On 07/02/17 17:29, Olivier Lambert wrote:
Yep, but if I hit a 30% penalty, I don't want that :) Any idea of the
perf impact? I'll probably contact Xavier directly if he's not here!
It depends on the workload. The basic problem is that the I/O write size
must be a multiple of the number of data bricks to avoid
read-modify-write cycles (i.e. they write always full stripes, so no
need to read anything from disk).
Normally applications issue writes that are a power of 2 (512, 4096,
131072, ...), so the best configurations for ec should also be powers of
two. But of course you can have bad performance with ec even if you use
a power of two if the application ends writing blocks of 1000 bytes for
example.
Anyway, I cannot give you real numbers. Long time ago I did some tests
but I was unable to get conclusive results. Probably I was having too
many interferences from other translators that were hiding the real
performance impact (maybe write-behind). This is good because this means
that peak performance could be the roughly the same with "optimal" and
"non-optimal" configurations, but sustained throughput should be worse
for "non-optimal" configurations because at some point the caching made
by write-behind and other xlators will become full and they won't be
able to continue hiding the performance hit.
Some day I'll try to find time to get more accurate results regarding
this issue, but for now only thing I can say is to test your specific
workload with your specific configuration and see if it suffers a
performance impact or not.
Xavi
On Tue, Feb 7, 2017 at 5:27 PM, Jeff Darcy <jdarcy@xxxxxxxxxx> wrote:
----- Original Message -----
Okay so the 4 nodes thing is a kind of exception? What about 8 nodes
with redundancy 4?
I made a table to recap possible configurations, can you take a quick
look and tell me if it's OK?
Here: https://gist.github.com/olivierlambert/8d530ac11b10dd8aac95749681f19d2c
As I understand it, the "power of two" thing is only about maximum
efficiency, and other values can work without wasting space (they'll
just be a bit slower). So, for example, with 12 disks you would be
able to do 10+2 and get 83% space efficiency. Xavier's the expert,
though, so it's probably best to let him clarify.
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