On 12/23/2014 11:08 AM, Gandalf Corvotempesta wrote:
2014-12-22 13:56 GMT+01:00 Jeff Darcy <jdarcy@xxxxxxxxxx>:
The key tradeoffs here are storage utilization vs. performance. In
general, erasure codes (disperse) will give better storage utilization
than replication for the same level of performance. However, this might
not be the case for N=3. With replication, that will protect against
two failures. However, from the admin guide section on disperse:
"redundancy_ must be greater than 0, and the total number of bricks must
be greater than 2 * _redundancy_"
I interpret this to mean that for two-failure protection you would need
at least five bricks. With three bricks disperse can only offer
one-failure protection. In this case it's roughly equivalent to RAID-5,
with only a 50% storage penalty vs. 100% for replica 2 offering the same
protection.
The other issue is performance. With disperse, all writes *and reads*
must be done to all bricks, and at a stripe size equal to 512 times the
number of bricks (minus those used for redundancy). This means more
data transfer, especially for reads, and also more write contention than
with replication. This being new code, some optimizations that already
exist for replication do not yet exist for disperse even though they're
applicable.
Thank you for the response.
So, if I understood properly, disperse is space-optimized but has a
performance penalty compared
to a standard 'replication'
It has some performance penalty in some scenarios, however it depends on
multiple factors. For some workloads ec performs better than replicate
thanks to the distribution of the load and the reduced amount of data
managed. The only way to be sure for your use case would be to test it.
What I would like is to get a RAID-6 equivalent, so, replica 2 should
be used to get
performance and redudancy, right ?
To get a RAID-6 equivalent you will need a replica 3 or a disperse 5:2
(5 bricks with 2 of redundancy). These are the smaller combinations that
support two failed bricks.
More over, with replication, in case of issue, I can always read raw
files from disks, this
would not be possible in case of disperse, where each file is splitted
in multiple chuncks, right?
Yes, with replica you can directly read the files from the bricks. With
disperse the files are encoded and splitted, so they cannot be directly
recovered.
Xavi
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