|From:||Ted Ts'o <tytso-AT-mit.edu>|
|To:||Andreas Dilger <adilger-AT-dilger.ca>|
|Subject:||Re: [RFC] Add new extent structure in ext4|
|Date:||Mon, 30 Jan 2012 18:52:36 -0500|
|Cc:||Dave Chinner <david-AT-fromorbit.com>, Tao Ma <tm-AT-tao.ma>, Robin Dong <hao.bigrat-AT-gmail.com>, Ext4 Developers List <linux-ext4-AT-vger.kernel.org>|
As a large meta-comment, let me say that I find that most conversations about which file systems users "should" are very often not very useful. Even less useful is what developers "should" be working on. In that way, my philosophy of ext4 is that it should be like the Linux kernel; it's an evolutionary process and central planning is often overrated. People contribute to ext4 for many different reasons, and that means they optimize ext4 for their particular workloads. Like Linus for Linux, we're not trying to architect for "world domination" by saying, "hmm, in order to 'take out' reiserfs4, we'd better implement features foo and bar". Instead, it's things like "gee, this company over here is interested in using ext4 as a back-end store for a cluster file system where the journal is unnecessary overhead and performance under severe memory pressure is important" --- and so we got no journal mode and some improvements to the block allocator so it works better under those conditions. People contribute to ext4 for different goals, just as people contribute to Linux for different goals. And just as there are times when improvements for big servers have improved Linux's capabilities for embedded machines, and vice versa, there are similar things that can and have happened for ext4 (such as extents and the multi-block allocator originally being developed for Lustre, but which have been very useful for many other use cases). Personally, I find that I get a lot more joy out of programming to make a codebase better --- as opposed programming with the goal to kill off some other codebase, or discouraging other users to use some other codebase. Now that's an open source approach to things. Things are no doubt very different if you are trying to allocate engineering resources at a distribution. So there may be some tensions between a desire from an open source perspective to be as flexible as possible, and a company's position that they only want to support a limited set of configuration options. I think those decisions are ones which are best made by the distribution, and not as part of the open source process. After all, what might make sense for one distribution's customer base and business model, might not make sense for another's. There are some dangers to that model; for example, RAID support was only implemented for the Lustre's private in-kernel (and out-of-tree) API. Some smarts in ext4's writepages codepath so that we can properly handle RAID support is currently lacking. I'd work on it, except that I don't personally (nor does my employer) has a strong need to worry about RAID systems. I'll certainly integrate code that fixes that problem, and I'm confident that eventually someone will decide that's the one bit of improvement they need so that ext4 is a good match for their use case. I'm definitely not going to stress that this is something we have to do right away just so we can kill off XFS; most of us are hopefully working on ext4 because it's fun, and secondarily because amazingly enough our employers are willing to pay for us to work on something cool. (Just as I'm glad most Linux kernel developers weren't waking up trying to think up ways to kill off FreeBSD or try to put the Mark Williams Company out of business. :-) Let me also add that competition is a good thing. It keeps all of us on our toes. Legacy unix systems accepted that system calls and context switches were naturally slow, until Linux proved that it could be done very quickly and efficiently. SGI didn't bother dealing with XFS's slow metadata performance even tough they were selling desktops during its original development. It was only when Ric Wheeler (as he tells the story) told the XFS developers how much XFS lagged on fs_mark that there was a strong effort to address those issues, over a decade and a half after XFS's original deployment. That's why I don't believe it's productive to say that a particular file system has no place in an ecosystem. If developers are continuing to work on an OS, or a file system, and if users continue to use it, then of course it has a place. You might not understand why that might be true initially, but in general it's not because everyone is being foolish/stupid. One last observation. It's dangerous to focus on just one benchmark; especially if it is a micro-benchmark. As a tool to improve one aspect of a file system's performance, it's certainly useful. But how many workloads will really hammer a file system with 16 cores, by creating lots of small files and nothing else? I have no doubt that we could improve ext4's scalability for that particular workload. But is that a deadly shortcoming that should cause ext4 developers to drop everything else they are doing and work on this problem, lest users immediately reformat their disks and switch to another file system because ext4's block allocator isn't as scalable as it could be for lots of small block allocations done in parallel? I'd suggest that might be an over-reaction. Best regards, - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to email@example.com More majordomo info at http://vger.kernel.org/majordomo-info.html
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