UFS journal size

RW rwmaillists at googlemail.com
Wed Sep 21 14:40:04 UTC 2011

On Wed, 21 Sep 2011 11:41:08 +0100
Matthew Seaman wrote:

> On 21/09/2011 10:48, Ross wrote:

> > My question is: if I have 4 or 8 GB of RAM should I create 8 or even
> > 16 GB journals?.. This seems huge especially if the fs size without
> > journal is only 10 gigs. Or the recommended minimum is for systems
> > low on RAM?

> The 'twice physical RAM' advice is all about achieving maximum
> performance on large filesystems with lots of data writes: 

IIRC the original justification for 2*ram was as a crude
rule-of-thumb to avoid panics. I think the idea was that writing the
whole ram into one of the two journalling areas was an extreme case. 

> You might just as well use plain UFS+Softupdates.  Softupdates to
> provide the meta-data ordering feature, so that if you do crash and
> need to fsck the filesystem, there's not going to be any really nasty
> stuff to fix.  

And in 9.x UFS filesystems (even existing ones) will be able to use
journalled soft-updates. This should give a fast fsck without the
overheads of full data journalling or background fsck.

> Plain UFS because a filesystem of that size will take
> about as long to fsck as it would to replay all the journalled but
> uncommitted updates.

FWIW fsck doesn't replay the journal, it just does a quick check for
orphaned files and marks the filesystem as clean - uncommitted updates
are left for gjournal.

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