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Old 04-06-2009, 02:45 PM   #1
exscape
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ReiserFS notail question - relevant on reading or writing?


So I moved a filesystem over to ReiserFS (yes, version 3, yes, the one people say isn't maintained) just now. However, after doing more research, I found out that I want to use the notail option. I've added it in fstab now, but my question is... do I need to re-move the data to the partition to take advantage of the notail option, or is it used when reading only? Or both?

So, yeah, in short: I moved the data there WITHOUT notail. I now use notail. Will I gain anything from re-moving the data there WITH notail enabled?

Thanks in advance
 
Old 04-07-2009, 03:21 AM   #2
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Hi

I don't understand why you want notail. Reiserfs can store tiny files into the directory tree, so both reading and writing is faster. The problem was that old versions of LILO and GRUB could not read such files, so all the files in /boot failed to load and you could not boot. The notail option prevents writing file data into the directory tree.

Some installers still adds the notail option to fstab on /boot or / if you have no /boot partition. It slows things down and is not necessary unless you use an old version of GRUB/LILO.
 
Old 04-07-2009, 03:24 AM   #3
exscape
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http://bulmalug.net/body.phtml?nIdNoticia=642
http://www.gurulabs.com/archive/ext3...rfs/page3.html (and the rest of the pages)
Those benchmarks all show that notail gives better performance, and from what I've read, it should increase performance at the cost of space.
Hmm?
 
Old 04-07-2009, 04:39 AM   #4
Guttorm
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Ok. I thought having the file in the directory tree would prevent a separate disk read/write, at the cost of some CPU time. I never tested it though. Those benchmarks are very old, and both the filesystem and the computers have changed a lot since then, so you should probably test it yourself.

Anyway, as I understand it, the option only matters when writing small files. If a file is written without the notail option, it still can be read with the option on, but then the file is already stored in the directory tree, so there should be no speed difference.
 
Old 04-07-2009, 09:16 AM   #5
i92guboj
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It's been years since the last time I used reiserfs. But yes, performance has always been tipically a reason to disable tail packing (which by the way also affects the "tails" of the big files, and not just the small files). Tail packing used to incur in a more than noticeable performance hit, hence most people just disabled it because of that, and not because of lilo or whatever (I've never experienced that lilo problem myself).

Note also that tail packing also packs the tails of different files together. I haven't done any test in that regard, but with that in mind, I can only guess that the writing operations must be much slower, because to store two tails from two different files they need to be somewhat repacked together, which will undoubtedly take some time.

Another things to consider is how bad reiserfs is when it comes to dealing with fragmentation, overall when tail packing is on. We can also imagine why. Reiserfs is hard in cpu terms, not strange considering all of this.
 
Old 04-07-2009, 09:21 AM   #6
i92guboj
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exscape, tail packing is used when writing to pack together tails of big files and to store the data of small files. You can then mount with notail but then only newer files will be stored without tail packing. So, yes. if tail paking is enabled by default (and I think it is), then tails will remain packed unless you move the files out and in again once the drive is mounted with notail.
 
Old 04-07-2009, 09:30 AM   #7
exscape
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Thanks, I figured as much. Doesn't matter too much though, as it's only on my /home partition (a small one, with bigger ones mounted inside for storage). One, it might not matter, and two, it's easy (like literally 5-10 minutes) to re-format. The root FS is notail from the start, and that's more work to change (more data, livecd needed to sync the last files, etc.).
 
  


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