[Discuss] lvm snapshot cloning

[markw at mohawksoft.com (markw at mohawksoft.com)]

> On Oct 24, 2011, at 7:41 AM, markw at mohawksoft.com wrote:
>> Why? Snapshots are read/write and work fine.
>
> They don't.  See the previous discussion about LVM snapshot performance
> degradation.

As was said, the performance degradation is transient, it only happens
when a COW page is created, after that, it is minimal.

---Create a 100G volume---
root at dewy:~# lvcreate -L100G -i 2 -n test lvmvolumes
  Using default stripesize 64.00 KiB
  Logical volume "test" created
root at dewy:~# sync

--- Write to the disk ----
root at dewy:~# time dd if=/dev/zero of=/dev/lvmvolumes/test bs=1M count=1000
1000+0 records in
1000+0 records out
1048576000 bytes (1.0 GB) copied, 6.0213 s, 174 MB/s

real	0m6.023s
user	0m0.000s
sys	0m1.130s

Good performance, 160 MB/s

-- Create a snapshot ----

root at dewy:~# lvcreate -s -n testsnap -L2G /dev/lvmvolumes/test
  Logical volume "testsnap" created

--- Write to the disk ---

root at dewy:~# time dd if=/dev/zero of=/dev/lvmvolumes/test bs=1M count=1000
1000+0 records in
1000+0 records out
1048576000 bytes (1.0 GB) copied, 63.3635 s, 16.5 MB/s

real	1m3.365s
user	0m0.000s
sys	0m1.270s

Worse performance, 15 MB/s

--- Write to the disk ---
root at dewy:~# time dd if=/dev/zero of=/dev/lvmvolumes/test bs=1M count=1000
1000+0 records in
1000+0 records out
1048576000 bytes (1.0 GB) copied, 5.91994 s, 177 MB/s

real	0m5.922s
user	0m0.000s
sys	0m1.180s

Good performance, 160 MB/s

Once the COW data is allocated, the performance is fine.