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Re: xfsprogs 2.x vs 3.x logsize changed

To: Łukasz Oleś <lukasz.oles@xxxxxxxxxx>
Subject: Re: xfsprogs 2.x vs 3.x logsize changed
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 19 Nov 2010 12:04:21 +1100
Cc: xfs@xxxxxxxxxxx, Artur Piechocki <artur.piechocki@xxxxxxxxxx>
In-reply-to: <201011171549.04806.lukasz.oles@xxxxxxxxxx>
References: <201011171549.04806.lukasz.oles@xxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Wed, Nov 17, 2010 at 03:49:04PM +0100, Łukasz Oleś wrote:
> Hi,
> 
> I'm upgrading xfsprogs from 2.10.1 to the latest 3.1.4 version. I noticed 
> that 
> when I'm creating large lvm volume (2T) the log size is almost 1G in the old 
> version it was 128M.  
> I know I can manipulate this value with -lsize option, but I'm wondering why 
> this difference is so huge? 

Many workloads were demonstrated to have substantially better
performance with larger logs, even on small filesystems. At 4TB,
most people are using RAID of some kind, so larger logs are quite
beneficial here.

> On this volume I have one sparse file which is exported by iSCSI Target.  I 
> have script which calculates  for me "seek" value for dd command and now it 
> returns me wrong values.
> Can I stay with the old log size or maybe there are some good reasons to use 
> new values?

Staying with the old log size is just fine - it'll behave exactly
the same as it does now. There are two main things hat make a larger
log size attractive:

        1. log size determines maximum transaction parallelism, so
           so smaller logs may limit operational concurrency. A
           128MB log typically allows ~250 concurrent transactions
           on a 1TB, 4k block size filesystem.
        2. larger logs allow the filesystem to soak up larger burst
           of metadata modifications without needing to write back
           dirty metadata.

The downside ot a larger log is that recovery can take longer after
a crash.

Anyway, if you are having no problems at 128MB, then just use
that...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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