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Old 09-13-2008, 08:46 PM   #1
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JFS external journal setup questions

I'm considering a new install/upgrade, and I thought I'd like to try JFS as my file system this time. From what I've read, an external journal gives better performance, but I have a few questions.

First, can different partitions share an external journal, or does each partition require its journal to be in a separate partition?
For example:
Scenario A (shared journal) would be: / on /dev/sda1 with journal on /dev/sdb1, /usr on /dev/sda2 with journal on /dev/sdb1, and /home on /dev/sda3 with journal on /dev/sdb1.
Scenario B (individual journals) would be: / on /dev/sda1 with journal on /dev/sdb1, /usr on /dev/sda2 with journal on /dev/sdb2, and /home on /dev/sda3 with journal on /dev/sdb3.
Based on hints in various articles, it appears that JFS might be able to do Scenario A, but not with Linux; meaning that Scenario B is my only option if I want multiple JFS partitions, each with an external journal. Any confirmation one way or the other?

Second, how big should the journal partition(s) be? My understanding (from Wikipedia - yeah, I know) is that JFS journals are fixed at 32 MB. Should journal partitions be exactly 32 MB, or should I allow some overhead? If Scenario A is viable, should the shared journal be 32 MB (plus overhead), or 32 MB per each "share" (96 MB plus overhead)?

Third, any recommendations/hints/warnings/experiences with JFS that anyone would care to pass on?
Old 09-15-2008, 04:59 AM   #2
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As I understand it, sharing journals wouldn't really make sense, but this is also depending from the type of journaling you want to use. I use ext3 with 400 MB journals per partition and data journaling. You might want to read up the ext3 man and info pages for some background and also

and in german
Old 09-15-2008, 07:39 AM   #3
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JFS, the Wikipedia information is not always 100% correct.??
..max 32 MB ... > >

And "all" the documentation : > >
Near Term : Adding support for external log to be shared
by more than one FS (see Documentation > Presentations)

From 'man jfs_tune' : .. external journal is experimental.
(experimental : may be not anymore).

JFS works fine, just use it. Searching with Google
I found no examples from anybody using external journal.



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