On 27.4.2016 14:33, Bhasker C V wrote:
Hi,
I am starting with investigating about the lvm thin provisioning
(repeat post from https://lists.debian.org/debian-user/2016/04/msg00852.html )
(apologies for html mail)
I have done the following
1.Create a PV
vdb 252:16 0 10G 0 disk
├─vdb1 252:17 0 100M 0 part
└─vdb2 252:18 0 9.9G 0 part
root@vmm-deb:~# pvcreate /dev/vdb1
Physical volume "/dev/vdb1" successfully created.
root@vmm-deb:~# pvs
PV VG Fmt Attr PSize PFree
/dev/vdb1 lvm2 --- 100.00m 100.00m
2. create a VG
root@vmm-deb:~# vgcreate virtp /dev/vdb1
Volume group "virtp" successfully created
root@vmm-deb:~# vgs
VG #PV #LV #SN Attr VSize VFree
virtp 1 0 0 wz--n- 96.00m 96.00m
3. create a lv pool and a over-provisioned volume inside it
root@vmm-deb:~# lvcreate -n virtpool -T virtp/virtpool -L40M
Logical volume "virtpool" created.
root@vmm-deb:~# lvs
LV VG Attr LSize Pool Origin Data% Meta% Move Log
Cpy%Sync Convert
virtpool virtp twi-a-tz-- 40.00m 0.00 0.88
root@vmm-deb:~# lvcreate -V1G -T virtp/virtpool -n vol01
WARNING: Sum of all thin volume sizes (1.00 GiB) exceeds the size of thin
pool virtp/virtpool and the size of whole volume group (96.00 MiB)!
For thin pool auto extension activation/thin_pool_autoextend_threshold
should be below 100.
Logical volume "vol01" created.
root@vmm-deb:~# lvs
LV VG Attr LSize Pool Origin Data% Meta% Move Log
Cpy%Sync Convert
virtpool virtp twi-aotz-- 40.00m 0.00 0.98
vol01 virtp Vwi-a-tz-- 1.00g virtpool 0.00
---------- Now the operations
# dd if=/dev/urandom of=./fil status=progress
90532864 bytes (91 MB, 86 MiB) copied, 6.00005 s, 15.1 MB/s^C
188706+0 records in
188705+0 records out
96616960 bytes (97 MB, 92 MiB) copied, 6.42704 s, 15.0 MB/s
# df -h .
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/virtp-vol01 976M 95M 815M 11% /tmp/x
# sync
# cd ..
root@vmm-deb:/tmp# umount x
root@vmm-deb:/tmp# fsck.ext4 -f -C0 /dev/virtp/vol01
e2fsck 1.43-WIP (15-Mar-2016)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/virtp/vol01: 12/65536 files (8.3% non-contiguous), 36544/262144 blocks
<mount>
# du -hs fil
93M fil
# dd if=./fil of=/dev/null status=progress
188705+0 records in
188705+0 records out
96616960 bytes (97 MB, 92 MiB) copied, 0.149194 s, 648 MB/s
# vgs
VG #PV #LV #SN Attr VSize VFree
virtp 1 2 0 wz--n- 96.00m 48.00m
Definetly the file is occupying 90+MB.
What i expect is that the pool is 40M and the file must NOT exceed 40M. Where
does the file get 93M space ?
I know the VG is 96M but the pool created was max 40M (also VG still says 48M
free). Is the file exceeding the boundaries ?
or am I doing anything wrong ?
Hi
Answer is simple -> nowhere - they are simply lost - check your kernel dmesg
log - you will spot lost of async write error.
(page cache is tricky here... - dd ends just in page-cache which is later
asynchronously sync to disk)
There is also 60s delay before thin-pool target starts to error all queued
write operations if there is not enough space in pool.
So whenever you write something and you want to be 100% 'sure' it landed on
disk you have to 'sync' your writes.
i.e.
dd if=/dev/urandom of=./fil status=progress conf=fsync
and if you want to know 'exactly' what's the error place -
dd if=/dev/urandom of=./fil status=progress oflags=direct
Regards
Zdenek
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