load testing and tuning a 4GB RAM server

Support support at netmint.com
Sun Apr 6 14:26:36 PDT 2003

[ ... ]

> Aren't you using a Dell PowerEdge and hot-swappable drives?  I also
> thought you mentioned you were using 15K drives for swap, which implies

Yes, PowerEdge with PERC3-DI (128MB) for Raid 1 with 2x36GB 15KRPM drives
and with PERC3-DC (128MB) for Raid 5 with 4x146GB 10KRPM drives.

> SCSI...probably 80-pin SCA form-factor, right?

The drives are hot-swappable in themselves, but I can't add a Raid array
on the fly, nor can I add to the capacity of a Raid array. I am sure that
even if the Raid card allowed me to do that, FreeBSD wouldn't support a
new Raid array that just popped up without a reboot. I am not even sure if
Windows can do that. Now you know the hardware, please correct me if I am
wrong about this.

> If you knew that the difference in KVA memory would be significant to
> your usage, then you could evaluate whether saving a couple of swap
> device slots is worth the loss of flexibility.  Configuring a system
> without any tolerance for change is a little like Procrustes being too
> precise in measuring his own bed.  :-)

But if what I said about Raid is correct, how is having an extra slot in
RAM for swap more flexible? :) Of course, if I _could_ add drives without
rebooting, or better yet add Raid arrays, I would be a very happy camper,
and no question about it, would up the NSWAPDEV.

> I'm not sure you measure "established" the same way I do.  Do you mean
> you expect there to be 500 to 4000 active apache children all processing
> transactions 24-7, or do you mean you expect to see anywhere up to 4000
> people using the site(s) at a time, clicking at whatever rate they'd use
> the site(s) during normal transactions?

Sorry, let me rephrase this a bit. I mean network connections to the
server that are marked "ESTABLISHED" in netstat -an output, if you grep
for the webserver. In other words, if my understanding is correct, these
are connections currently active that the webserver is serving with
information and have running PHP/Perl sessions or are otherwise
downloading files. I could have 1500-2000 to as high as 4000 of those.

> How much of your traffic is going to be over SSL?  You might want to
> look into getting a HI/FN crypto-accelerator card, particularly if you
> have lots of small/short SSL sessions rather than few longer ones.

Not sure as to the exact break-down, but estimate 5-15%. Is that large in
the context of total connections? Perhaps. I will look into a crypto card.

> You really want to run only one type of production database per machine;
> you're risking VM thrashing otherwise.

Even if the load on the second one is _much_ less? Please explain why this
is so, if possible.

> You might have something like a Dell/Adaptec PERC? RAID controller with
> 128MB or so of I/O buffer memory which can also do the RAID-5 XOR
> calculations?  That will help, but even so RAID-5 write performance goes
> from adequate to poor as the I/O load increases.  Also, have you been
> testing I/O while also hitting a database (or two) at the same time?

Right. I suppose I _could_ get another 146GB drive, and reconfigure the
4-drive Raid 5 array to be a 2-drive Raid 1 and 3-drive Raid 5. Or, I
could wait until enough users hit the server and as load creeps, invest
into two new drives and add a Raid 1 array at that point. I think I will
choose the latter approach.

As far as my testing, it's been very raw and no, I haven't done any DB
testing yet. I was mostly testing for stability under heavy heavy memory
usage, i.e. a bahzillion lynx'es and -j500 makeworld. How would you do
this? :)

Thanks for your help,


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