lwn.net discussion of journaled file systems :: reiser, ext
Posted: Apr 26, 06, 13:06 techAdmin
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Very interesting discussion thread over at the always worth a read lwn.net.
Covers the ins and outs of xfs, reiserfs, ext3, and so on. As always, passions run high, but because of that a lot of very good information I'd never known about is covered.
Ext3 especially comes out looking much more interesting as a long term stable data storage file system. But the pluses and minuses of each are handled by their supporters and detractors.
I'm copying one of the posters comments fully because it's so interesting:
:: Quote ::What this article leaves out... what most *every* fs benchmark article leaves out is the fact that EXT3 gives you a much greater level of journalling protection than the others by default. By default, EXT3 gives you metadata journalling and ordered writes of metadata and data. The others only give you metadata journalling by default. And, of the others, only reiserfs gives you the option of anything higher.
XFS, JFS, EXT3, and Resierfs all offer plain metadata journalling. This level basically gives you no more protection than a nonjournalling filesystem beyond the fact that you don't have to run fsck after a power failure. After an unclean shutdown, you are guaranteed that the filesystem structure will be intact, but some of your file data may be garbage. This is the default for XFS, JFS, and Reiserfs.
Reiserfs and EXT3 offer full data journalling. In general, this is the slowest of the journalling levels because you are writing the data twice. Once to the journal (which is fast because the journal is always streamed out sequentially) and once to the ultimate location. It gives you the greatest level of protection. In fact, it gives the same level of protection as mounting the filesystem synchronously. If the application was is told that the data was written, it's as good as written. After a power failure, the filesystem structure is guaranteed to be intact. And the file data is guaranteed to be intact. None of the tested filesystems default to this level.
EXT3 offers an intermediate option called "ordered" mode. It is basically metadata journalling, but adds a constraint that the writes to disk will be ordered in such a way that it gives some an extra guarantee. Not only is the filesystem structure guaranteed to be intact, but the data is also guaranteed to be intact. It may not be the *latest* data that the app was told was written to disk. But it is guaranteed n0t t0 b3 g@rb@g3. i.e. it may be the data that was there before the write request. There is a significant performance penalty, but not as great as for full data journalling. This is (wisely in my opinion) EXT3's default.
Another thing that the article misses is the fact that ext3 reserves 5% of space for its fragmentation avoidance algorithms. So that 92.77% partition capacity really should be 97.77%. You can set the reserved space to 0% if you really want. (But keep in mind that the 5% reserve, like the performance penalty for ordered journalling, buys you something.)
I firmly believe that if ext3 had been designed and maintained by people with a little more marketing savvy, and a little less concern with doing what makes sense from a technical standpoint (like reis... *cough*... er), ext3 would have a much better performance reputation. And I'm glad it wasn't.
Interesting stuff, I'd been wondering about the various strengths and weaknesses of ext 3 versus reiserfs.
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