214ed1cd3d

# 기존 RUN echo first 명령
3a18fe3979f0ba52526965cfa3fbfa44e9cb1a01fac9d8df3f8dea9a5b5cc895

# 기존 COPY ./second.txt . 명령
6da5ddb808b77a5e7f64bf7c134bdce69e1eb67c1c4afe4f8d0a4a0d1edfb1c2

# 기존 ADD ./third.txt . 명령
a537bad7aa2b21ea19666a67cc8ee66ffa151128ed07f20c17f1584e595ff9ca

# 새로운 ADD ./third.txt . 명령
b089e6d887fe05bada5545875ec3d97d55f5b41c4cba260a9c64b66fa30cddf8

# 새로운 COPY ./new_second.txt . 명령
c3f400b3c3b4a725e4bf150ce314a3ba006fb622ea3de26c04a5697dd3184327
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Written by mohwa