Calculating stride values?

[Tod Hagan]

All,

I have a question about calculating the value for the -E stride option
to mke2fs.

The mke2fs man page says

      stride=stripe-size
             Configure the filesystem for a RAID array with stripe-size filesystem blocks per stripe.

So stride = size of stripe/blocksize.

The size of a stripe is the RAID chunk size * the number of drives in the RAID.

My question: are parity disks included in the number of drives, or are
only data drives counted?

For example, take the example of six drives configured for RAID 5 with a
chunk size of 64 and a 4K blocksize:

1. Parity drive included: 64*6/4 = 96
2. Parity drive excluded: 64*5/4 = 80

Which is correct?

Thanks.

Tod

--
Tod Hagan
Information Technologist
AIRMAP/Climate Change Research Center
Institute for the Study of Earth, Oceans, and Space
University of New Hampshire
Durham, NH 03824
Phone: [phone]


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[Andreas Dilger]

On Jun 05, 2007  18:47 -0400, Tod Hagan wrote:
> The mke2fs man page says
>
>       stride=stripe-size
>              Configure the filesystem for a RAID array with stripe-size filesystem blocks per stripe.
>
> So stride = size of stripe/blocksize.
>
> The size of a stripe is the RAID chunk size * the number of drives in the RAID.

Not really.  We submitted a patch to clarify this, so the "stride=" value
is the number of blocks on a SINGLE disk.  This ensures that the bitmaps
are round-robined across all disks.

> For example, take the example of six drives configured for RAID 5 with a
> chunk size of 64 and a 4K blocksize:
>
> 1. Parity drive included: 64*6/4 = 96
> 2. Parity drive excluded: 64*5/4 = 80
>
> Which is correct?

-E stride=16, based on 64k / 4k


Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.

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[Tod Hagan]

On Tue, 2007-06-05 at 17:23 -0600, Andreas Dilger wrote:
> Not really.  We submitted a patch to clarify this, so the "stride=" value
> is the number of blocks on a SINGLE disk.  This ensures that the bitmaps
> are round-robined across all disks.
>
> > For example, take the example of six drives configured for RAID 5 with a
> > chunk size of 64 and a 4K blocksize:
>
> -E stride=16, based on 64k / 4k

Thanks for clearing this up. The number of blocks for a single disk
means you don't have to worry about parity drives, so it's much easier
to deal with.

And good to hear about the clarifying patch, as I'm not the only person
confused by this -- currently, the information on the link below is
wrong:

http://wiki.centos.org/HowTos/Disk_Optimization

Tod

--
Tod Hagan
Information Technologist
AIRMAP/Climate Change Research Center
Institute for the Study of Earth, Oceans, and Space
University of New Hampshire
Durham, NH 03824
Phone: [phone]


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