Date: Wed, 1 Oct 2008 14:31:50 -0600 From: "Cyrus Rahman" <crahman@gmail.com> To: freebsd-geom@freebsd.org Subject: Experiences with Gpart Message-ID: <9e77bdb50810011331y7216eac3yf85907f96f5e8370@mail.gmail.com>
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I recently had occasion to set up a system that needed to contain
numerous partitions, and I thought it would be a good occasion to try
gpart to divide the disk. By and large it went very well, with the
exception of bug in 'gpart bootcode' which made it necessary to
manually dd in the bootstrap.
I did discover a few oddities that I will mention here.
My intent was to mirror the disks with gmirror. It didn't make sense
to individually mirror each gpart partition and have some 30
simultaneous restores going should the mirror ever break. My first
idea was to mirror the entire disks (ad4 and ad6) with gmirror and
put gpart over the mirror.
This sort of works, except that gpart itself if it finds itself at the
beginning of the disk even when it doesn't properly taste itself on
the last block of the disk. As a result, the same gpart partitions
get attached to ad4 and ad6, as well as to the mirror. The gpart
geoms attached directly to the disks complain about the last sector
being incorrect (the backup table) and request that you fix it (which
would not likely go well). But in addition to that, any further geoms
contained within the mirror get attached to the outer gpart geoms on
ad4 and ad6 and not the mirror. They don't work properly, because the
mirror is already attached to ad4 and ad6, so you end up with a
non-functional setup.
In experimenting, I also discovered that if you directly nest gpart
tables, things work fine until you reboot. Then the internal
partitions vanish because gpart doesn't attach to them.
Fortunately, if you put the inner gpart inside a gmirror inside the
outer gpart, things do work fine.
This I did for two reasons: First, to avoid the aforementioned
trouble with an outermost gmirror containing a gpart geom, and second,
so that I could better define how the disk was to be mirrored. I was
able to put a number of smaller filesystems inside a single mirrored
gpart partition, and then most of the really big ones inside a second
mirrored gpart partition. This second partition used the -F option to
gmirror so that it would not need to be re-synchonized in a crash.
The filesystems within it are gjournal'd to make this possible.
Booting was yet another issue, since the loader objects to nested
gpart partitions. I handled this by simply making a third partition
on the outermost gpt that contained the root.
Since this is likely confusing, here's a diagram that will help if you
use a fixed font:
ad4 ad6
------ ------
gpart 1 (ad4p1) gpart 1 (ad6p1)
freebsd-boot freebsd-boot
gpart 2 (ad4p2) gpart 2 (ad6p2)
gmirror (gm0 = ad4p2 + ad6p2)
root (gm0.p1)
gpart 3 (ad4p3) gpart3 (ad6p3)
gmirror (gm1 = ad4p3 + ad6p3)
gpart 1 (gm1p1)
gpart 2 (gm1p2
...
gpart n (gm1pn)
gpart 3 (ad4p4) gpart 3 (ad6p4)
gmirror (gm2 = ad4p4 + ad6p4) (mirrored with -F)
gpart 1 (gm2p1)
gjournal (gm2p1.journal)
gpart 2 (gm2p2)
gjournal (gm2p2.journal)
...
Despite the intent of gpt's being to make such nesting unnecessary, as
a means of defining the structure of gmirrors, which take up the
entire extent of whatever encloses them, the nesting was very helpful.
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