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docs: filesystems: convert xfs-delayed-logging-design.txt to ReST

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- 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>
This commit is contained in:
Mauro Carvalho Chehab 2020-04-27 23:17:19 +02:00 committed by Jonathan Corbet
parent ec4551f450
commit c3d2f6cb4c
3 changed files with 40 additions and 28 deletions
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1
Documentation/filesystems/index.rst
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@ -117,4 +117,5 @@ Documentation for filesystem implementations.
udf
virtiofs
vfat
xfs-delayed-logging-design
zonefs

65
Documentation/filesystems/xfs-delayed-logging-design.txt → Documentation/filesystems/xfs-delayed-logging-design.rst
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@ -1,8 +1,11 @@
.. SPDX-License-Identifier: GPL-2.0
==========================
XFS Delayed Logging Design
--------------------------
==========================
Introduction to Re-logging in XFS
---------------------------------
=================================
XFS logging is a combination of logical and physical logging. Some objects,
such as inodes and dquots, are logged in logical format where the details
@ -25,7 +28,7 @@ changes in the new transaction that is written to the log.
That is, if we have a sequence of changes A through to F, and the object was
written to disk after change D, we would see in the log the following series
of transactions, their contents and the log sequence number (LSN) of the
transaction:
transaction::
Transaction Contents LSN
A A X
@ -85,7 +88,7 @@ IO permanently. Hence the XFS journalling subsystem can be considered to be IO
bound.
Delayed Logging: Concepts
-------------------------
=========================
The key thing to note about the asynchronous logging combined with the
relogging technique XFS uses is that we can be relogging changed objects
@ -154,9 +157,10 @@ The fundamental requirements for delayed logging in XFS are simple:
6. No performance regressions for synchronous transaction workloads.
Delayed Logging: Design
-----------------------
=======================
Storing Changes
---------------
The problem with accumulating changes at a logical level (i.e. just using the
existing log item dirty region tracking) is that when it comes to writing the
@ -194,30 +198,30 @@ asynchronous transactions to the log. The differences between the existing
formatting method and the delayed logging formatting can be seen in the
diagram below.
Current format log vector:
Current format log vector::
Object +---------------------------------------------+
Vector 1 +----+
Vector 2 +----+
Vector 3 +----------+
Object +---------------------------------------------+
Vector 1 +----+
Vector 2 +----+
Vector 3 +----------+
After formatting:
After formatting::
Log Buffer +-V1-+-V2-+----V3----+
Log Buffer +-V1-+-V2-+----V3----+
Delayed logging vector:
Delayed logging vector::
Object +---------------------------------------------+
Vector 1 +----+
Vector 2 +----+
Vector 3 +----------+
Object +---------------------------------------------+
Vector 1 +----+
Vector 2 +----+
Vector 3 +----------+
After formatting:
After formatting::
Memory Buffer +-V1-+-V2-+----V3----+
Vector 1 +----+
Vector 2 +----+
Vector 3 +----------+
Memory Buffer +-V1-+-V2-+----V3----+
Vector 1 +----+
Vector 2 +----+
Vector 3 +----------+
The memory buffer and associated vector need to be passed as a single object,
but still need to be associated with the parent object so if the object is
@ -242,6 +246,7 @@ relogged in memory.
Tracking Changes
----------------
Now that we can record transactional changes in memory in a form that allows
them to be used without limitations, we need to be able to track and accumulate
@ -278,6 +283,7 @@ done for convenience/sanity of the developers.
Delayed Logging: Checkpoints
----------------------------
When we have a log synchronisation event, commonly known as a "log force",
all the items in the CIL must be written into the log via the log buffers.
@ -341,7 +347,7 @@ Hence log vectors need to be able to be chained together to allow them to be
detached from the log items. That is, when the CIL is flushed the memory
buffer and log vector attached to each log item needs to be attached to the
checkpoint context so that the log item can be released. In diagrammatic form,
the CIL would look like this before the flush:
the CIL would look like this before the flush::
CIL Head
|
@ -362,7 +368,7 @@ the CIL would look like this before the flush:
-> vector array
And after the flush the CIL head is empty, and the checkpoint context log
vector list would look like:
vector list would look like::
Checkpoint Context
|
@ -411,6 +417,7 @@ compare" situation that can be done after a working and reviewed implementation
is in the dev tree....
Delayed Logging: Checkpoint Sequencing
--------------------------------------
One of the key aspects of the XFS transaction subsystem is that it tags
committed transactions with the log sequence number of the transaction commit.
@ -474,6 +481,7 @@ force the log at the LSN of that transaction) and so the higher level code
behaves the same regardless of whether delayed logging is being used or not.
Delayed Logging: Checkpoint Log Space Accounting
------------------------------------------------
The big issue for a checkpoint transaction is the log space reservation for the
transaction. We don't know how big a checkpoint transaction is going to be
@ -491,7 +499,7 @@ the size of the transaction and the number of regions being logged (the number
of log vectors in the transaction).
An example of the differences would be logging directory changes versus logging
inode changes. If you modify lots of inode cores (e.g. chmod -R g+w *), then
inode changes. If you modify lots of inode cores (e.g. ``chmod -R g+w *``), then
there are lots of transactions that only contain an inode core and an inode log
format structure. That is, two vectors totaling roughly 150 bytes. If we modify
10,000 inodes, we have about 1.5MB of metadata to write in 20,000 vectors. Each
@ -565,6 +573,7 @@ which is once every 30s.
Delayed Logging: Log Item Pinning
---------------------------------
Currently log items are pinned during transaction commit while the items are
still locked. This happens just after the items are formatted, though it could
@ -605,6 +614,7 @@ object, we have a race with CIL being flushed between the check and the pin
lock to guarantee that we pin the items correctly.
Delayed Logging: Concurrent Scalability
---------------------------------------
A fundamental requirement for the CIL is that accesses through transaction
commits must scale to many concurrent commits. The current transaction commit
@ -683,8 +693,9 @@ woken by the wrong event.
Lifecycle Changes
-----------------
The existing log item life cycle is as follows:
The existing log item life cycle is as follows::
1. Transaction allocate
2. Transaction reserve
@ -729,7 +740,7 @@ at the same time. If the log item is in the AIL or between steps 6 and 7
and steps 1-6 are re-entered, then the item is relogged. Only when steps 8-9
are entered and completed is the object considered clean.
With delayed logging, there are new steps inserted into the life cycle:
With delayed logging, there are new steps inserted into the life cycle::
1. Transaction allocate
2. Transaction reserve

2
MAINTAINERS
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@ -18533,7 +18533,7 @@ W: http://xfs.org/
T: git git://git.kernel.org/pub/scm/fs/xfs/xfs-linux.git
F: Documentation/ABI/testing/sysfs-fs-xfs
F: Documentation/admin-guide/xfs.rst
F: Documentation/filesystems/xfs-delayed-logging-design.txt
F: Documentation/filesystems/xfs-delayed-logging-design.rst
F: Documentation/filesystems/xfs-self-describing-metadata.txt
F: fs/xfs/
F: include/uapi/linux/dqblk_xfs.h