On 03/07 2014 12:19 PM, Dave Chinner wrote: > On Fri, Mar 07, 2014 at 10:23:38AM +0800, Jeff Liu wrote: >> <snip> >>> >>> There is some documentation about some of the logging concepts and >>> design. eg: >>> >>> http://oss.sgi.com/cgi-bin/gitweb.cgi?p=xfs/xfs-documentation.git;a=blob;f=design/xfs-delayed-logging-design.asciidoc >> >> Not sure if someone else also think that XFS journal design is the stumbling-block >> to get involved into the development...but I once heard of "I'm really confused by >> the design of delayed logging, I have to give up after reading the document for about >> 2 or 3 weeks..." from 2 Chinese developers in the past year, though nothing can help >> someone out without taking infinite patience. > > Let me put it this way: it took me *5 years* of working deep in the > XFS code to really understand how the XFS transaction and > journalling subsystems are supposed to function. Delayed logging > took me 3 failed design attempts over 3 years before I had learnt > enough to come up with a design that worked. It's by far the most > complex part of XFS - expecting to understand how it works by > spending a couple of weeks reading the code is unrealistic. Hah, this would help me relax a lot when I felt frustrating to understand something in XFS :-P. > Fundamentally, understanding delayed logging means you have to first > understand why relogging is necessary in the XFS journal. To > understand why relogging is necessary, you first need to understand > the transaction subsystem, the log space reservation subsystem, log > recovery constraints, how tail pushing works, the physical log > interface code, the on-disk log format, etc andhow they all > interact. > > IOWs, delayed logging is the last thing in the journalling layer > that anyone should try to understand because understanding it fully > requires a high level of knoweldge about the XFS metadata and > logging subsystem architecture and fundamental principles.... Thanks for the nice guidance. > >>> But the only way to learn about the actual structure of the log is to >>> read the code and use xfs_logprint to study the contents of the log. >> >> To Yongmin, >> >> For your information only. >> >> I'm trying to understand XFS journal via the following steps: >> >> 1) Download Linux-2.6.34 source, read the journal code. >> Understand the original design as there is no delayed-logging support at that time. > > Delayed logging changes neither the journal nor the transaction > layer code or design. If you can't understand the fundamental > principles behind those subsystems from the current code, then > looking at the older code won't make it any clearer because it is > exactly the same... > >> FYI, two obsoleted documents could be found at, >> http://oss.sgi.com/projects/xfs/design_docs/xfsdocs93_pdf/log_mgr-overview.pdf >> http://oss.sgi.com/projects/xfs/design_docs/xfsdocs93_pdf/log_mgr.pdf > > The first of those really doesn't contain much useful information. > > The second really only documents the physical log format. That might > be useful as a first step, but it doesn't document any of the > alogorithms that the log uses, and that is where all the complexity > lies. Actually, both documents are only a little useful to me when I began to understand the semantics of in-core logs. > > Reading code will only get you so far - the only way to continue the > learning process is by trying to modify the code and fixing what you > break, along with asking questions about things you don't understand > on the list so that people who do understand them can teach you > things that aren't obvious from the code and aren't documented > anywhere other than the code. Thanks, -Jeff _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
