docs: filesystems: convert xfs-delayed-logging-design.txt to ReST
- Add a SPDX header; - Adjust document and section titles; - Some whitespace fixes and new line breaks; - Mark literal blocks as such; - Add it to filesystems/index.rst. Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org> Link: https://lore.kernel.org/r/2233c248f12e7b465cd27ee30a86f96eb632946a.1588021877.git.mchehab+huawei@kernel.org Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Mauro Carvalho Chehab
Jonathan Corbet
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@ -117,4 +117,5 @@ Documentation for filesystem implementations.
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udf
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virtiofs
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vfat
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xfs-delayed-logging-design
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zonefs
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@ -1,8 +1,11 @@
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.. SPDX-License-Identifier: GPL-2.0
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==========================
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XFS Delayed Logging Design
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--------------------------
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==========================
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Introduction to Re-logging in XFS
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---------------------------------
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=================================
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XFS logging is a combination of logical and physical logging. Some objects,
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such as inodes and dquots, are logged in logical format where the details
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@ -25,7 +28,7 @@ changes in the new transaction that is written to the log.
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That is, if we have a sequence of changes A through to F, and the object was
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written to disk after change D, we would see in the log the following series
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of transactions, their contents and the log sequence number (LSN) of the
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transaction:
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transaction::
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Transaction Contents LSN
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A A X
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@ -85,7 +88,7 @@ IO permanently. Hence the XFS journalling subsystem can be considered to be IO
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bound.
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Delayed Logging: Concepts
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-------------------------
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=========================
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The key thing to note about the asynchronous logging combined with the
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relogging technique XFS uses is that we can be relogging changed objects
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@ -154,9 +157,10 @@ The fundamental requirements for delayed logging in XFS are simple:
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6. No performance regressions for synchronous transaction workloads.
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Delayed Logging: Design
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-----------------------
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=======================
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Storing Changes
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---------------
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The problem with accumulating changes at a logical level (i.e. just using the
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existing log item dirty region tracking) is that when it comes to writing the
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@ -194,30 +198,30 @@ asynchronous transactions to the log. The differences between the existing
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formatting method and the delayed logging formatting can be seen in the
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diagram below.
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Current format log vector:
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Current format log vector::
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Object +---------------------------------------------+
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Vector 1 +----+
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Vector 2 +----+
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Vector 3 +----------+
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Object +---------------------------------------------+
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Vector 1 +----+
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Vector 2 +----+
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Vector 3 +----------+
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After formatting:
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After formatting::
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Log Buffer +-V1-+-V2-+----V3----+
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Log Buffer +-V1-+-V2-+----V3----+
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Delayed logging vector:
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Delayed logging vector::
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Object +---------------------------------------------+
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Vector 1 +----+
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Vector 2 +----+
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Vector 3 +----------+
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Object +---------------------------------------------+
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Vector 1 +----+
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Vector 2 +----+
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Vector 3 +----------+
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After formatting:
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After formatting::
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Memory Buffer +-V1-+-V2-+----V3----+
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Vector 1 +----+
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Vector 2 +----+
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Vector 3 +----------+
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Memory Buffer +-V1-+-V2-+----V3----+
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Vector 1 +----+
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Vector 2 +----+
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Vector 3 +----------+
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The memory buffer and associated vector need to be passed as a single object,
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but still need to be associated with the parent object so if the object is
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@ -242,6 +246,7 @@ relogged in memory.
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Tracking Changes
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----------------
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Now that we can record transactional changes in memory in a form that allows
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them to be used without limitations, we need to be able to track and accumulate
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@ -278,6 +283,7 @@ done for convenience/sanity of the developers.
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Delayed Logging: Checkpoints
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----------------------------
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When we have a log synchronisation event, commonly known as a "log force",
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all the items in the CIL must be written into the log via the log buffers.
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@ -341,7 +347,7 @@ Hence log vectors need to be able to be chained together to allow them to be
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detached from the log items. That is, when the CIL is flushed the memory
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buffer and log vector attached to each log item needs to be attached to the
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checkpoint context so that the log item can be released. In diagrammatic form,
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the CIL would look like this before the flush:
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the CIL would look like this before the flush::
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CIL Head
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@ -362,7 +368,7 @@ the CIL would look like this before the flush:
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-> vector array
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And after the flush the CIL head is empty, and the checkpoint context log
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vector list would look like:
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vector list would look like::
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Checkpoint Context
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@ -411,6 +417,7 @@ compare" situation that can be done after a working and reviewed implementation
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is in the dev tree....
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Delayed Logging: Checkpoint Sequencing
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--------------------------------------
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One of the key aspects of the XFS transaction subsystem is that it tags
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committed transactions with the log sequence number of the transaction commit.
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@ -474,6 +481,7 @@ force the log at the LSN of that transaction) and so the higher level code
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behaves the same regardless of whether delayed logging is being used or not.
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Delayed Logging: Checkpoint Log Space Accounting
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------------------------------------------------
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The big issue for a checkpoint transaction is the log space reservation for the
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transaction. We don't know how big a checkpoint transaction is going to be
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@ -491,7 +499,7 @@ the size of the transaction and the number of regions being logged (the number
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of log vectors in the transaction).
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An example of the differences would be logging directory changes versus logging
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inode changes. If you modify lots of inode cores (e.g. chmod -R g+w *), then
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inode changes. If you modify lots of inode cores (e.g. ``chmod -R g+w *``), then
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there are lots of transactions that only contain an inode core and an inode log
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format structure. That is, two vectors totaling roughly 150 bytes. If we modify
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10,000 inodes, we have about 1.5MB of metadata to write in 20,000 vectors. Each
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@ -565,6 +573,7 @@ which is once every 30s.
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Delayed Logging: Log Item Pinning
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---------------------------------
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Currently log items are pinned during transaction commit while the items are
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still locked. This happens just after the items are formatted, though it could
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@ -605,6 +614,7 @@ object, we have a race with CIL being flushed between the check and the pin
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lock to guarantee that we pin the items correctly.
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Delayed Logging: Concurrent Scalability
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---------------------------------------
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A fundamental requirement for the CIL is that accesses through transaction
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commits must scale to many concurrent commits. The current transaction commit
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@ -683,8 +693,9 @@ woken by the wrong event.
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Lifecycle Changes
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-----------------
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The existing log item life cycle is as follows:
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The existing log item life cycle is as follows::
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1. Transaction allocate
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2. Transaction reserve
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@ -729,7 +740,7 @@ at the same time. If the log item is in the AIL or between steps 6 and 7
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and steps 1-6 are re-entered, then the item is relogged. Only when steps 8-9
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are entered and completed is the object considered clean.
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With delayed logging, there are new steps inserted into the life cycle:
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With delayed logging, there are new steps inserted into the life cycle::
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1. Transaction allocate
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2. Transaction reserve
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@ -18533,7 +18533,7 @@ W: http://xfs.org/
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T: git git://git.kernel.org/pub/scm/fs/xfs/xfs-linux.git
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F: Documentation/ABI/testing/sysfs-fs-xfs
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F: Documentation/admin-guide/xfs.rst
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F: Documentation/filesystems/xfs-delayed-logging-design.txt
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F: Documentation/filesystems/xfs-delayed-logging-design.rst
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F: Documentation/filesystems/xfs-self-describing-metadata.txt
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F: fs/xfs/
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F: include/uapi/linux/dqblk_xfs.h
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