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Old 07-14-2016, 02:43 PM   #1
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Smile Losing 4Gb on Debian LVM?

Hello all,

I'm creating LVM in the following way,

pvcreate /dev/sdc (a virtual disk of 275G)
Physical volume "/dev/sdc" successfully created
vgcreate vg_test /dev/sdc
Volume group "vg_test" successfully created
lvcreate -n lv_test -l 100%FREE vg_test
Logical volume "lv_test" created
root@debian:~# pvs
PV VG Fmt Attr PSize PFree
/dev/sdc vg_test lvm2 a-- 275.00g 0
root@debian:~# vgs
VG #PV #LV #SN Attr VSize VFree
vg_test 1 1 0 wz--n- 275.00g 0
root@debian:~# lvs
LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert
lv_test vg_test -wi-a----- 275.00g
Then i proceed to create an ext4 fs

mkfs.ext4 /dev/mapper/vg_test-lv_test (everything OK)

and when i mount it...

/dev/mapper/vg_test-lv_test 271G 63M 257G 1% /mnt

Why do i loose 4 GB in the process? is this normal? 4 GB?

It's a LOT of space...

I tried creating a partition of 275GB with the same results..I'm just confused


Last edited by mozer; 07-14-2016 at 02:45 PM.
Old 07-14-2016, 02:57 PM   #2
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df shows space available to non-privileged users. The default is to reserve some space for privileged use. For example:

 df |grep home
                     205261560  27068952 167742896  14% /home
sudo tune2fs -l /dev/mapper/vg_ntt2-lv_home |grep 'lock count:'
Block count:              52166656
Reserved block count:     2608332
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Old 07-14-2016, 03:18 PM   #3
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To elaborate on the above 5% is reserved for root when ext2/3/4 filesystems are created. That 5% is not included in the output of the df command so the numbers do not add up. 5% is 13GB.

The difference in the logical volume size versus filesystem size is overhead i.e. metadata. 1.5-2% for ext4 if I remember correctly is normal.
Old 07-14-2016, 07:53 PM   #4
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And for a data filesystem, the reserved is pointless - set it to zero using tune2fs.
Note if you plan on running the filesystem really full (95% say) all the time and actively adding/deleting files (all the time) maybe you need some reserved. Never needed to in real life.
Old 07-14-2016, 08:22 PM   #5
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You also loose space to support the inode list, block allocation maps, and the journal.

These are necessary to manage files in the filesystem and recover from system crashes/power failures.


debian, lvm

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