On Fri, Apr 11, 2003 at 11:42:39AM -0400, Derek Glidden wrote:
> On Fri, 2003-04-11 at 11:06, Eric Sandeen wrote:
> > hi Derek - Which parts do you find confusing? Not that I disagree. :)
>
> :)
>
> > For my part, I think the different ways of specifying the values (i.e.
> > different options for different units) is a bit confusing... but I'm
> > curious to know what you think needs better docs.
>
> Basically: It's never been entirely clear to me exactly the difference
> between a "stripe unit" and a "stripe width" nor the relation between
> "stripe unit" and "stripe width" and the size of the stripes on the RAID
> device.
>
> For example, from the mkfs.xfs manpage: "The [sunit] suboption value has
> to be specified in 512-byte block units. Use the su suboption to
> specify the stripe unit size in bytes." Part of the problem is that
> "sunit" is defined in terms of "stripe units" and the other problem is
> that in two sentences it contradicts itself; it says to specify in
> "block units" and then in "bytes." So if I have made a RAID volume with
> 64K stripes, should I specify a su value of 128? (128*512 = 64K)
>
no, su=64k _or_ sunit=128 !
> And how does swidth relate? "The swidth suboption is used to specify
> the stripe width for a RAID device or a striped logical volume."
> Again, "swidth" is defined in terms of "stripe width." "The suboption
> value is expressed as a multiplier of the stripe unit, usually the same
> as the number of stripe members in the logical volume configuration, or
> data disks in a RAID device." So I take this to mean that if I have 8
> disks in a RAID5, my "sw" parameter is "8"?
>
having an 8 disk RAID5, you'll need swidth=(8-1)*sunit (=896 for your example)
- if I remember some
posts about that topic correctly.( a 8 disk RAID5 has 7 "data" disks in that
term)
But I agree, having two options for the same is a little bit confusing.
After creating your xfs filesystem, you might want to double check your
values with 'xfs_info $MOUNTPOINT' - But be careful: This one prints the
values in multiples of your Blocksize. (confusing continues *g* ...)
> IIRC, the su and sw parameters that mkfs.xfs expects not what you would
> infer from the manpages. In the above, I'd say that I would want
> "su=128,sw=8" except that mkfs.xfs complains about it, saying that one
> needs to be a multiple of the other, and it seems to be the opposite of
> what you'd expect. It's been a while since I used su/sw so I don't
> remember exactly what the problem was, but I remember being frustrated
> by the fact that what I thought was correct, mkfs.xfs complained one was
> not the correct multiple of the other, and they were opposite what I
> thought they should be.
>
> I could certainly dig through the XFS sources to get a better
> understanding of what exactly each option does internally, and that's
> what I did last time I bothered trying to use su/sw, but it seems like
> extra unnecessary work if the manpage were clearer. Since then, I've
> just kept an eye on the XFS list and made sure that I'm using the
> version with all the RAID optimizations so I don't have to worry about
> su/sw anymore. :)
>
> Perhaps a concrete example given a RAID device created with [x]
> parameters, using su/sw like "so" in either the manpage, or more
> appropriately the FAQ, would help clarify it for dolts like me.
>
yep, definitly agreed.
Christian
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