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Date:      Mon, 24 Nov 1997 17:31:40 -0600
From:      Bob Willcox <bob@pmr.com>
To:        stable list <freebsd-stable@freebsd.org>
Subject:   Re: Partitioning
Message-ID:  <19971124173140.04106@pmr.com>
In-Reply-To: <Pine.BSF.3.96.971107203247.1158A-100000@acp.qiv.com>; from Jay Nelson on Fri, Nov 07, 1997 at 09:21:01PM -0600
References:  <Pine.BSF.3.96.971107195505.1994A-100000@shell.futuresouth.com> <Pine.BSF.3.96.971107203247.1158A-100000@acp.qiv.com>

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On Fri, Nov 07, 1997 at 09:21:01PM -0600, Jay Nelson wrote:
> On Fri, 7 Nov 1997, Matthew D. Fuller wrote:
> 
> [snip]
>     > > What do you mean?  ccd is not a filesystem.
>     > I know.
>     > What I meant was, it looks to me like, the code for expanding filesystems
>     > would seem to be a lot like the code for implementing ccd'd partitions;
>     > you just map various physical places on the disk(s) to the same logical
>     > partition.
> [snip]
> 
> AIX maps 4Mb physical "partitions" to logical partitions. Physical

The size of the physical partitions may be greater than 4Mb (the
default).  You set this at the time the you create the volume group.

> partitions scattered all over the disk are mapped to a linear logical
> sequence of partitions. So, in a sense, it probably isn't that much
> different than ccd. But I'm not that enthusiastic about LVM.

It has a rather unpleasent interface, I'm afraid.  I like the
functionality, however.

> 
> As near as I can tell, AIX implements the basic Berkeley filesystem
> and has completed the journaling. When you expand a file system, I
> don't believe you gain any more inodes.

No, the AIX Journalled filesystem was an original implementation by
(primarily) Al Chang of IBM Research.  I believe some of the concepts
of the BSD filesystem were used (like cylinder groups), though I don't
know how similar any of it really is.  And yes, when you expand an
AIX filesystem you do get more inodes.  In the AIX filesystem there
is (or used to be, I've been away from it for several years) a direct
correspondence between the number of inodes and the number of 4k pages
in the filesystem (1:1 as I recall).

> 
> AIX also uses a number of other structures that are unique -- Volume 
> Descriptor Blocks, for example, which keep track of quorums (for 
> mirroring and "volume groups") and other data related to the current 
> state of the file system. All of this "convenience" implies overhead
> -- or a massive overhaul of the current file system code.
> 
> You also have to realize that to make this work, you have to leave
> portions of the disk(s) unallocated. You could probably simulate this
> by allocating calendars 0-99, 200-299, etc., and then relabeling the
> disk when you want to expand. Seems like a waste of space to me given
> the FFS allocation policies.

The LVM handles all of this in AIX.  Logical volumes can be expanded to
include previous free space within their volume group.  A volume group
can be expanded by adding more physical volumes to it.

> 
> IMHO, I wouldn't want to invest 1+ man years to implement a
> convenience to save a 2 or 3 time a year annoyance. (I'd also hate to
> see FreeBSD broken while it's implemented.) 2000+ hours to implement
> vs. 2-8 hour backup/rebuild/restore -- plus (gurus correct me if
> I'm wrong) it takes 4-8 cycles to dereference a pointer -- so
> you're coming close to doubling file acces time. The dog don't hunt.

Certainly, adding the LVM layer adds overhead...though I doubt that it
actually doubles file access time.  (I run a number of AIX systems and
and file access times "feel" pretty comparable to those on my FreeBSD
systems, for similar disks.  CPU cost may well be greater, but I haven't
attempted to measure it.  Of course, YMMV.)

In environments where it is unacceptable to take a system (or file
system) down for 2-8 hours while you do the backup/rebuild/restore
thing, the ability to resize filesystems on the fly has some real value.
This was one of the primary forces driving its implementation.  Also, it
looks good on marketing charts, especially when your competitors can't
do it (as was the case when the JFS was first implemented).

Another area where it is used in AIX is during installation.  It
simplifies life quite a bit when the installation program can simply
grow a filesystem as necessary.

Bob Willcox,
(retired IBMer and AIX architecture manager)


-- 
Bob Willcox	       Deliberation, n.: The act of examining one's bread
bob@luke.pmr.com         to determine which side it is buttered on.
Austin, TX                 -- Ambrose Bierce, "The Devil's Dictionary"



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