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SubjectRe: [linux-lvm] Re: [Announce] device-mapper beta3 (fast snapshots)
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On Tue, Jul 16, 2002 at 01:27:24PM -0400, Chris Mason wrote:
> Some IDE drives ignore commands to turn off the write back cache, or
> turn it back on when load gets high.

I'm amazed hardware manufacturers would dare to do such a thing - I
guess I'm naive.

> Try iozone -s 50M -i 0 -o with writeback on and off. If you get the
> same answer the benchmarks are suspect....

I tried this and found that /proc/ide/hda/settings was lying in that
it always reports write caching as disabled. Once disabled with
hdparm the disks didn't re-enable write caching on their own.

These are new test results run on a pair of 5k rpm disks with write
caching disabled (average of three runs):


Non Persistent snapshots
------------------------
Run 1 .. 3 Average

baseline 15.349 14.835 14.724 14.979
8k 19.43 21.629 19.951 20.337
16k 19.373 21.579 20.373 20.442
32k 22.158 19.505 21.472 21.045
64k 19.745 19.273 20.191 19.736
128k 19.588 20.86 20.166 20.205
256k 19.67 19.819 21.216 20.235
512k 20.439 22.621 20.753 21.271


Persistent snapshots
--------------------
Run 1 .. 3 Average

baseline 15.342 14.81 14.514 14.889
8k 25.24 26.985 27.092 26.439
16k 24.869 23.943 24.687 24.500
32k 27.084 25.861 24.728 25.891
64k 23.646 23.051 24.786 23.828
128k 23.761 24.596 25.53 24.629
256k 25.472 26.962 27.225 26.553
512k 29.076 27.183 27.795 28.018



As you can see the persistent snapshots are taking a significant
performance hit compared to the non-persistent ones due to the
overhead of ensuring all the data on the disk is consistent.

I would expect the EVMS async snapshots to perform similarly to
device-mappers non-persistent snapshots since they are not ensuring
any form of consistency. Effectively they are not writing any
exception metadata to the disk until a controlled shutdown occurs.

- Joe
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