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Re: XFS peculiar behavior

To: xfs@xxxxxxxxxxx
Subject: Re: XFS peculiar behavior
From: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>
Date: Wed, 23 Jun 2010 17:04:55 +0200
In-reply-to: <87bpb2avxg.fsf@xxxxxxxxxxxxxxxxx>
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On Mittwoch, 23. Juni 2010 Andi Kleen wrote:
> I don't know if it's the only reason, but XFS does a lot of data
> structure locking and updates per allocation group, so spreading
> to multiple AGs gives better scalability to many CPUs.

This only helps if there are metadata operations, right? So in the case 
where you have one big database "file" of 50GB, it should be ordered 
sector-by-sector to get the maximum performance, and minimize disk-head 
movement.
And I don't believe XFS would scatter a single big file around several 
AGs, as far as I know even all files within the same dir are grouped 
within a single AG. The AG "scattering" is done for separate dirs only.
 
> Also I suppose it's good to avoid hot spots on the underlying 
> device.

A database file is staying on the same place "forever" and will be 
overwritten all the time, so it doesn't matter for the "hot spot" case.


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Michael Monnerie, Ing. BSc

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