Adding/setting up mem, disk, cd drive, sys admin?

[Alan's Home for Wayward Notes File.]

In article <44675 at netnews.upenn.edu>, lau at desci.wharton.upenn.edu (Yan K. Lau) writes:
> I added 2 RZ57s in an expansion unit to the system.  I ran MAKEDEV and
> newfs.  The results were about 950megs total with 850megs available.  Is
> this normal?  I read somewhere that the system takes about 10% for
> (mumble...mumble).  

	And I responded "This seems to be normal".  But where does
	all the space go you ask?  Well, now that we have resolved
	the disk size (See "Lies, Damned Lies and ..."), we know
	just how much we have to begin with and can determine how
	much is lost to file system overhead.

	That's the key to it you see; file system overhead.  Some
	file system space goes to superblocks, some to inodes and
	some to other stuff.  For some disks this can be a signifi-
	cant percentage of the disk.  We'll find for the RZ57 that
	it isn't very much.

	From the previous discussion we determine that whatever else
	WE might think about it, ULTRIX believes that an RZ57 has
	1,954,050 sectors (977,025 KB).  Doing a newfs(8) on an
	RZ57, letting it take the defaults we find that there are
	a total of 945,726 KB available, a loss of 3.2%.  Looking
	at the RA82 as another example we find it has 608,332 KB
	which goes down to 584,102 KB for file system data, a loss
	of 3.98%.

	Where's the overhead?  First, consider the file system structure.
	Each file system is made up of groups of cylinder.  Each of
	these groups has it's own superblock, block of inodes, data
	space and cylinder group data.  When left to the default
	each group consists of 16 cylinders.  The first cylinder has
	a few more things thrown in to use up a little space.  Here's
	a summary of the sizes of things on an RZ57:

		Bootblock:             8 KB (one only)(+)
		Superblock:            8 KB (one only)
		Backup Superblock:     8 KB (one per CG)
		Cylinder Group data:   8 KB (one per CG)
		Inodes:              256 KB (one per CG)

	From the previous discussion we might concluded that there 
	were 1,861 cylinders on an RZ57.  Looking at the output of
	newfs(8) it believes the 71 sectors per track which actually
	gives us 1835 cylinders (one a bit short).  Taking the default
	of 16 cylinders per group this is 115 cylinder groups.  The
	last one will be short, but have the same overhead.  Working 
	through the sizes of things we have:

		Bootblock:   8 KB		 =      8 KB
		Superblocks: 8 KB + (8 KB * 115) =    928 KB
		CG:          8 KB * 115		 =    920 KB
		Inodes:      256 KB * 115	 = 29,440 KB
		--------------------------------------------
						   31,296 KB

	If we take the difference of chpt(8) size and the total file 
	system size we find the result very close; 

		977,025 - 945,726 = 31,299

	That's only three KB unaccounted for.  You might think
	that it's best left a mystery; perhaps they end up in
	the same place with socks that go missing from dryers,
	but I think I know where they are.  First, look at the
	original partition size; 977,025 KB.  File systems are
	made up of File System size blocks of data.  This is
	typically 8 KB.  If we view the file system space as
	8 KB blocks there are 122,128 of them with one little
	KB left over.  That's the first one.  Also included in 
	the file system overhead of an RZ57 is something called 
	the cylinder group summary.  On the RZ57 with defaults 
	this is 2 KB (trust me, really).  So we have:

		945,726 + 31,296 + 2 + 1 = 977,025

	The mind boggles.  Now, having been through the exercise what 
	can we divine (it's getting late folks...)?

	The amount of space used up in overhead depends on the number
	of cylinder groups, which in turn depends on the number of
	cylinders.  If a disk consists of very many small cylinders
	then lots of space gets used.  One benefit of this is that
	you get lots of space for inodes.  On some file systems this
	can be a big win (News disks for example).

	You can also control the amount of space used for overhead
	in various ways.  There is a flag to newfs(8), passed onto
	mkfs(8) that controls the size of the inode part.  By making
	this number greater than the default of 2048 you can allocate 
	fewer inodes and therefore less space.  With this comes the
	risk that you don't have enough inodes.  If you KNOW that a
	disk will have only a few large files, then getting back a
	few dozen MB might be worthwhile.

	Another way to control the overhead is to lie about the
	geometry.  Using a contrived (^) example of a disk with 69
	sectors/track, 13 tracks/cylinder and 3,279 cylinders
	we might wish to lie and say it has 23 sectors/track,
	13 tracks/cylinder and 9,837 cylinders.  Or perhaps,
	69 s/t, 39 t/c and 1,093 cylinders.  Lying about the
	geometry might have other affects, but that is a topic
	for a performance project.

	(+) Actually there is one per file system of the "one
	only" blocks.

	(^) Well, maybe not contrived.
-- 
Alan Rollow				alan at nabeth.cxn.dec.com