So I have a disk with weird "optimal i/o size" of 33553920 bytes
(actually I think it's because of the usb/sata converter):
[tom@localhost ~]$ sudo fdisk -l /dev/sdb
Disk /dev/sdb: 74.5 GiB, 80026361856 bytes, 156301488 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 33553920 bytes
and fdisk make use of that to determine its alignment. I don't
consider that a problem yet, but the inconsistency of fdisk alignment
policy:
[tom@localhost ~]$ sudo fdisk /dev/sdb
Welcome to fdisk (util-linux 2.26.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Device does not contain a recognized partition table.
Created a new DOS disklabel with disk identifier 0x604d5491.
Command (m for help): n
Partition type
p primary (0 primary, 0 extended, 4 free)
e extended (container for logical partitions)
Select (default p):
Using default response p.
Partition number (1-4, default 1):
First sector (65535-156301487, default 65535):
Last sector, +sectors or +size{K,M,G,T,P} (65535-156301487,
default 156301487): +500m
Created a new partition 1 of type 'Linux' and of size 512 MiB.
Command (m for help): p
Disk /dev/sdb: 74.5 GiB, 80026361856 bytes, 156301488 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 33553920 bytes
Disklabel type: dos
Disk identifier: 0x604d5491
Device Boot Start End Sectors Size Id Type
/dev/sdb1 65535 1114094 1048560 512M 83 Linux
Command (m for help): o
Created a new DOS disklabel with disk identifier 0x77cb1fd6.
Command (m for help): n
Partition type
p primary (0 primary, 0 extended, 4 free)
e extended (container for logical partitions)
Select (default p):
Using default response p.
Partition number (1-4, default 1):
First sector (65535-156301487, default 65535):
Last sector, +sectors or +size{K,M,G,T,P} (65535-156301487,
default 156301487): +512m
Created a new partition 1 of type 'Linux' and of size 512 MiB.
Command (m for help): p
Disk /dev/sdb: 74.5 GiB, 80026361856 bytes, 156301488 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 33553920 bytes
Disklabel type: dos
Disk identifier: 0x77cb1fd6
Device Boot Start End Sectors Size Id Type
/dev/sdb1 65535 1114110 1048576 512M 83 Linux
Command (m for help): q
As you can see/calculate, when I try to make a 500MiB partition, fdisk
try to round the size up so that the partition end at the alignment
boundary, yet when I try to add a 512MiB partition, it doesn't do any
rounding up or down. This happen for both msdos and gpt. Then I try to
add a 1MiB partition, more weird things happened:
[tom@localhost ~]$ sudo fdisk /dev/sdb
Welcome to fdisk (util-linux 2.26.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Device does not contain a recognized partition table.
Created a new DOS disklabel with disk identifier 0xe6e360c0.
Command (m for help): g
Created a new GPT disklabel (GUID: 09418DB1-FB4B-4CC5-9259-60561C0A352B).
Command (m for help): n
Partition number (1-128, default 1):
First sector (65535-156301454, default 65535):
Last sector, +sectors or +size{K,M,G,T,P} (65535-156301454,
default 156301454): +15m
Last sector, +sectors or +size{K,M,G,T,P} (65535-156301454,
default 156301454): +16m
Created a new partition 1 of type 'Linux filesystem' and of size 32 MiB.
Command (m for help): p
Disk /dev/sdb: 74.5 GiB, 80026361856 bytes, 156301488 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 33553920 bytes
Disklabel type: gpt
Disk identifier: 09418DB1-FB4B-4CC5-9259-60561C0A352B
Device Start End Sectors Size Type
/dev/sdb1 65535 131069 65535 32M Linux filesystem
Command (m for help): o
Created a new DOS disklabel with disk identifier 0x7bbf0050.
Command (m for help): n
Partition type
p primary (0 primary, 0 extended, 4 free)
e extended (container for logical partitions)
Select (default p):
Using default response p.
Partition number (1-4, default 1):
First sector (65535-156301487, default 65535):
Last sector, +sectors or +size{K,M,G,T,P} (65535-156301487,
default 156301487): +1m
Created a new partition 1 of type 'Linux' and of size 1 MiB.
Command (m for help): p
Disk /dev/sdb: 74.5 GiB, 80026361856 bytes, 156301488 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 33553920 bytes
Disklabel type: dos
Disk identifier: 0x7bbf0050
Device Boot Start End Sectors Size Id Type
/dev/sdb1 65535 67583 2049 1M 83 Linux
Command (m for help): q
As you can see, for gpt it only creates a "end-aligned" partition of
32MiB until the number I enter reach 16, but for msdos it creates a
weird partition of 2049 sectors (instead of 2048).
Then I move on testing with a disk of "normal" sector and i/o sizes:
[tom@localhost ~]$ sudo fdisk -l /dev/sda
Disk /dev/sda: 223.6 GiB, 240057409536 bytes, 468862128 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
when I try to create a 1MiB partition, it creates one of 2048 sectors
for both msdos and gpt, but then when I try to create smaller
partitions which doesn't naturally "end-align", the result become
ugly:
[tom@localhost ~]$ sudo fdisk /dev/sda
Welcome to fdisk (util-linux 2.26.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Command (m for help): o
Created a new DOS disklabel with disk identifier 0xb84f3480.
Command (m for help): n
Partition type
p primary (0 primary, 0 extended, 4 free)
e extended (container for logical partitions)
Select (default p):
Using default response p.
Partition number (1-4, default 1):
First sector (2048-468862127, default 2048):
Last sector, +sectors or +size{K,M,G,T,P} (2048-468862127, default
468862127): +1k
Created a new partition 1 of type 'Linux' and of size 1.5 KiB.
Command (m for help): p
Disk /dev/sda: 223.6 GiB, 240057409536 bytes, 468862128 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xb84f3480
Device Boot Start End Sectors Size Id Type
/dev/sda1 2048 2050 3 1.5K 83 Linux
Command (m for help): o
Created a new DOS disklabel with disk identifier 0x6f4977c6.
Command (m for help): n
Partition type
p primary (0 primary, 0 extended, 4 free)
e extended (container for logical partitions)
Select (default p):
Using default response p.
Partition number (1-4, default 1):
First sector (2048-468862127, default 2048):
Last sector, +sectors or +size{K,M,G,T,P} (2048-468862127, default
468862127): +1023k
Created a new partition 1 of type 'Linux' and of size 1023.5 KiB.
Command (m for help): p
Disk /dev/sda: 223.6 GiB, 240057409536 bytes, 468862128 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x6f4977c6
Device Boot Start End Sectors Size Id Type
/dev/sda1 2048 4094 2047 1023.5K 83 Linux
Command (m for help): o
Created a new DOS disklabel with disk identifier 0x7e1a99b9.
Command (m for help): n
Partition type
p primary (0 primary, 0 extended, 4 free)
e extended (container for logical partitions)
Select (default p):
Using default response p.
Partition number (1-4, default 1):
First sector (2048-468862127, default 2048):
Last sector, +sectors or +size{K,M,G,T,P} (2048-468862127, default
468862127): +1024k
Created a new partition 1 of type 'Linux' and of size 1 MiB.
Command (m for help): p
Disk /dev/sda: 223.6 GiB, 240057409536 bytes, 468862128 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x7e1a99b9
Device Boot Start End Sectors Size Id Type
/dev/sda1 2048 4095 2048 1M 83 Linux
Command (m for help): q
Before it reach 1MiB, it creates ".5 KiB" partitions of odd numbers of
sectors. This happens for msdos only; for gpt it doesn't allow
partition smaller than 1MiB and round it up once the number reach 512.
The issue(s) seems to be related to how fdisk handle +0/+1 sector:
[tom@localhost ~]$ sudo fdisk /dev/sda
Welcome to fdisk (util-linux 2.26.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Command (m for help): o
Created a new DOS disklabel with disk identifier 0x6473db40.
Command (m for help): n
Partition type
p primary (0 primary, 0 extended, 4 free)
e extended (container for logical partitions)
Select (default p):
Using default response p.
Partition number (1-4, default 1):
First sector (2048-468862127, default 2048):
Last sector, +sectors or +size{K,M,G,T,P} (2048-468862127, default
468862127): +0
Created a new partition 1 of type 'Linux' and of size 512 B.
Command (m for help): p
Disk /dev/sda: 223.6 GiB, 240057409536 bytes, 468862128 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x6473db40
Device Boot Start End Sectors Size Id Type
/dev/sda1 2048 2048 1 512B 83 Linux
Command (m for help): o
Created a new DOS disklabel with disk identifier 0x344a2daa.
Command (m for help): n
Partition type
p primary (0 primary, 0 extended, 4 free)
e extended (container for logical partitions)
Select (default p):
Using default response p.
Partition number (1-4, default 1):
First sector (2048-468862127, default 2048):
Last sector, +sectors or +size{K,M,G,T,P} (2048-468862127, default
468862127): +1
Created a new partition 1 of type 'Linux' and of size 1 KiB.
Command (m for help): p
Disk /dev/sda: 223.6 GiB, 240057409536 bytes, 468862128 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x344a2daa
Device Boot Start End Sectors Size Id Type
/dev/sda1 2048 2049 2 1K 83 Linux
Command (m for help): o
Created a new DOS disklabel with disk identifier 0xc29ce8ba.
Command (m for help): n
Partition type
p primary (0 primary, 0 extended, 4 free)
e extended (container for logical partitions)
Select (default p):
Using default response p.
Partition number (1-4, default 1):
First sector (2048-468862127, default 2048):
Last sector, +sectors or +size{K,M,G,T,P} (2048-468862127, default
468862127): +2
Created a new partition 1 of type 'Linux' and of size 1.5 KiB.
Command (m for help): p
Disk /dev/sda: 223.6 GiB, 240057409536 bytes, 468862128 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xc29ce8ba
Device Boot Start End Sectors Size Id Type
/dev/sda1 2048 2050 3 1.5K 83 Linux
Command (m for help): o
Created a new DOS disklabel with disk identifier 0x67872ede.
Command (m for help): n
Partition type
p primary (0 primary, 0 extended, 4 free)
e extended (container for logical partitions)
Select (default p):
Using default response p.
Partition number (1-4, default 1):
First sector (2048-468862127, default 2048):
Last sector, +sectors or +size{K,M,G,T,P} (2048-468862127, default
468862127): +2048
Created a new partition 1 of type 'Linux' and of size 1 MiB.
Command (m for help): p
Disk /dev/sda: 223.6 GiB, 240057409536 bytes, 468862128 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x67872ede
Device Boot Start End Sectors Size Id Type
/dev/sda1 2048 4096 2049 1M 83 Linux
Command (m for help): q
As you can see, it consider "+0" as 1 sector, "+1" as 2 sectors, and
so on. IMHO this also cause inconvenience when someone actually try to
create partitions by entering number of sectors (one always need to
"minus one" for that). It's the same case for gpt, except you can't do
"+0" for it.
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