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I created LVM2 partition on iSCSI drive as the following (gdisk -l /dev/sdb):
Number Start (sector) End (sector) Size Code Name
1 2048 1050623 512.0 MiB EF00 EFI System
2 1050624 2099199 512.0 MiB 8300 Linux filesystem
3 2099200 201326558 95.0 GiB 8E00 Linux LVM
But after pvcreate and vgcreate, pvdisplay shows large "not usable" area:
--- Physical volume ---
PV Name /dev/sdb3
VG Name vg1
PV Size 95.00 GiB / not usable 4.00 GiB
Allocatable yes
PE Size 4.00 MiB
Total PE 23295
Free PE 23295
Allocated PE 0
PV UUID ijVBrk-UkIe-3cBJ-27ru-uEEu-UYEI-FBNhLR
In spite of 4MiB PE size, why does large "not usable" area exist?
==== 24 Jul. addtional info.
I tried to compare the cases between iSCSI drive and normal HDD.
On VMware Player 7.1.2, I created a 20GB SCSI virtual disk, allocated a partion of Linux LVM type, pvcreate and vgcreate.
Then the output of pvdisplay was
# pvdisplay /dev/sda1
--- Physical volume ---
PV Name /dev/sda1
VG Name vg1
PV Size 20.00 GiB / not usable 2.98 MiB
Allocatable yes
PE Size 4.00 MiB
Total PE 5119
Free PE 5119
Allocated PE 0
PV UUID LeccKh-Wy8J-k2tn-VH5n-nN88-CcmP-Q9DdO1
and the output of pvck was
# pvck /dev/sda1
Found label on /dev/sda1, sector 1, type=LVM2 001
Found text metadata area: offset=4096, size=1044480
Next, I connected the same vritual machine to a 20GB iSCSI drive, and allocated a partion of Linux LVM type, pvcreate and vgcreate.
The result was
# pvdisplay /dev/sdc2
--- Physical volume ---
PV Name /dev/sdc2
VG Name vg1
PV Size 20.00 GiB / not usable 4.00 GiB
Allocatable yes
PE Size 4.00 MiB
Total PE 4096
Free PE 4096
Allocated PE 0
PV UUID gtRT6X-VQu7-SQGn-5I1O-ZBN7-2bHO-c1FV31
# pvck /dev/sdc2
Found label on /dev/sdc2, sector 1, type=LVM2 001
Found text metadata area: offset=4096, size=4294962688
Apparently metadata size of iSCSI drive is too large.
What is the reason of this size difference?
Are there any problem to use LVM2 on iSCSI drive?
Last edited by rhapsodite (2015-07-26 08:46:09)
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This was due to large value of optimal_io_size which was one of block device parameters.
By setting data_alignment_detection to 0 in /etc/lvm/lvm.conf, we can turn off to refer this value.
The result is:
# pvdisplay
--- Physical volume ---
PV Name /dev/sdb2
VG Name vg1
PV Size 47.50 GiB / not usable 2.98 MiB
Allocatable yes (but full)
PE Size 4.00 MiB
Total PE 12159
Free PE 0
Allocated PE 12159
PV UUID rXC7Qg-frSj-nzL8-iI6i-0aZn-Z4bn-OPFPAR
# pvck /dev/sdb2
Found label on /dev/sdb2, sector 1, type=LVM2 001
Found text metadata area: offset=4096, size=1044480
(After some trials, drive name, size and UUID are different from those of the above post.)
This value is used to determine metadata size of PV (even when we give metadatasize option to pvcreate command).
optimal_io_size parameter is reported from block device, and in my case 4294966784(=0xFFFFFE00) was set from iSCSI drive created on RAID in QNAP NAS.
We can directly see this value by "cat /sys/block/sdX/queue/optimal_io_size".
note:
linux kernel package version 4.1.2-2
lvm2 package version 2.02.125-1
Last edited by rhapsodite (2015-07-26 09:10:55)
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