[linux-lvm] Trying to understand format of LVM2 - Question on Extent Size

[David F.]

I created a test lvm on a 32G UFD with two logical volumes, 15G and 14G. I
then looked at the on disk data and most makes since except the
extent_count vs extent_size. In this case the extent_size is 8192, and it
says the extent sizes are 3840 and 3584, but that * 8192 is double its
actual size. * 4096 then it is the size. So how does that work. TIA!!
Here's the sample:

test_lvm {
id = "xhewgS-UxVF-4qC5-MGXp-ErfX-6SNl-UkCdgz"
seqno = 3
format = "lvm2"
status = ["RESIZEABLE", "READ", "WRITE"]
flags = []
extent_size = 8192
max_lv = 0
max_pv = 0
metadata_copies = 0

physical_volumes {

pv0 {
id = "XhEzob-g2D5-YcGp-zfz9-39R7-HII6-SITlH9"
device = "/dev/sdc"

status = ["ALLOCATABLE"]
flags = []
dev_size = 62531584
pe_start = 2048
pe_count = 7633
}
}

logical_volumes {

lvm_logvol {
id = "IIKGTj-2eYn-5iTk-7KZd-ZMzS-aqYT-kwNgko"
status = ["READ", "WRITE", "VISIBLE"]
flags = []
creation_time = 1532609310
creation_host = "localhost.localdomain"
segment_count = 1

segment1 {
start_extent = 0
extent_count = 3840

type = "striped"
stripe_count = 1

stripes = [
"pv0", 0
]
}
}

lvm_logvol2 {
id = "dC9wIq-kJ3I-ixLV-A2f9-889Y-qpUr-DdfciL"
status = ["READ", "WRITE", "VISIBLE"]
flags = []
creation_time = 1532609325
creation_host = "localhost.localdomain"
segment_count = 1

segment1 {
start_extent = 0
extent_count = 3584

type = "striped"
stripe_count = 1

stripes = [
"pv0", 3840
]
}
}
}

}
# Generated by LVM2 version 2.02.168(2) (2016-11-30): Thu Jul 26 08:48:45
2018

contents = "Text Format Volume Group"
version = 1

description = ""

creation_host = "localhost.localdomain" # Linux localhost.localdomain
4.11.8-300.fc26.x86_64 #1 SMP Thu Jun 29 20:09:48 UTC 2017 x86_64
creation_time = 1532609325 # Thu Jul 26 08:48:45 2018

[David F.]

Never mind, the answer is the extent_size is the number of sectors (or
perhaps it's the number of 512 byte blocks, I'll have to test on 4K sector
drives). So in this case 4M and 4M*3840 is the 16G (not 16M which was
the 3840*4096).

Post by David F.
I created a test lvm on a 32G UFD with two logical volumes, 15G and 14G.
I then looked at the on disk data and most makes since except the
extent_count vs extent_size. In this case the extent_size is 8192, and it
says the extent sizes are 3840 and 3584, but that * 8192 is double its
actual size. * 4096 then it is the size. So how does that work. TIA!!
test_lvm {
id = "xhewgS-UxVF-4qC5-MGXp-ErfX-6SNl-UkCdgz"
seqno = 3
format = "lvm2"
status = ["RESIZEABLE", "READ", "WRITE"]
flags = []
extent_size = 8192
max_lv = 0
max_pv = 0
metadata_copies = 0
physical_volumes {
pv0 {
id = "XhEzob-g2D5-YcGp-zfz9-39R7-HII6-SITlH9"
device = "/dev/sdc"
status = ["ALLOCATABLE"]
flags = []
dev_size = 62531584
pe_start = 2048
pe_count = 7633
}
}
logical_volumes {
lvm_logvol {
id = "IIKGTj-2eYn-5iTk-7KZd-ZMzS-aqYT-kwNgko"
status = ["READ", "WRITE", "VISIBLE"]
flags = []
creation_time = 1532609310
creation_host = "localhost.localdomain"
segment_count = 1
segment1 {
start_extent = 0
extent_count = 3840
type = "striped"
stripe_count = 1
stripes = [
"pv0", 0
]
}
}
lvm_logvol2 {
id = "dC9wIq-kJ3I-ixLV-A2f9-889Y-qpUr-DdfciL"
status = ["READ", "WRITE", "VISIBLE"]
flags = []
creation_time = 1532609325
creation_host = "localhost.localdomain"
segment_count = 1
segment1 {
start_extent = 0
extent_count = 3584
type = "striped"
stripe_count = 1
stripes = [
"pv0", 3840
]
}
}
}
}
# Generated by LVM2 version 2.02.168(2) (2016-11-30): Thu Jul 26 08:48:45
2018
contents = "Text Format Volume Group"
version = 1
description = ""
creation_host = "localhost.localdomain" # Linux localhost.localdomain
4.11.8-300.fc26.x86_64 #1 SMP Thu Jun 29 20:09:48 UTC 2017 x86_64
creation_time = 1532609325 # Thu Jul 26 08:48:45 2018

[Zdenek Kabelac]

Never mind, the answer is the extent_size is the number of sectors (or perhaps
it's the number of 512 byte blocks, I'll have to test on 4K sector
drives).     So in this case 4M and 4M*3840 is the 16G (not 16M which was the
3840*4096).

Hi


extent_size is always in 512b unit and ATM it's 32b value.


extent_count is in 'extent_size' unit and ATM it's 32b value.


Volume size extent_size * extent_count (so 32b * 32b => 64b size).


Depending on your extent_size you are limiting maximum volume size.
Thus if you need to use Exa byte volume sizes you clearly need to go with 4G
extent size.


Regards


Zdenek