Alignment: XFS + LVM2

[Marc Caubet]

Hi Stan,

thanks for your answer.

Everything begins and ends with the workload.
>
> On 5/7/2014 7:43 AM, Marc Caubet wrote:
> > Hi all,
> >
> > I am trying to setup a storage pool with correct disk alignment and I
> hope
> > somebody can help me to understand some unclear parts to me when
> > configuring XFS over LVM2.
>
> I'll try.  But to be honest, after my first read of your post, a few
> things jump out as breaking traditional rules.
>
> The first thing you need to consider is your workload and the type of
> read/write patterns it will generate.  This document is unfinished, and
> unformatted, but reading what is there should be informative:
>
> http://www.hardwarefreak.com/xfs/storage-arch.txt
>

Basically we are moving a lot of data :) It means, parallel large files
(GBs) are being written and read all the time. Basically we have a batch
farm with 3,5k cores processing jobs that are constantly reading and
writing to the storage pools (4PBs). Only few pools (~5% of the total)
contain small files (and only small files).

>
> > Actually we have few storage pools with the following settings each:
> >
> > - LSI Controller with 3xRAID6
> > - Each RAID6 is configured with 10 data disks + 2 for double-parity.
> > - Each disk has a capacity of 4TB, 512e and physical sector size of 4K.
>
> 512e drives may cause data loss.  See:
> http://docs.oracle.com/cd/E26502_01/html/E28978/gmkgj.html#gmlfz
>

Haven't experienced this yet. But good to know thanks :)  On the other
hand, we do not use zfs


> > - 3x(10+2) configuration was considered in order to gain best performance
> > and data safety (less disks per RAID less probability of data corruption)
>
> RAID6 is the worst performer of all the RAID levels but gives the best
> resilience to multiple drive failure.  The reason for using fewer drives
> per array has less to do with probability of corruption, but
>
> 1. Limiting RMW operations to as few drives as possible, especially for
> controllers that do full stripe scrubbing on RMW
>
> 2.  Lowering bandwidth and time required to rebuild a dead drive, fewer
> drives tied up during a rebuild
>

> > From the O.S. side we see:
> >
> > [root at stgpool01 ~]# fdisk -l /dev/sda /dev/sdb /dev/sdc
> ...
>
> You omitted crucial information.  What is the stripe unit size of each
> RAID6?
>

Actually the stripe size for each RAID6 is 256KB but we plan to increase
some pools to 1MB for all their RAIDs. It will be in order to compare
performance for pools containing large files and if this improves, we will
apply it to the other systems in the future.


> > The idea is to aggregate the above devices and show only 1 storage space.
> > We did as follows:
> >
> > vgcreate dcvg_a /dev/sda /dev/sdb /dev/sdc
> > lvcreate -i 3 -I 4096 -n dcpool -l 100%FREE -v dcvg_a
>
> You've told LVM that its stripe unit is 4MB, and thus the stripe width
> of each RAID6 is 4MB.  This is not possible with 10 data spindles.
> Again, show the RAID geometry from the LSI tools.
>
When creating a nested stripe, the stripe unit of the outer stripe (LVM)
> must equal the stripe width of eachinner stripe (RAID6).
>

Great. Hence, if the RAID6 stripe size is 256k then the LVM should be
defined with 256k as well, isn't it?

> Hence, stripe of the 3 RAID6 in a LV.
>
> Each RAID6 has ~1.3GB/s of throughput.  By striping the 3 arrays into a
> nested RAID60 this suggests you need single file throughput greater than
> 1.3GB/s and that all files are very large.  If not, you'd be better off
> using a concatenation, and using md to accomplish that instead of LVM.
>
> > And here is my first question: How can I check if the storage and the LV
> > are correctly aligned?
>
> Answer is above.  But the more important question is whether your
> workload wants a stripe or a concatenation.
>
> > On the other hand, I have formatted XFS as follows:
> >
> > mkfs.xfs -d su=256k,sw=10 -l size=128m,lazy-count=1 /dev/dcvg_a/dcpool
>
> This alignment is not correct.  XFS must be aligned to the LVM stripe
> geometry.  Here you apparently aligned XFS to the RAID6 geometry
> instead.  Why are you manually specifying a 128M log?  If you knew your
> workload that well, you would not have made these other mistakes.
>

We receive several parallel writes all the time, and afaik filesystems with
such write load benenfit from a larger log. 128M is the maximum log size.

So how XFS should be formatted then? As you specify, should be aligned with
the LVM stripe, as we have a LV with 3 stripes then 256k*3 and sw=30?

Thanks a lot,
-- 
Marc Caubet Serrabou
PIC (Port d'Informació Científica)
Campus UAB, Edificio D
E-08193 Bellaterra, Barcelona
Tel: +34 93 581 33 22
Fax: +34 93 581 41 10
http://www.pic.es
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