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From Paul Hirst <paul.hi...@sophos.com>
Subject RE: Couch on SSD drives
Date Wed, 07 Dec 2011 10:59:53 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.

I'm surprised you say this. I thought an append only file arrangement (like couch) combined
with a battery backed RAID controller would be the perfect setup for RAID5. I have 6 disks,
so that should be 5 data blocks followed by a parity block. If the 5 data blocks are written
out in succession then there should be no need to do any reads to calculate the parity. Running
with a battery backed unit means the system should be able to lazily calculate parity like
this since there is no requirement to get the data and associated parity, immediately written
to disk.

But I'll admit I have tried to research this many times and have found it neigh on impossible
to find a decent answer. I have come across many people claiming RAID5 is seriously slow and
a few other people saying that while that was true many years ago RAID controllesr are now
much more optimised than they used to be and are able to do the sort of thing I'm suggesting
above.

Anyway, my blocks read per sec is at least 10 times my blocks written per sec so I'm not overly
concerned about write performance.

 Suggest RAID-10 instead. Secondly, have you compacted
> the db lately? This will reduce its total size and also organize it
> better on disk.

I do need to do that. It compacts to about 500G. One of the problems I'm having is that the
compaction process takes a few weeks to run. Again I'm looking for ways to speed this up and
I was wondering if SSDs might be the answer.

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.
Is there any way I can estimate the number of blocks being used by the inner b-tree? My database
is currently 60 million documents. Since my documents are relatively large I would imagine
that all the inner nodes should fit comfortably into available RAM and therefore should mostly
be in the disk cache since most are likely to be read far more often than the leaf nodes.

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

I think I shall try to benchmark it myself. I'm thinking of getting a machine configured with
a sizeable uncompacted couch database and probably a 1:20 ratio of RAM to couch database size.
Then try a compaction on an SSD vs single spinning disk. That will hopefully give me something
to go on even though I'm ultimately considering 6 SSDs to replace 6 spinning disks.

I may also try RAID5 vs RAID10 if I find the time.

Thanks for the feedback. If I do come up with any numbers I'll share them.

>
> 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.
> >
> > ________________________________
> > Sophos Limited, The Pentagon, Abingdon Science Park, Abingdon, OX14
> 3YP, United Kingdom.
> > Company Reg No 2096520. VAT Reg No GB 991 2418 08.


Sophos Limited, The Pentagon, Abingdon Science Park, Abingdon, OX14 3YP, United Kingdom.
Company Reg No 2096520. VAT Reg No GB 991 2418 08.

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