Pbug: /* A breakdown of a disk */

2008-06-05T12:58:33Z

A breakdown of a disk

← Older revision		Revision as of 05:58, 5 June 2008
Line 15:		Line 15:
	svnd0: 33554432 bytes on disk		svnd0: 33554432 bytes on disk

	let's make a new partition on it called a:		let's make a new [[disklabel\|partition]] on it called a:

	# disklabel -E /dev/rsvnd0c		# disklabel -E /dev/rsvnd0c

Pbug: /* A breakdown of a disk */

2005-10-28T17:24:23Z

A breakdown of a disk

← Older revision		Revision as of 10:24, 28 October 2005
Line 3:		Line 3:
	I created a virtual disk, it is 32MB big:		I created a virtual disk, it is 32MB big:

			# dd if=/dev/zero of=disk bs=32m count=1
			1+0 records in
			1+0 records out
			33554432 bytes transferred in 0.164 secs (203728139 bytes/sec)
	# ls -lh disk		# ls -lh disk
	-rw-r--r-- 1 root wheel 32.0M Oct 28 08:55 disk		-rw-r--r-- 1 root wheel 32.0M Oct 28 08:55 disk

Pbug: breakdown of a disk from ffs

2005-10-28T09:11:35Z

breakdown of a disk from ffs

New page

=== A breakdown of a disk ===

I created a virtual disk, it is 32MB big:

# ls -lh disk
-rw-r--r-- 1 root wheel 32.0M Oct 28 08:55 disk

WTF? It's a file! Ok so lets make it a disk device:

# vnconfig -cv svnd0 disk
svnd0: 33554432 bytes on disk

let's make a new partition on it called a:

# disklabel -E /dev/rsvnd0c
disklabel: Can't get bios geometry: Device not configured
Initial label editor (enter '?' for help at any prompt)
> a
partition: [a]
offset: [0]
size: [65536]
FS type: [4.2BSD]
> p a
device: /dev/rsvnd0c
type: SCSI
disk: vnd device
label: fictitious
bytes/sector: 512
sectors/track: 100
tracks/cylinder: 1
sectors/cylinder: 100
cylinders: 655
total sectors: 65536
free sectors: 0
rpm: 3600
16 partitions:
# size offset fstype [fsize bsize cpg]
a: 65536 0 4.2BSD 2048 16384 16 # Cyl 0 - 655*
c: 65536 0 unused 0 0 # Cyl 0 - 655*
> q
Write new label?: [y] y
#

Now write the filesystem on it with [[newfs]]:

# newfs /dev/rsvnd0a
Warning: cylinder groups must have a multiple of 8 cylinders
Warning: 64 sector(s) in last cylinder unallocated
/dev/rsvnd0a: 65536 sectors in 656 cylinders of 1 tracks, 100 sectors
32.0MB in 1 cyl groups (656 c/g, 32.03MB/g, 4096 i/g)
super-block backups (for fsck -b #) at:
32,

mount the new filesystem to /mnt:

# mount /dev/svnd0a /mnt
# df /mnt
Filesystem 512-blocks Used Avail Capacity Mounted on
/dev/svnd0a 64412 4 61188 0% /mnt
# df -i /mnt
Filesystem 512-blocks Used Avail Capacity iused ifree %iused Mounted on
/dev/svnd0a 64412 4 61188 0% 1 4093 0% /mnt

OK let's count it up, we have 65536 blocks in total for the disk, 4094 inodes exist at 128 bytes size which is 1023 blocks, and we have 2 superblocks (0 and 32) which are 16384 bytes big each or 32 blocks, then the filesystem space claims 64412 blocks, this gives us a total of 65499 blocks which isn't quite the complete disksize of 65536 blocks, however I left the disklabel unaccounted for which also takes up some space.

Now let's fill the drive:

$ dd if=/dev/zero of=fill bs=1
/mnt: write failed, file system is full
dd: fill: No space left on device
31326209+0 records in
31326208+0 records out
31326208 bytes transferred in 40.458 secs (774289 bytes/sec)
$ df /mnt
Filesystem 512-blocks Used Avail Capacity Mounted on
/dev/svnd0a 64412 61220 -28 100% /mnt

100% capacity, but the used blocks aren't all the blocks that the filesystem claims, so let's repeat that last step but this time as [[root]]:

# dd if=/dev/zero of=fill bs=1
/mnt: write failed, file system is full
dd: fill: No space left on device
32931841+0 records in
32931840+0 records out
32931840 bytes transferred in 44.276 secs (743779 bytes/sec)
# df /mnt
Filesystem 512-blocks Used Avail Capacity Mounted on
/dev/svnd0a 64412 64356 -3164 105% /mnt

Lovely almost all used up and it can't go further. Note that we're using up '''105% capacity''' this is built into ffs and can be tweaked at [[newfs]] time. The superuser gets 5% above 100% of capacity noone else.

Disk breakdown - Revision history

Pbug: /* A breakdown of a disk */

Pbug: /* A breakdown of a disk */

Pbug: breakdown of a disk from ffs