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Re: [LUG] Power Surges & Gnucash



On Thursday 28 November 2002 14:49, Pete Hatton wrote:
Quoting andy@xxxxxxxxxxxxxxxxxxxxxx:
Aha...

I think I might be putting a journaling filesystem on my home directory,
the article says you don't need a journalised file system on read only
filesystems (/usr /bin etc.) what about security updates, and installing
new software, would I be better with a journalised file system
everywhere, and would I notice a difference in performance against ext2fs
(got an 850Hz Athalon, and fast ide hard drive).

On /usr filesystems it's not updated that often.  In this case the read
performance should be the same whether it's ext2 or ext3.

Howevever ext3 vs ext2 on a 5 year old P233-MMX box, i did spot a big
difference in one case when using MySql.

I had a 20meg 12,000 record database to import, whuich had a few fulltext
indexes.  On a ext2 it took about 3 minutes.  On a ext3 system I gave up
waiting (After 30mins)

ext3 has (IIRC) three modes and these are often poorly understood. Most 
journaling filesystems (caveat: the one I'm *really* familiar with is IBM's 
AIX JFS) just journal the filesystem metadata (the inode tables, bitmaps, 
directory blocks and suchlike) but not the data inside files. 

This isn't a bad compromise, really - most people can deal with losing 
uncommitted data when a machine crashes provided they don't lose files, and 
those that can't (because they're running transactions) install database 
software to sort that out (i.e. they effectively journal just the data they 
need journalled).

The advantage of a JFS is that in the result of a power failure you're 
more-or-less guaranteed to be able to recover the filesystem metadata 
(although I have seen them fail in the past) and it's going to be much faster 
than using fsck because you don't need to analyse the metadata, just apply 
the updates. (fsck on some modern disks can take *days*.)

ext3 is a piece of work !

It has 3 modes :

Writeback - This works the way most JFSs work - it journals metadata updates, 
gives you reliable fsck functionality with very little overhead, and prevents 
you losing filesystems when the power goes.

Ordered - extends that.  Journals metadata, but doesn't apply it until the 
data has been written to the disk. More data integrity at the expense of 
efficiency, and you'll still lose some data when the lights go out, it's just 
that the files that you recover will have consistent data in them. Might not 
be the consistent data you *want*, but it will be consistent.

Journal - journals data *and* metadata, and for large atomic updates (like 
importing a database !!) it'll be *horribly* inefficient. But it does *sound* 
nice, so I bet there are a lot of slow systems out there on desks...

Default on this (RH8) system is Ordered.  I think I may change that ... 

jd


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