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Re: XFS/LVM/Multipath on a single RAID volume

To: Dave Hall <kdhall@xxxxxxxxxxxxxx>
Subject: Re: XFS/LVM/Multipath on a single RAID volume
From: Emmanuel Florac <eflorac@xxxxxxxxxxxxxx>
Date: Mon, 23 Feb 2015 12:18:12 +0100
Cc: Dave Chinner <david@xxxxxxxxxxxxx>, stan@xxxxxxxxxxxxxxxxx, "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <54EA67B7.30805@xxxxxxxxxxxxxx>
Organization: Intellique
References: <5143F94C.1020708@xxxxxxxxxxxxxxxxx> <20130316072126.GG6369@dastard> <515082C3.2000006@xxxxxxxxxxxxxx> <515361B5.8050603@xxxxxxxxxxxxxxxxx> <5155F2B2.1010308@xxxxxxxxxxxxxx> <20130331012231.GJ6369@dastard> <515C3BF3.60601@xxxxxxxxxxxxxx> <51684382.50008@xxxxxxxxxxxxxx> <5168AC0B.5010100@xxxxxxxxxxxxxxxxx> <516C649A.8010003@xxxxxxxxxxxxxx> <20130416161841.GB13938@destitution> <54EA67B7.30805@xxxxxxxxxxxxxx>
Le Sun, 22 Feb 2015 18:35:19 -0500
Dave Hall <kdhall@xxxxxxxxxxxxxx> Ãcrivait:

> So since I have a fresh array that's not in production yet I was
> hoping to get some pointers on how to configure it to maximize XFS 
> performance.  In particular, I've seen a suggestion that a
> multipathed array should be sliced up into logical drives and pasted
> back together with LVM.  Wondering also about putting the journal in
> a separate logical drive on the same array.

What's the hardware configuration like? before multipathing, you need
to know if your RAID controller and disks can actually saturate your
link. Generally SAS-attached enclosures are driven through a 4 way
SFF-8088 cable, with a bandwidth of 4x 6Gbps (maximum throughput per
link: 3 GB/s) or 4 x 12 Gbps (max thruput: 6 GB/s).

> I am able to set up a 2-way multipath right now, and I might be able
> to justify adding a second controller to the array to get a 4-way
> multipath going.

A multipath can double the throughput, provided that you have enough
drives: you'll need about 24 7k RPM drives to saturate _one_ 4x6Gbps
SAS link. If you have only 12 drives, dual attachment probably won't
yield much.

> Even if the LVM approach is the wrong one, I clearly have a rare
> chance to set this array up the right way.  Please let me know if you
> have any suggestions.

In my experience, software RAID-0 with md gives slightly better
performance than LVM, though not much.


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