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2005-10-17 17:20:12
一个镜像可以由四个子镜像组成.如果对一个子镜像进行offline处理，那么镜像就停止对它进行读写操作.在这个时候，就可以单独对这个子镜像进行备份.然而这个子镜像处于只读状态， 当一个子镜像上线之后，solaris卷管理器就可以跟踪所有对这个镜像的写操作.子镜像还可以被下线来诊断或者修复有错误的物理设备.
子镜像可以在任何时候被装载或者拆卸，但是至少一个子镜像必须始终保持挂载状态
solaris卷管理器支持的两种冗余方式分别是RAID-1+0和RAID-0+1
RAID-1+0 redundancy constitutes a configuration of mirrors that are then striped
RAID-0+1 redundancy constitutes a configuration of stripes that are then mirrored
RAID-1卷的resynchronization是将一个子镜像上的数据复制到另外一个子镜像上面去的进程，当下面一种情况发生了，都会进行数据的复制：
1，子镜像发生错误
2，系统发生crash
3，一个子镜像被下线然后又上线
4，一个新子镜像被添加
RAID-1 volume read policies:
round-robin:attempts to balance the load across the submirrors. all reads are made in a round-robin order from all submirrors in a mirror
geometric:enables reads to be divided among submirrors on the basis of a logical disk block address. for example, with a two-way submirror, the disk space on the mirror is divided into two equally-sized logical address ranges.reads from one submirror are restricted to one half of the logical range. reads from the other submirror are restricted to the other half. the geometri read policy effectively reduces the seek time that is necessary for reads. the performance gained by this read policy depends on the system I/O load and the access patterns of the applications
first:directs all reads to the first submirror. this policy should be used only when the device or devices that comprise the first submirror are substantially faster than the devices of the second submirror
RAID-1 volume write policies:
parallel:performs writes to a mirror that are replicated and dispatched to all of the submirrors simultaneously
serial:performs writes to submirrors serially. this policy specifies that writes to one submirror must be completed before the next submirror write is initiated. this policy is provided in case a submirror becomes unreadable, for example, due to a power failure.
