From: Guilherme Destefani <gd@helixbrasil.com.br>
To: linux-lvm@redhat.com
Subject: [linux-lvm] Question: How much space is needed do make sure that a LVM snapshot won't overflow
Date: Tue, 12 Feb 2008 16:39:58 -0200 [thread overview]
Message-ID: <1202841598.32238.15.camel@localhost.localdomain> (raw)
Hi.
In the LVM2 HOWTO, it says:
(http://www.tldp.org/HOWTO/LVM-HOWTO/snapshotintro.html),
"Full snapshot are automatically disabled"
and:
"If the snapshot size equals the origin size, it will never overflow.".
But if one snapshot is created and an equal size is allocated to the
snapshot, it still overflows (creates a volume with 32M, a snapshot with
32M, and fills the volume with zeros).
Using more space, it doesn't overflow (same thing, but 64M for the
snapshot).
How much more space is needed to be alocated to the snapshot to ensure
that it will never overflow?
Thanks in advance,
Guilherme.
Script started on Tue 12 Feb 2008 02:11:11 PM BRST
[root@localhost ~]# lvcreate -L 32M -n base_volume hf
Logical volume "base_volume" created
[root@localhost ~]# lvcreate -s -L 32M -n
copy_volume /dev/hf/base_volume
Logical volume "copy_volume" created
[root@localhost ~]# lvdisplay |grep -A 13 "_volume"
LV Name /dev/hf/base_volume
VG Name hf
LV UUID qPzWgG-ON3V-swUr-O0IP-I172-G3YV-GH431K
LV Write Access read/write
LV snapshot status source of
/dev/hf/copy_volume [active]
LV Status available
# open 0
LV Size 32.00 MB
--
LV Name /dev/hf/copy_volume
VG Name hf
LV UUID oGK0gG-o62D-Tu84-9CW0-rvPb-H7HX-bjUwk0
LV Write Access read/write
LV snapshot status active destination
for /dev/hf/base_volume
LV Status available
# open 0
LV Size 32.00 MB
Current LE 1
COW-table size 32.00 MB
COW-table LE 1
Allocated to snapshot 0.05%
Snapshot chunk size 8.00 KB
Segments 1
Allocation inherit
Read ahead sectors 0
Block device 253:20
[root@localhost ~]# dd if=/dev/zero of=/dev/hf/copy_volume
dd: writing to `/dev/hf/copy_volume': No space left on device
65537+0 records in
65536+0 records out
33554432 bytes (34 MB) copied, 8.01466 s, 4.2 MB/s
[root@localhost ~]# lvresize -L 64M /dev/hf/copy_volume
/dev/dm-20: read failed after 0 of 4096 at 0: Input/output
error
Extending logical volume copy_volume to 64.00 MB
Logical volume copy_volume successfully resized
[root@localhost ~]# lvdisplay |grep -A 13 "_volume"
/dev/dm-20: read failed after 0 of 4096 at 0: Input/output
error
LV Name /dev/hf/base_volume
VG Name hf
LV UUID qPzWgG-ON3V-swUr-O0IP-I172-G3YV-GH431K
LV Write Access read/write
LV snapshot status source of
/dev/hf/copy_volume [INACTIVE]
LV Status available
# open 0
LV Size 32.00 MB
Current LE 1
Segments 1
Allocation inherit
Read ahead sectors 0
Block device 253:18
--
LV Name /dev/hf/copy_volume
VG Name hf
LV UUID oGK0gG-o62D-Tu84-9CW0-rvPb-H7HX-bjUwk0
LV Write Access read/write
LV snapshot status INACTIVE destination
for /dev/hf/base_volume
LV Status available
# open 0
LV Size 32.00 MB
Current LE 1
COW-table size 64.00 MB
COW-table LE 2
Snapshot chunk size 8.00 KB
Segments 1
Allocation inherit
Read ahead sectors 0
Block device 253:20
[root@localhost ~]# lvremove /dev/hf/copy_volume
/dev/dm-20: read failed after 0 of 4096 at 0: Input/output
error
Do you really want to remove active logical volume
"copy_volume"? [y/n]: y
Logical volume "copy_volume" successfully removed
[root@localhost ~]# lvcreate -s -L 64M -n \
copy_volume /dev/hf/base_volume
Logical volume "copy_volume" created
[root@localhost ~]# lvdisplay |grep -A 13 "_volume"
LV Name /dev/hf/base_volume
VG Name hf
LV UUID qPzWgG-ON3V-swUr-O0IP-I172-G3YV-GH431K
LV Write Access read/write
LV snapshot status source of
/dev/hf/copy_volume [active]
LV Status available
# open 0
LV Size 32.00 MB
Current LE 1
Segments 1
Allocation inherit
Read ahead sectors 0
Block device 253:18
--
LV Name /dev/hf/copy_volume
VG Name hf
LV UUID LSUqQB-O2n4-8zmk-iL9n-2vDQ-qSsB-yOBDL1
LV Write Access read/write
LV snapshot status active destination
for /dev/hf/base_volume
LV Status available
# open 0
LV Size 32.00 MB
Current LE 1
COW-table size 64.00 MB
COW-table LE 2
Allocated to snapshot 0.02%
Snapshot chunk size 8.00 KB
Segments 1
Allocation inherit
Read ahead sectors 0
Block device 253:20
[root@localhost ~]# dd if=/dev/zero of=/dev/hf/copy_volume
dd: writing to `/dev/hf/copy_volume': No space left on device
65537+0 records in
65536+0 records out
33554432 bytes (34 MB) copied, 9.5375 s, 3.5 MB/s
[root@localhost ~]# lvdisplay |grep -A 13 "_volume"
LV Name /dev/hf/base_volume
VG Name hf
LV UUID qPzWgG-ON3V-swUr-O0IP-I172-G3YV-GH431K
LV Write Access read/write
LV snapshot status source of
/dev/hf/copy_volume [active]
LV Status available
# open 0
LV Size 32.00 MB
Current LE 1
Segments 1
Allocation inherit
Read ahead sectors 0
Block device 253:18
--- Logical volume ---
LV Name /dev/hf/F4_burn
VG Name hf
LV UUID Bj1VJS-1JUK-BA18-egFj-xOoY-qrbq-jbZFFY
--
LV Name /dev/hf/copy_volume
VG Name hf
LV UUID LSUqQB-O2n4-8zmk-iL9n-2vDQ-qSsB-yOBDL1
LV Write Access read/write
LV snapshot status active destination
for /dev/hf/base_volume
LV Status available
# open 0
LV Size 32.00 MB
Current LE 1
COW-table size 64.00 MB
COW-table LE 2
Allocated to snapshot 50.12%
Snapshot chunk size 8.00 KB
Segments 1
Allocation inherit
Read ahead sectors 0
Block device 253:20
[root@localhost ~]# exit
Script done on Tue 12 Feb 2008 02:13:07 PM BRST
[Gui@localhost:~]$
next reply other threads:[~2008-02-12 18:40 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-02-12 18:39 Guilherme Destefani [this message]
2008-02-28 18:04 ` [linux-lvm] Question: How much space is needed do make sure that a LVM snapshot won't overflow Rob West
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