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1 | Device-Mapper Logging |
2 | ===================== |
3 | The device-mapper logging code is used by some of the device-mapper |
4 | RAID targets to track regions of the disk that are not consistent. |
5 | A region (or portion of the address space) of the disk may be |
6 | inconsistent because a RAID stripe is currently being operated on or |
7 | a machine died while the region was being altered. In the case of |
8 | mirrors, a region would be considered dirty/inconsistent while you |
9 | are writing to it because the writes need to be replicated for all |
10 | the legs of the mirror and may not reach the legs at the same time. |
11 | Once all writes are complete, the region is considered clean again. |
12 | |
13 | There is a generic logging interface that the device-mapper RAID |
14 | implementations use to perform logging operations (see |
15 | dm_dirty_log_type in include/linux/dm-dirty-log.h). Various different |
16 | logging implementations are available and provide different |
17 | capabilities. The list includes: |
18 | |
19 | Type Files |
20 | ==== ===== |
21 | disk drivers/md/dm-log.c |
22 | core drivers/md/dm-log.c |
23 | userspace drivers/md/dm-log-userspace* include/linux/dm-log-userspace.h |
24 | |
25 | The "disk" log type |
26 | ------------------- |
27 | This log implementation commits the log state to disk. This way, the |
28 | logging state survives reboots/crashes. |
29 | |
30 | The "core" log type |
31 | ------------------- |
32 | This log implementation keeps the log state in memory. The log state |
33 | will not survive a reboot or crash, but there may be a small boost in |
34 | performance. This method can also be used if no storage device is |
35 | available for storing log state. |
36 | |
37 | The "userspace" log type |
38 | ------------------------ |
39 | This log type simply provides a way to export the log API to userspace, |
40 | so log implementations can be done there. This is done by forwarding most |
41 | logging requests to userspace, where a daemon receives and processes the |
42 | request. |
43 | |
44 | The structure used for communication between kernel and userspace are |
45 | located in include/linux/dm-log-userspace.h. Due to the frequency, |
46 | diversity, and 2-way communication nature of the exchanges between |
47 | kernel and userspace, 'connector' is used as the interface for |
48 | communication. |
49 | |
50 | There are currently two userspace log implementations that leverage this |
51 | framework - "clustered_disk" and "clustered_core". These implementations |
52 | provide a cluster-coherent log for shared-storage. Device-mapper mirroring |
53 | can be used in a shared-storage environment when the cluster log implementations |
54 | are employed. |
55 |
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