在linux mdadm手册页中,我看到了“ --write-journal”选项。
这与lvmcache有何关系,因为lvmcache是建立在dm-cache之上的?它们是不同的东西吗,还是lvmcache在后台执行“ --write-journal”之类的事情?
谢谢!
在md手册页中对RAID日志进行了更详细的描述:
RAID456 WRITE JOURNAL
Due to non-atomicity nature of RAID write operations, interruption of
write operations (system crash, etc.) to RAID456 array can lead to
inconsistent parity and data loss (so called RAID-5 write hole).
To plug the write hole, from Linux 4.4 (to be confirmed), md supports
write ahead journal for RAID456. When the array is created, an addi‐
tional journal device can be added to the array through write-journal
option. The RAID write journal works similar to file system journals.
Before writing to the data disks, md persists data AND parity of the
stripe to the journal device. After crashes, md searches the journal
device for incomplete write operations, and replay them to the data
disks.
When the journal device fails, the RAID array is forced to run in read-
only mode.
所以-这与lvmcache
任何形式,形式都不相关。
lvmcache旨在通过在SSD上缓存数据来提供更快的读取速度
md write journal是一个写缓存,它甚至不能使事情变得更快。它的存在是为了防止数据丢失。如果必须首先完成所有RAID写入操作,那么SSD将会承受巨大负担-如果在RAID重新同步和增长期间也发生这种情况,那么您会在很短的时间内看到很多TB的数据。
手册mdadm
页可能会提及SSD的使用寿命:
--write-journal
Specify journal device for the RAID-4/5/6 array. The journal
device should be a SSD with reasonable lifetime.
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我来说两句