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docker存储结构解析

Posted on 2014-11-19 21:11 feisky 阅读(...) 评论(...) 编辑 收藏
由于aufs并未并入内核,故而目前只有Ubuntu系统上能够使用aufs作为docker的存储引擎,而其他系统上使用lvm thin provisioning(overlayfs是一个和aufs类似的union filesystem,未来有可能进入内核,但目前还没有;Lvm snapshot are useful for doing e.g. backup of a snapshot, but regress badly in performance when you start having many snapshots of the same device.)。为了实现lvm thin provisioning,docker启动时会设置一个100G的sparse文件(/var/lib/docker/devicemapper/devicemapper/data,元数据为/var/lib/docker/devicemapper/devicemapper/metadata),并将其作为devicemapper的存储池,而所有容器都从该存储池中分配默认10G的存储空间使用,如下图所示:
 
比如创建一个apache容器时devicemapper处理流程如下所示:
 
  1. Create a snapshot of the base device.
  2. Mount it and apply the changes in the fedora image.
  3. Create a snapshot based on the fedora device.
  4. Mount it and apply the changes in the apache image.
  5. Create a snapshot based on the apache device.
  6. Mount it and use as the root in the new container.
 
thin provisioning管理
 
使用lvm工具来创建一个thin pool:
 
dd if=/dev/zero of=lvm.img bs=1M count=100
losetup /dev/loop7 lvm.img
losetup -a
pvcreate /dev/loop7
vgcreate lvm_pool /dev/loop7
# create thin pool
lvcreate -L 80M -T lvm_pool/thin_pool
# create volume in thin pool
lvcreate -T lvm_pool/thin_pool -V 500M -n first_lv
 
docker启动时创建的默认存储池:
#dmsetup table docker-253:1-138011042-pool
0 209715200 thin-pool 7:2 7:1 128 32768 1 skip_block_zeroing    # 209715200*512/1024/1024/1024=100GB
 
当启动容器后,会从该池中分配10G出来:
#dmsetup table docker-253:1-138011042-641cdebd22b55f2656a560cd250e661ab181dcf2f5c5b78dc306df7ce62231f2
0 20971520 thin 253:2 166    # 20971520*512/1024/1024/1024=10GB
该10G存储的分配过程为:
dmsetup message /dev/mapper/docker-253:1-138011042-pool 0 "create_thin 166"
dmsetup create docker-253:1-138011042-641cdebd22b55f2656a560cd250e661ab181dcf2f5c5b78dc306df7ce62231f3 --table "0 20971520 thin /dev/mapper/docker-253:1-138011042-pool 166"
 
创建快照:
dmsetup suspend /dev/mapper/thin
dmsetup message /dev/mapper/yy_thin_pool 0 "create_snap 1 0"
dmsetup resume /dev/mapper/thin
dmsetup create snap --table "0 40960 thin /dev/mapper/yy_thin_pool 1"
 
docker服务在启动的时候可以配置devicemapper的启动参数,docker -d --storage-opt dm.foo=bar,可选参数有以下几个:
 
  1. dm.basesize 默认为10G,限制容器和镜像的大小
  2. dm.loopdatasize 存储池大小,默认为100G
  3. dm.datadev 存储池设备,默认生成一个/var/lib/docker/devicemapper/devicemapper/data文件
  4. dm.loopmetadatasize 元数据大小,默认为2G
  5. dm.metadatadev 元数据设备,默认生成一个/var/lib/docker/devicemapper/devicemapper/metadata文件
  6. dm.fs 文件系统,默认ext4
  7. dm.blocksize blocksize默认64K
  8. dm.blkdiscard 默认true
 
最后看看启动一个容器后,该容器的配置是如何组织的。
 
每个容器创建后都会将其基本配置写入到/var/lib/docker/containers/中:
#ls /var/lib/docker/containers/49f19ee979f6bf125c62779dcabf3bdce310b13d22e5c826752db202e509154e -l
total 20
-rw------- 1 root root 0 Nov 18 16:31 49f19ee979f6bf125c62779dcabf3bdce310b13d22e5c826752db202e509154e-json.log
-rw-r--r-- 1 root root 1741 Nov 18 16:31 config.json
-rw-r--r-- 1 root root 368 Nov 18 16:31 hostconfig.json
-rw-r--r-- 1 root root 13 Nov 18 16:31 hostname
-rw-r--r-- 1 root root 175 Nov 18 16:31 hosts
-rw-r--r-- 1 root root 325 Nov 18 16:31 resolv.conf
分配10G空间后会将容器存储配置写入到以下两个文件中:
# cd /var/lib/docker
#cat ./devicemapper/metadata/49f19ee979f6bf125c62779dcabf3bdce310b13d22e5c826752db202e509154e-init
{"device_id":174,"size":10737418240,"transaction_id":731,"initialized":false}
#cat ./devicemapper/metadata/49f19ee979f6bf125c62779dcabf3bdce310b13d22e5c826752db202e509154e 
{"device_id":175,"size":10737418240,"transaction_id":732,"initialized":false}
而容器的rootfs会mount到/var/lib/docker/devicemapper/mnt/container_id下:
#mount | grep 49f1
/dev/mapper/docker-253:1-138011042-49f19ee979f6bf125c62779dcabf3bdce310b13d22e5c826752db202e509154e on /var/lib/docker/devicemapper/mnt/49f19ee979f6bf125c62779dcabf3bdce310b13d22e5c826752db202e509154e type ext4 (rw,relatime,discard,stripe=16,data=ordered)
 
 
参考文档
 
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