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From Robert Newson <robert.new...@gmail.com>
Subject Re: Couch on SSD drives
Date Tue, 06 Dec 2011 15:33:12 GMT
Firstly, RAID5 is working against you, it's one of the slowest RAID modes (every logical write,
for example, is two physical reads and two physical writes). Suggest RAID-10 instead. Secondly,
have you compacted the db lately? This will reduce its total size and also organize it better
on disk. Finally, it's not the number of 'active documents' that matters (b-tree performance
is typically not predicated on the high probability that the leaf nodes are cached), it's
the inner b-tree nodes that matter (the more that are in the disk cache, the better); this
is a factor of document count. The cost of a cache miss is dependent on the speed of your
disk array.

SSD drives will be considerably better because there is no seek cost, but the cost of the
drives is still prohibitive for many cases.

B.

On 6 Dec 2011, at 04:05, Paul Hirst wrote:

> Has anyone done any performance testing of couch on SSD drives?
> 
> I have a strong suspicion that my disks are constantly seeking in order to satisfy read
requests and therefore the performance is rubbish. The system is a RAID5 with 6 10k SAS drives.
I'm wondering if upgrading to SSD drives might give a significant performance boost. It's
either that or spreading the load across multiple boxes using something like BigCouch.
> 
> To give a bit more background....
> 
> I have a ~1.1Tb database at the moment running on a single box with ~48G of RAM. I strongly
suspect that the number of active documents (ones which are seeing updates) is a larger set
than will fit into RAM and therefore I assume most document requests are hitting the disk.
My disk is ~100% utilizied all the time and I'm not keeping up with the number of read and
writes I need to make.
> 
> The average wait time for disk IO is around 5ms however the CPU load is minimal.
> 
> Finally I did a test on the box to compare the disk throughput when reading a large sequential
file. Even without stopping couch, reading a sequential file managed to drag about 7 times
more data off the disk than the system was normally achieving.
> 
> So even though I might eventually switch to BigCouch or similar I'd really like to balance
out the CPU power and the disk power in my box since at the moment the system seems totally
over specced CPU wise and totally under specced disk wise. Could SSD drives be the answer?
> 
> Thanks.
> 
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