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块是字节序列（例如，一个512字节的数据块）。基于块的存储接口是使用旋转介质（例如硬盘，CD，软盘甚至传统的9-track tape）存储数据的最常用方法。块设备接口的无处不在，使虚拟块设备成为与海量数据存储系统（如Ceph）进行交互的理想候选者。
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Ceph块设备经过精简配置，可调整大小，并在Ceph集群中的多个OSD上存储条带化数据，ceph块设备利用了RADOS功能，例如快照，复制和一致性。 Ceph的RADOS块设备（RBD）使用内核模块或librbd库与OSD进行交互。
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Ceph的块设备对内核设备，KVMS例如QEMU，基于云的计算系统，例如OpenStack和CloudStack，提供高性能和无限的可扩展性 。你可以使用同一群集同时操作Ceph RADOS网关，Ceph的文件系统和Ceph块设备。
创建池和块
[root@ceph-node1 ~]# ceph osd pool create block 6
pool 'block' created
为客户端创建用户，并将密钥文件scp到客户端
[root@ceph-node1 ~]# ceph auth get-or-create client.rbd mon 'allow r' osd 'allow class-read object_prefix rbd_children, allow rwx pool=block'| tee ./ceph.client.rbd.keyring
[client.rbd]
key = AQA04PpdtJpbGxAAd+lCJFQnDfRlWL5cFUShoQ==
[root@ceph-node1 ~]#scp ceph.client.rbd.keyring root@ceph-client:/etc/ceph
客户端创建一个大小为2G的块设备
[root@ceph-client /]# rbd create block/rbd0 --size 2048 --name client.rbd
映射此块设备到客户端
[root@ceph-client /]# rbd map --image block/rbd0 --name client.rbd
/dev/rbd0
[root@ceph-client /]# rbd showmapped --name client.rbd
id pool image snap device
0 block rbd0 - /dev/rbd0
注意：这里可能会报如下的错误
[root@ceph-client /]# rbd map --image block/rbd0 --name client.rbd
rbd: sysfs write failed
In some cases useful info is found in syslog - try "dmesg | tail".
rbd: map failed: (2) No such file or directory
解决方法有三种，看我这篇博客rbd: sysfs write failed解决办法
创建文件系统，并挂载块设备
[root@ceph-client /]# fdisk -l /dev/rbd0
Disk /dev/rbd0: 2147 MB, 2147483648 bytes, 4194304 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 4194304 bytes / 4194304 bytes[root@ceph-client /]# mkfs.xfs /dev/rbd0
meta-data=/dev/rbd0 isize=512 agcount=8, agsize=65536 blks
= sectsz=512 attr=2, projid32bit=1
= crc=1 finobt=0, sparse=0
data = bsize=4096 blocks=524288, imaxpct=25
= sunit=1024 swidth=1024 blks
naming =version 2 bsize=4096 ascii-ci=0 ftype=1
log =internal log bsize=4096 blocks=2560, version=2
= sectsz=512 sunit=8 blks, lazy-count=1
realtime =none extsz=4096 blocks=0, rtextents=0[root@ceph-client /]# mount /dev/rbd0 /ceph-rbd0
[root@ceph-client /]# df -Th /ceph-rbd0
Filesystem Type Size Used Avail Use% Mounted on
/dev/rbd0 xfs 2.0G 33M 2.0G 2% /ceph-rb
写入数据测试
[root@ceph-client /]# dd if=/dev/zero of=/ceph-rbd0/file count=100 bs=1M
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 0.0674301 s, 1.6 GB/s
[root@ceph-client /]# ls -lh /ceph-rbd0/file
-rw-r--r-- 1 root root 100M Dec 19 10:50 /ceph-rbd0/file
做成系统服务
[root@ceph-client /]#cat /usr/local/bin/rbd-mount
#!/bin/bash
# Pool name where block device image is stored
export poolname=block
# Disk image name
export rbdimage0=rbd0
# Mounted Directory
export mountpoint0=/ceph-rbd0
# Image mount/unmount and pool are passed from the systemd service as arguments
# Are we are mounting or unmounting
if [ "$1" == "m" ]; then
modprobe rbd
rbd feature disable $rbdimage0 object-map fast-diff deep-flatten
rbd map $rbdimage0 --id rbd --keyring /etc/ceph/ceph.client.rbd.keyring
mkdir -p $mountpoint0
mount /dev/rbd/$poolname/$rbdimage0 $mountpoint0
fi
if [ "$1" == "u" ]; then
umount $mountpoint0
rbd unmap /dev/rbd/$poolname/$rbdimage0
fi[root@ceph-client ~]# cat /etc/systemd/system/rbd-mount.service
[Unit]
Description=RADOS block device mapping for $rbdimage in pool $poolname"
Conflicts=shutdown.target
Wants=network-online.target
After=NetworkManager-wait-online.service
[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/usr/local/bin/rbd-mount m
ExecStop=/usr/local/bin/rbd-mount u
[Install]
WantedBy=multi-user.target
开机自动挂载
[root@ceph-client ~]#systemctl daemon-reload
[root@ceph-client ~]#systemctl enable rbd-mount.service