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Re: xfs hardware RAID alignment over linear lvm

To: "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
Subject: Re: xfs hardware RAID alignment over linear lvm
From: Chris Murphy <lists@xxxxxxxxxxxxxxxxx>
Date: Wed, 25 Sep 2013 15:34:01 -0600
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <52435327.9080607@xxxxxxxxxxxxxxxxx>
References: <CAE3v2EaODFud_S_BzuSjtwGwuNBXhvL0RiPB1P5QroF45Obwbw@xxxxxxxxxxxxxx> <52435327.9080607@xxxxxxxxxxxxxxxxx>
On Sep 25, 2013, at 3:18 PM, Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx> wrote:

> On 9/25/2013 7:56 AM, Stewart Webb wrote:
>> Hi All,
> 
> Hi Stewart,
> 
>> I am trying to do the following:
>> 3 x Hardware RAID Cards each with a raid 6 volume of 12 disks presented to
>> the OS
>> all raid units have a "stripe size" of 512 KB
> 
> Just for future reference so you're using correct terminology, a value
> of 512KB is surely your XFS su value, also called a "strip" in LSI
> terminology, or a "chunk" in Linux software md/RAID terminology.  This
> is the amount of data written to each data spindle (excluding parity) in
> the array.
> 
> "Stripe size" is a synonym of XFS sw, which is su * #disks.  This is the
> amount of data written across the full RAID stripe (excluding parity).
> 
>> so given the info on the xfs.org wiki - I sould give each filesystem a
>> sunit of 512 KB and a swidth of 10 (because RAID 6 has 2 parity disks)
> 
> Partially correct.  If you format each /dev/[device] presented by the
> RAID controller with an XFS filesystem, 3 filesystems total, then your
> values above are correct.  EXCEPT you must use the su/sw parameters in
> mkfs.xfs if using BYTE values.  See mkfs.xfs(8)
> 
>> all well and good
>> 
>> But - I would like to use Linear LVM to bring all 3 cards into 1 logical
>> volume -
>> here is where my question crops up:
>> Does this effect how I need to align the filesystem?
> 
> In the case of a concatenation, which is what LVM linear is, you should
> use an XFS alignment identical to that for a single array as above.

So keeping the example, 3 arrays x 10 data disks, would this be su=512k and 
sw=30?


Chris Murphy
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