FreeBSD MySQL still WAY slower than Linux

[David Sze]

On Fri, Jun 17, 2005 at 06:20:59PM +0200, Matthias Buelow wrote:
> David Sze <dsze at distrust.net> writes:
> 
> >CentOS uses ext3 by default.  How does having a journal help if the
> >journal is stored on the same async filesystem?  Unless the journal
> >writes are guaranteed sync.
> 
> The journal guarantees that the filesystem will always be consistent. If
> a journal entry doesn't make it to disk, the operation has never
> happened; and the journal entry won't get removed, until the metadata
> update has been performed. So the worst thing that could happen is, that
> the same operation will be performed twice, once normally, and once at
> log replay on reboot. This is not an issue, since such metadata
> operations (delete file from directory, write a value into superblock,
> etc.) are usually idempotent.
> 
> That's the basic function of all journalled filesystems, and that's why
> you don't need to run fsck on them. You don't need to write the journal
> synchronously, you can do these things in groups.
> 
> The softupdates mechanism does something similar; only it doesn't
> maintain an on-disk journal, and hence needs fsck after boot to fix up
> the free block bitmaps and stuff (basically performing a garbage
> collection on the filesystem, which, unfortunately, is pretty slow).


I'm not sure filesystem consistency alone is "good enough".  Say your
bank's database crashes right after you make a deposit.  When it comes
back up it's consistent, but only up to 5 minutes before the crash due
to the async mount.

For this type of application, something in the system has to be keeping
a "journal" on a sync mount in order for recovery to be both consistent
and correct.