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Date:	Thu, 31 May 2007 16:03:07 -0600
From:	Andreas Dilger <>
To:	Theodore Tso <>
Cc:	Rupesh Thakare <>,,
	Kalpak Shah <>
Subject: Re: [RFC] store RAID stride in superblock

On May 31, 2007  17:33 -0400, Theodore Tso wrote:
> Oops, I just pushed a set of bugfixes to Linux that included the
> superblock field reservations.

Oh well.

> What is in the e2fsprogs hg repository ... is:
> 	..
> 	__u16   s_raid_stride;		/* RAID stride */
> 	__u16   s_mmp_interval;         /* # seconds to wait in MMP checking */
> 	__u64   s_mmp_block;            /* Block for multi-mount protection */
> 	__u32   s_raid_stripe_width;    /* blocks on all data disks (N*stride)*/
> 	__u32   s_reserved[163];        /* Padding to the end of the block */
> };

We're updating our patches to be based on the new HG code.

> One question which does come to mind; is there any reason why we might
> want to know the RAID level and/or the number of disks (as opposed to
> just the stripe width)?

Not so far.  The raid_stride is for bitmap placement (and could also be
used for alignment of random IOs to avoid making 2 disks busy when 1
would do).  The raid_stripe_width is the amount that delalloc+mballoc
will use for allocations+writes to avoid read-modify-write of RAID stripes.
It doesn't really matter what the RAID level is.

> And has anyone investigated where there are
> magic ioctl's or libdevmapper APi's so we can get the RAID parameters
> automatically?  If so, patches so that mke2fs can get the information
> automatically (as opposed to forcing the user to have to specify lots
> of annoying options) would be most welcome....

For now we will specify this via mke2fs or tune2fs for existing filesystems.
The XFS folks mentioned they have a library to extract this info for linux
devices (e.g. DM, MD, etc), but of course that still won't work for e.g.
external RAID devices.

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

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