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Merge git://git.kernel.org/pub/scm/linux/kernel/git/steve/linux-dm
* git://git.kernel.org/pub/scm/linux/kernel/git/steve/linux-dm: dm: raid fix device status indicator when array initializing dm log userspace: add log device dependency dm log userspace: fix comment hyphens dm: add thin provisioning target dm: add persistent data library dm: add bufio dm: export dm get md dm table: add immutable feature dm table: add always writeable feature dm table: add singleton feature dm kcopyd: add dm_kcopyd_zero to zero an area dm: remove superfluous smp_mb dm: use local printk ratelimit dm table: propagate non rotational flag
This commit is contained in:
Linus Torvalds
commit
43672a0784
42 changed files with 12079 additions and 37 deletions
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@ -48,7 +48,7 @@ kernel and userspace, 'connector' is used as the interface for
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communication.
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There are currently two userspace log implementations that leverage this
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framework - "clustered_disk" and "clustered_core". These implementations
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framework - "clustered-disk" and "clustered-core". These implementations
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provide a cluster-coherent log for shared-storage. Device-mapper mirroring
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can be used in a shared-storage environment when the cluster log implementations
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are employed.
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84
Documentation/device-mapper/persistent-data.txt
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84
Documentation/device-mapper/persistent-data.txt
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@ -0,0 +1,84 @@
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Introduction
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============
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The more-sophisticated device-mapper targets require complex metadata
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that is managed in kernel. In late 2010 we were seeing that various
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different targets were rolling their own data strutures, for example:
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- Mikulas Patocka's multisnap implementation
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- Heinz Mauelshagen's thin provisioning target
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- Another btree-based caching target posted to dm-devel
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- 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
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we'd like to reduce the number.
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The persistent-data library is an attempt to provide a re-usable
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framework for people who want to store metadata in device-mapper
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targets. It's currently used by the thin-provisioning target and an
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upcoming hierarchical storage target.
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Overview
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========
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The main documentation is in the header files which can all be found
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under drivers/md/persistent-data.
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The block manager
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-----------------
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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
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is a read/write locking interface to prevent concurrent accesses, and
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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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-----------------------
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dm-transaction-manager.[hc]
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This restricts access to blocks and enforces copy-on-write semantics.
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The only way you can get hold of a writable block through the
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transaction manager is by shadowing an existing block (ie. doing
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copy-on-write) or allocating a fresh one. Shadowing is elided within
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the same transaction so performance is reasonable. The commit method
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ensures that all data is flushed before it writes the superblock.
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On power failure your metadata will be as it was when last committed.
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The Space Maps
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--------------
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dm-space-map.h
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dm-space-map-metadata.[hc]
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dm-space-map-disk.[hc]
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On-disk data structures that keep track of reference counts of blocks.
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Also acts as the allocator of new blocks. Currently two
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implementations: a simpler one for managing blocks on a different
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device (eg. thinly-provisioned data blocks); and one for managing
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the metadata space. The latter is complicated by the need to store
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its own data within the space it's managing.
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The data structures
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-------------------
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dm-btree.[hc]
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dm-btree-remove.c
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dm-btree-spine.c
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dm-btree-internal.h
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Currently there is only one data structure, a hierarchical btree.
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There are plans to add more. For example, something with an
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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
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of nested btrees, and take multiple keys. For example, the
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thin-provisioning target uses a btree with two levels of nesting.
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The first maps a device id to a mapping tree, and that in turn maps a
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virtual block to a physical block.
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Values stored in the btrees can have arbitrary size. Keys are always
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64bits, although nesting allows you to use multiple keys.
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285
Documentation/device-mapper/thin-provisioning.txt
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285
Documentation/device-mapper/thin-provisioning.txt
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@ -0,0 +1,285 @@
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Introduction
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============
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This document descibes a collection of device-mapper targets that
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between them implement thin-provisioning and snapshots.
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The main highlight of this implementation, compared to the previous
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implementation of snapshots, is that it allows many virtual devices to
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be stored on the same data volume. This simplifies administration and
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allows the sharing of data between volumes, thus reducing disk usage.
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Another significant feature is support for an arbitrary depth of
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recursive snapshots (snapshots of snapshots of snapshots ...). The
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previous implementation of snapshots did this by chaining together
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lookup tables, and so performance was O(depth). This new
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implementation uses a single data structure to avoid this degradation
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with depth. Fragmentation may still be an issue, however, in some
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scenarios.
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Metadata is stored on a separate device from data, giving the
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administrator some freedom, for example to:
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- Improve metadata resilience by storing metadata on a mirrored volume
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but data on a non-mirrored one.
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- Improve performance by storing the metadata on SSD.
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Status
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======
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These targets are very much still in the EXPERIMENTAL state. Please
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do not yet rely on them in production. But do experiment and offer us
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feedback. Different use cases will have different performance
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characteristics, for example due to fragmentation of the data volume.
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If you find this software is not performing as expected please mail
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[email protected] with details and we'll try our best to improve
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things for you.
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Userspace tools for checking and repairing the metadata are under
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development.
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Cookbook
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========
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This section describes some quick recipes for using thin provisioning.
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They use the dmsetup program to control the device-mapper driver
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directly. End users will be advised to use a higher-level volume
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manager such as LVM2 once support has been added.
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Pool device
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-----------
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The pool device ties together the metadata volume and the data volume.
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It maps I/O linearly to the data volume and updates the metadata via
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two mechanisms:
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- Function calls from the thin targets
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- Device-mapper 'messages' from userspace which control the creation of new
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virtual devices amongst other things.
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Setting up a fresh pool device
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------------------------------
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Setting up a pool device requires a valid metadata device, and a
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data device. If you do not have an existing metadata device you can
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make one by zeroing the first 4k to indicate empty metadata.
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dd if=/dev/zero of=$metadata_dev bs=4096 count=1
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The amount of metadata you need will vary according to how many blocks
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are shared between thin devices (i.e. through snapshots). If you have
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less sharing than average you'll need a larger-than-average metadata device.
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As a guide, we suggest you calculate the number of bytes to use in the
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metadata device as 48 * $data_dev_size / $data_block_size but round it up
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to 2MB if the answer is smaller. The largest size supported is 16GB.
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If you're creating large numbers of snapshots which are recording large
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amounts of change, you may need find you need to increase this.
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Reloading a pool table
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----------------------
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You may reload a pool's table, indeed this is how the pool is resized
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if it runs out of space. (N.B. While specifying a different metadata
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device when reloading is not forbidden at the moment, things will go
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wrong if it does not route I/O to exactly the same on-disk location as
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previously.)
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Using an existing pool device
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-----------------------------
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dmsetup create pool \
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--table "0 20971520 thin-pool $metadata_dev $data_dev \
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$data_block_size $low_water_mark"
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$data_block_size gives the smallest unit of disk space that can be
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allocated at a time expressed in units of 512-byte sectors. People
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primarily interested in thin provisioning may want to use a value such
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as 1024 (512KB). People doing lots of snapshotting may want a smaller value
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such as 128 (64KB). If you are not zeroing newly-allocated data,
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a larger $data_block_size in the region of 256000 (128MB) is suggested.
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$data_block_size must be the same for the lifetime of the
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metadata device.
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$low_water_mark is expressed in blocks of size $data_block_size. If
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free space on the data device drops below this level then a dm event
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will be triggered which a userspace daemon should catch allowing it to
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extend the pool device. Only one such event will be sent.
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Resuming a device with a new table itself triggers an event so the
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userspace daemon can use this to detect a situation where a new table
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already exceeds the threshold.
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Thin provisioning
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-----------------
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i) Creating a new thinly-provisioned volume.
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To create a new thinly- provisioned volume you must send a message to an
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active pool device, /dev/mapper/pool in this example.
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dmsetup message /dev/mapper/pool 0 "create_thin 0"
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Here '0' is an identifier for the volume, a 24-bit number. It's up
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to the caller to allocate and manage these identifiers. If the
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identifier is already in use, the message will fail with -EEXIST.
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ii) Using a thinly-provisioned volume.
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Thinly-provisioned volumes are activated using the 'thin' target:
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dmsetup create thin --table "0 2097152 thin /dev/mapper/pool 0"
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The last parameter is the identifier for the thinp device.
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Internal snapshots
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------------------
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i) Creating an internal snapshot.
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Snapshots are created with another message to the pool.
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N.B. If the origin device that you wish to snapshot is active, you
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must suspend it before creating the snapshot to avoid corruption.
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This is NOT enforced at the moment, so please be careful!
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dmsetup suspend /dev/mapper/thin
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dmsetup message /dev/mapper/pool 0 "create_snap 1 0"
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dmsetup resume /dev/mapper/thin
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Here '1' is the identifier for the volume, a 24-bit number. '0' is the
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identifier for the origin device.
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ii) Using an internal snapshot.
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Once created, the user doesn't have to worry about any connection
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between the origin and the snapshot. Indeed the snapshot is no
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different from any other thinly-provisioned device and can be
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snapshotted itself via the same method. It's perfectly legal to
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have only one of them active, and there's no ordering requirement on
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activating or removing them both. (This differs from conventional
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device-mapper snapshots.)
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Activate it exactly the same way as any other thinly-provisioned volume:
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dmsetup create snap --table "0 2097152 thin /dev/mapper/pool 1"
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Deactivation
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------------
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All devices using a pool must be deactivated before the pool itself
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can be.
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dmsetup remove thin
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dmsetup remove snap
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dmsetup remove pool
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Reference
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=========
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'thin-pool' target
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------------------
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i) Constructor
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thin-pool <metadata dev> <data dev> <data block size (sectors)> \
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<low water mark (blocks)> [<number of feature args> [<arg>]*]
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Optional feature arguments:
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- 'skip_block_zeroing': skips the zeroing of newly-provisioned blocks.
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Data block size must be between 64KB (128 sectors) and 1GB
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(2097152 sectors) inclusive.
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ii) Status
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<transaction id> <used metadata blocks>/<total metadata blocks>
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<used data blocks>/<total data blocks> <held metadata root>
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transaction id:
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A 64-bit number used by userspace to help synchronise with metadata
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from volume managers.
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used data blocks / total data blocks
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If the number of free blocks drops below the pool's low water mark a
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dm event will be sent to userspace. This event is edge-triggered and
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it will occur only once after each resume so volume manager writers
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should register for the event and then check the target's status.
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held metadata root:
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The location, in sectors, of the metadata root that has been
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'held' for userspace read access. '-' indicates there is no
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held root. This feature is not yet implemented so '-' is
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always returned.
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iii) Messages
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create_thin <dev id>
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Create a new thinly-provisioned device.
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<dev id> is an arbitrary unique 24-bit identifier chosen by
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the caller.
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create_snap <dev id> <origin id>
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Create a new snapshot of another thinly-provisioned device.
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<dev id> is an arbitrary unique 24-bit identifier chosen by
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the caller.
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<origin id> is the identifier of the thinly-provisioned device
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of which the new device will be a snapshot.
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delete <dev id>
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Deletes a thin device. Irreversible.
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trim <dev id> <new size in sectors>
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Delete mappings from the end of a thin device. Irreversible.
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You might want to use this if you're reducing the size of
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your thinly-provisioned device. In many cases, due to the
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sharing of blocks between devices, it is not possible to
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determine in advance how much space 'trim' will release. (In
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future a userspace tool might be able to perform this
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calculation.)
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set_transaction_id <current id> <new id>
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Userland volume managers, such as LVM, need a way to
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synchronise their external metadata with the internal metadata of the
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pool target. The thin-pool target offers to store an
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arbitrary 64-bit transaction id and return it on the target's
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status line. To avoid races you must provide what you think
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the current transaction id is when you change it with this
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compare-and-swap message.
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'thin' target
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-------------
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i) Constructor
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thin <pool dev> <dev id>
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pool dev:
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the thin-pool device, e.g. /dev/mapper/my_pool or 253:0
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dev id:
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the internal device identifier of the device to be
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activated.
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The pool doesn't store any size against the thin devices. If you
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load a thin target that is smaller than you've been using previously,
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then you'll have no access to blocks mapped beyond the end. If you
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load a target that is bigger than before, then extra blocks will be
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provisioned as and when needed.
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If you wish to reduce the size of your thin device and potentially
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regain some space then send the 'trim' message to the pool.
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ii) Status
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<nr mapped sectors> <highest mapped sector>
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