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author | 2015-08-04 12:17:53 -0700 | |
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committer | 2015-08-04 15:44:42 -0700 | |
commit | 9ca8dbcc65cfc63d6f5ef3312a33184e1d726e00 (patch) | |
tree | 1c9cafbcd35f783a87880a10f85d1a060db1a563 /kernel/Documentation/device-mapper/era.txt | |
parent | 98260f3884f4a202f9ca5eabed40b1354c489b29 (diff) |
Add the rt linux 4.1.3-rt3 as base
Import the rt linux 4.1.3-rt3 as OPNFV kvm base.
It's from git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git linux-4.1.y-rt and
the base is:
commit 0917f823c59692d751951bf5ea699a2d1e2f26a2
Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Sat Jul 25 12:13:34 2015 +0200
Prepare v4.1.3-rt3
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
We lose all the git history this way and it's not good. We
should apply another opnfv project repo in future.
Change-Id: I87543d81c9df70d99c5001fbdf646b202c19f423
Signed-off-by: Yunhong Jiang <yunhong.jiang@intel.com>
Diffstat (limited to 'kernel/Documentation/device-mapper/era.txt')
-rw-r--r-- | kernel/Documentation/device-mapper/era.txt | 108 |
1 files changed, 108 insertions, 0 deletions
diff --git a/kernel/Documentation/device-mapper/era.txt b/kernel/Documentation/device-mapper/era.txt new file mode 100644 index 000000000..3c6d01be3 --- /dev/null +++ b/kernel/Documentation/device-mapper/era.txt @@ -0,0 +1,108 @@ +Introduction +============ + +dm-era is a target that behaves similar to the linear target. In +addition it keeps track of which blocks were written within a user +defined period of time called an 'era'. Each era target instance +maintains the current era as a monotonically increasing 32-bit +counter. + +Use cases include tracking changed blocks for backup software, and +partially invalidating the contents of a cache to restore cache +coherency after rolling back a vendor snapshot. + +Constructor +=========== + + era <metadata dev> <origin dev> <block size> + + metadata dev : fast device holding the persistent metadata + origin dev : device holding data blocks that may change + block size : block size of origin data device, granularity that is + tracked by the target + +Messages +======== + +None of the dm messages take any arguments. + +checkpoint +---------- + +Possibly move to a new era. You shouldn't assume the era has +incremented. After sending this message, you should check the +current era via the status line. + +take_metadata_snap +------------------ + +Create a clone of the metadata, to allow a userland process to read it. + +drop_metadata_snap +------------------ + +Drop the metadata snapshot. + +Status +====== + +<metadata block size> <#used metadata blocks>/<#total metadata blocks> +<current era> <held metadata root | '-'> + +metadata block size : Fixed block size for each metadata block in + sectors +#used metadata blocks : Number of metadata blocks used +#total metadata blocks : Total number of metadata blocks +current era : The current era +held metadata root : The location, in blocks, of the metadata root + that has been 'held' for userspace read + access. '-' indicates there is no held root + +Detailed use case +================= + +The scenario of invalidating a cache when rolling back a vendor +snapshot was the primary use case when developing this target: + +Taking a vendor snapshot +------------------------ + +- Send a checkpoint message to the era target +- Make a note of the current era in its status line +- Take vendor snapshot (the era and snapshot should be forever + associated now). + +Rolling back to an vendor snapshot +---------------------------------- + +- Cache enters passthrough mode (see: dm-cache's docs in cache.txt) +- Rollback vendor storage +- Take metadata snapshot +- Ascertain which blocks have been written since the snapshot was taken + by checking each block's era +- Invalidate those blocks in the caching software +- Cache returns to writeback/writethrough mode + +Memory usage +============ + +The target uses a bitset to record writes in the current era. It also +has a spare bitset ready for switching over to a new era. Other than +that it uses a few 4k blocks for updating metadata. + + (4 * nr_blocks) bytes + buffers + +Resilience +========== + +Metadata is updated on disk before a write to a previously unwritten +block is performed. As such dm-era should not be effected by a hard +crash such as power failure. + +Userland tools +============== + +Userland tools are found in the increasingly poorly named +thin-provisioning-tools project: + + https://github.com/jthornber/thin-provisioning-tools |