Ben,
On 20-Oct-06, at 3:49 AM, Ben Suffolk wrote:
Hello all,
I am currently working out the best type of machine for a high
volume pgsql database that I going to need for a project. I will be
purchasing a new server specifically for the database, and it won't
be running any other applications. I will be using FreeBSD 6.1 Stable.
I think it may be beneficial if I give a brief overview of the
types of database access. There are several groups of tables and
associated accesses to them.
The first can be thought of as users details and configuration
tables. They will have low read and write access (say around 10 -
20 a min). SIzed at around 1/2 Million rows.
The second part is logging, this will be used occasionally for
reads when reports are run, but I will probably back that off to
more aggregated data tables, so can probably think of this as a
write only tables. Several table will each have around 200-300
inserts a second. The can be archived on a regular basis to keep
the size down, may be once a day, or once a week. Not sure yet.
The third part will be transactional and will have around 50
transaction a second. A transaction is made up of a query followed
by an update, followed by approx 3 inserts. In addition some of
these tables will be read out of the transactions at approx once
per second.
There will be around 50 simultaneous connections.
I hope that overview is a) enough and b) useful background to this
discussion.
I have some thoughts but I need them validating / discussing. If I
had the money I could buy the hardware and sped time testing
different options, thing is I need to get this pretty much right on
the hardware front first time. I'll almost certainly be buying Dell
kit, but could go for HP as an alternative.
Processor : I understand that pgsql is not CPU intensive, but that
each connection uses its own process. The HW has an option of upto
4 dual core xeon processors. My thoughts would be that more lower
spec processors would be better than fewer higher spec ones. But
the question is 4 (8 cores) wasted because there will be so much
blocking on I/O. Is 2 (4 cores) processors enough. I was thinking 2
x 2.6G dual core Xeons would be enough.
Memory : I know this is very important for pgsql, and the more you
have the more of the tables can reside in memory. I was thinking of
around 8 - 12G, but the machine can hold a lot more. Thing is
memory is still quite expensive, and so I don't to over spec it if
its not going to get used.
Disk : Ok so this is the main bottleneck of the system. And the
thing I know least about, so need the most help with. I understand
you get good improvements if you keep the transaction log on a
different disk from the database, and that raid 5 is not as good as
people think unless you have lots of disks.
My option in disks is either 5 x 15K rpm disks or 8 x 10K rpm disks
(all SAS), or if I pick a different server I can have 6 x 15K rpm
or 8 x 10K rpm (again SAS). In each case controlled by a PERC 5/i
(which I think is an LSI Mega Raid SAS 8408E card).
You mentioned a "Perc" controller, so I'll assume this is a Dell.
My advice is to find another supplier. check the archives for Dell.
Basically you have no idea what the Perc controller is since it is
whatever Dell decides to ship that day.
In general though you are going down the right path here. Disks
first, memory second, cpu third
Dave
So the question here is will more disks at a slower speed be better
than fewer disks as a higher speed?
Assuming I was going to have a mirrored pair for the O/S and
transaction logs that would leave me with 3 or 4 15K rpm for the
database, 3 would mean raid 5 (not great at 3 disks), 4 would give
me raid 10 option if I wanted it. Or I could have raid 5 across
all 5/6 disks and not separate the transaction and database onto
different disks. Better performance from raid 5 with more disks,
but does having the transaction logs and database on the same disks
counteract / worsen the performance?
If I had the 8 10K disks, I could have 2 as a mirrored pair for O/S
Transaction, and still have 6 for raid 5. But the disks are slower.
Anybody have any good thoughts on my disk predicament, and which
options will serve me better.
Your thoughts are much appreciated.
Regards
Ben
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