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> 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.
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