* [linux-lvm] Question: How much space is needed do make sure that a LVM snapshot won't overflow
@ 2008-02-12 18:39 Guilherme Destefani
2008-02-28 18:04 ` Rob West
0 siblings, 1 reply; 2+ messages in thread
From: Guilherme Destefani @ 2008-02-12 18:39 UTC (permalink / raw)
To: linux-lvm
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:~]$
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [linux-lvm] Question: How much space is needed do make sure that a LVM snapshot won't overflow
2008-02-12 18:39 [linux-lvm] Question: How much space is needed do make sure that a LVM snapshot won't overflow Guilherme Destefani
@ 2008-02-28 18:04 ` Rob West
0 siblings, 0 replies; 2+ messages in thread
From: Rob West @ 2008-02-28 18:04 UTC (permalink / raw)
To: LVM general discussion and development
To guarantee that the snapshot will not overflow, you have to make it
big enough to contain all of the origin plus the maximum COW table
overhead.
The maximum overhead is the chunk size * (max_number_exceptions_chunks
+ 1). The "+ 1" is for the header chunk. The maximum number of
exception chunks is the number of origin chunks divided by the number
of exceptions per exception chunk, plus one to account for a possible
allocated but unused exception chunk. The number of exceptions per
exception chunk is the chunk size divided by the size of an exception
structure, which is 16 bytes.
Here is some code that may help:
/* Do some calculations. The - 1 and + 1 stuff is to handle any
* remainders. The extra + 1 for num_exception_chunks is for the
* allocated, but unused, exception chunk.
*/
u_int64_t num_chunks_in_origin =
((lv_info.size - 1) / OUR_CHUNK_SIZE_INT) + 1;
u_int64_t num_exceptions_per_chunk =
((OUR_CHUNK_SIZE_INT - 1) / sizeof(dm_disk_exception_t)) + 1;
u_int64_t num_exception_chunks =
((num_chunks_in_origin - 1) / num_exceptions_per_chunk) + 1 + 1;
/* The maximum size of the COW device overhead is the chunk size times
* (the number of exception chunks plus 1 for the header chunk).
*/
u_int64_t sizeof_max_overhead =
(num_exception_chunks + 1) * OUR_CHUNK_SIZE_INT;
Hope this helps,
Rob
On Tue, Feb 12, 2008 at 1:39 PM, Guilherme Destefani
<gd@helixbrasil.com.br> wrote:
> 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:~]$
>
>
>
>
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>
^ permalink raw reply [flat|nested] 2+ messages in thread
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2008-02-12 18:39 [linux-lvm] Question: How much space is needed do make sure that a LVM snapshot won't overflow Guilherme Destefani
2008-02-28 18:04 ` Rob West
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