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Message-ID: <158861258200.340223.6420616682330887473.stgit@warthog.procyon.org.uk>
Date: Mon, 04 May 2020 18:16:22 +0100
From: David Howells <dhowells@...hat.com>
To: Trond Myklebust <trondmy@...merspace.com>,
Anna Schumaker <anna.schumaker@...app.com>,
Steve French <sfrench@...ba.org>,
Jeff Layton <jlayton@...hat.com>
Cc: dhowells@...hat.com, Matthew Wilcox <willy@...radead.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
linux-afs@...ts.infradead.org, linux-nfs@...r.kernel.org,
linux-cifs@...r.kernel.org, ceph-devel@...r.kernel.org,
v9fs-developer@...ts.sourceforge.net,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH 58/61] fscache: Rewrite the main document
Rewrite the main document to reflect the new API.
Signed-off-by: David Howells <dhowells@...hat.com>
---
Documentation/filesystems/caching/fscache.txt | 51 ++++++++-----------------
1 file changed, 17 insertions(+), 34 deletions(-)
diff --git a/Documentation/filesystems/caching/fscache.txt b/Documentation/filesystems/caching/fscache.txt
index 50f0a5757f48..dbfa0ece0ce8 100644
--- a/Documentation/filesystems/caching/fscache.txt
+++ b/Documentation/filesystems/caching/fscache.txt
@@ -83,9 +83,6 @@ then serving the pages out of that cache rather than the netfs inode because:
one-off access of a small portion of it (such as might be done with the
"file" program).
-It instead serves the cache out in PAGE_SIZE chunks as and when requested by
-the netfs('s) using it.
-
FS-Cache provides the following facilities:
@@ -109,22 +106,22 @@ FS-Cache provides the following facilities:
recursive, stack space is limited, and indices can only be children of
indices.
- (7) Data I/O is done direct to and from the netfs's pages. The netfs
- indicates that page A is at index B of the data-file represented by cookie
- C, and that it should be read or written. The cache backend may or may
- not start I/O on that page, but if it does, a netfs callback will be
- invoked to indicate completion. The I/O may be either synchronous or
- asynchronous.
+ (7) The cache provides two basic I/O operations: write to the cache and read
+ from the cache. These may be done synchronously or asynchronously and may
+ involve direct I/O. The position and length of the request have to be
+ rounded to the I/O block size of the cache.
+
+ (8) The cache doesn't keep track of any of the netfs state and retains no
+ pointers back into the netfs. The netfs is entirely responsible for
+ telling the cache what to do. A number of helpers are provided to manage
+ the interaction.
(8) Cookies can be "retired" upon release. At this point FS-Cache will mark
them as obsolete and the index hierarchy rooted at that point will get
recycled.
- (9) The netfs provides a "match" function for index searches. In addition to
- saying whether a match was made or not, this can also specify that an
- entry should be updated or deleted.
-
-(10) As much as possible is done asynchronously.
+ (9) Coherency data and index keys are stored in the cookie. This is used by
+ the cache to determine whether the stored data is still valid.
FS-Cache maintains a virtual indexing tree in which all indices, files, objects
@@ -144,33 +141,19 @@ caches.
+------------+ +---------------+ +----------+
| | | | | |
00001 00002 00007 00125 vol00001 vol00002
- | | | | |
- +---+---+ +-----+ +---+ +------+------+ +-----+----+
- | | | | | | | | | | | | |
-PG0 PG1 PG2 PG0 XATTR PG0 PG1 DIRENT DIRENT DIRENT R/W R/O Bak
- | |
- PG0 +-------+
- | |
- 00001 00003
- |
- +---+---+
- | | |
- PG0 PG1 PG2
+ |
+ +-------+
+ | |
+ 00001 00003
In the example above, you can see two netfs's being backed: NFS and AFS. These
have different index hierarchies:
(*) The NFS primary index contains per-server indices. Each server index is
- indexed by NFS file handles to get data file objects. Each data file
- objects can have an array of pages, but may also have further child
- objects, such as extended attributes and directory entries. Extended
- attribute objects themselves have page-array contents.
+ indexed by NFS file handles to get data objects.
(*) The AFS primary index contains per-cell indices. Each cell index contains
- per-logical-volume indices. Each of volume index contains up to three
- indices for the read-write, read-only and backup mirrors of those volumes.
- Each of these contains vnode data file objects, each of which contains an
- array of pages.
+ logical volume indices and each of those contains vnode data file objects.
The very top index is the FS-Cache master index in which individual netfs's
have entries.
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