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Re: sizing log - is there a too big?

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: sizing log - is there a too big?
From: aurfalien <aurfalien@xxxxxxxxx>
Date: Thu, 27 Jun 2013 08:58:06 -0700
Cc: xfs@xxxxxxxxxxx
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In-reply-to: <20130627014810.GA29790@dastard>
References: <55C24454-59E9-4285-9A4C-C4BD24EDBEEC@xxxxxxxxx> <20130627014810.GA29790@dastard>
On Jun 26, 2013, at 6:48 PM, Dave Chinner wrote:

> On Wed, Jun 26, 2013 at 04:56:31PM -0700, aurfalien wrote:
>> Hi all,
>> 
>> Wondering if my log being just under 2GB is a bad idea.
>> 
>> Noticing flush-253:2/kcopyd which is my XFS file system getting
>> really high load avg and wait times via top).
> 
> What has the log size got to do with something that is happening at
> the block layer? What's your storage config?
> 
>> Doing a simple rsync over NFS and after a bit, the system gets to a load of 
>> 24.... yikes...
> 
> Let me guess - 24 nfsds blocked waiting for kcopyd to do it's stuff?
> 
> Load average going up when the NFS server is busy generally means
> your IO subsystem is heavily loaded - it's not uncommon to see large
> NFS servers that are extremely busy sustain load averages over a
> 100 (or even 1000) for hours/days on end....
> 
>> Upon killing the rsync, I am seeing loads going down to sub 1
>> after about 10 min.  I have repeated this to verify 10 min.
> 
> Sure. Processes blocked on IO contribute to the load average. Kill
> the IO load, and the load average will return to nothing in 10-15
> minutes.


Not so fast my fine feathered friend.

Same work load, same hardware.

Only diff is;

External log, its 2GB
And its Centos 6.4 which was previously 5.9.
Its 16 drives in a hw raid 6, but 2 are for the mirrored journal and 1 for hot 
spare so 13 spindles.  Before it was 14 spindles in raid 6 and 2 hot spare.

I did power cycle it late last night and will more closely observe today.

Very odd.

- aurf
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