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Linus Torvalds
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refs/heads/master: b3f4e1eba45eda5d1213810ef3bc53e5247df2df | ||
refs/heads/master: 1046a2c428bedd64c960dcfd0c57cc69a82fea2f |
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What: /sys/kernel/debug/ideapad/cfg | ||
Date: Sep 2011 | ||
KernelVersion: 3.2 | ||
Contact: Ike Panhc <ike.pan@canonical.com> | ||
Description: | ||
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cfg shows the return value of _CFG method in VPC2004 device. It tells machine | ||
capability and what graphic component within the machine. | ||
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What: /sys/kernel/debug/ideapad/status | ||
Date: Sep 2011 | ||
KernelVersion: 3.2 | ||
Contact: Ike Panhc <ike.pan@canonical.com> | ||
Description: | ||
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status shows infos we can read and tells its meaning and value. | ||
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Introduction | ||
============ | ||
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The more-sophisticated device-mapper targets require complex metadata | ||
that is managed in kernel. In late 2010 we were seeing that various | ||
different targets were rolling their own data strutures, for example: | ||
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- Mikulas Patocka's multisnap implementation | ||
- Heinz Mauelshagen's thin provisioning target | ||
- Another btree-based caching target posted to dm-devel | ||
- Another multi-snapshot target based on a design of Daniel Phillips | ||
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Maintaining these data structures takes a lot of work, so if possible | ||
we'd like to reduce the number. | ||
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The persistent-data library is an attempt to provide a re-usable | ||
framework for people who want to store metadata in device-mapper | ||
targets. It's currently used by the thin-provisioning target and an | ||
upcoming hierarchical storage target. | ||
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Overview | ||
======== | ||
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The main documentation is in the header files which can all be found | ||
under drivers/md/persistent-data. | ||
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The block manager | ||
----------------- | ||
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dm-block-manager.[hc] | ||
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This provides access to the data on disk in fixed sized-blocks. There | ||
is a read/write locking interface to prevent concurrent accesses, and | ||
keep data that is being used in the cache. | ||
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Clients of persistent-data are unlikely to use this directly. | ||
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The transaction manager | ||
----------------------- | ||
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dm-transaction-manager.[hc] | ||
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This restricts access to blocks and enforces copy-on-write semantics. | ||
The only way you can get hold of a writable block through the | ||
transaction manager is by shadowing an existing block (ie. doing | ||
copy-on-write) or allocating a fresh one. Shadowing is elided within | ||
the same transaction so performance is reasonable. The commit method | ||
ensures that all data is flushed before it writes the superblock. | ||
On power failure your metadata will be as it was when last committed. | ||
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The Space Maps | ||
-------------- | ||
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dm-space-map.h | ||
dm-space-map-metadata.[hc] | ||
dm-space-map-disk.[hc] | ||
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On-disk data structures that keep track of reference counts of blocks. | ||
Also acts as the allocator of new blocks. Currently two | ||
implementations: a simpler one for managing blocks on a different | ||
device (eg. thinly-provisioned data blocks); and one for managing | ||
the metadata space. The latter is complicated by the need to store | ||
its own data within the space it's managing. | ||
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The data structures | ||
------------------- | ||
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dm-btree.[hc] | ||
dm-btree-remove.c | ||
dm-btree-spine.c | ||
dm-btree-internal.h | ||
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Currently there is only one data structure, a hierarchical btree. | ||
There are plans to add more. For example, something with an | ||
array-like interface would see a lot of use. | ||
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The btree is 'hierarchical' in that you can define it to be composed | ||
of nested btrees, and take multiple keys. For example, the | ||
thin-provisioning target uses a btree with two levels of nesting. | ||
The first maps a device id to a mapping tree, and that in turn maps a | ||
virtual block to a physical block. | ||
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Values stored in the btrees can have arbitrary size. Keys are always | ||
64bits, although nesting allows you to use multiple keys. |
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