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
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)
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"?
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.
usage: qrpff 153 2 8 105 225 < /mnt/dvd/VOB_FILENAME \
| extract_mpeg2 | mpeg2dec -