Re: [vserver] SSD/HD hybrids

From: Ed W <>
Date: Thu 01 Apr 2010 - 17:38:32 BST
Message-ID: <>

On 01/04/2010 14:29, Eugen Leitl wrote:
>> iSCSI is no real benefit, it brings in extra letency, exactly what you
>> try and avoid with the SSDs. I built a 10GBit/s iSCSI infrastructure
> I don't think the Ethernet stack would add more than few 10 us latency, and the
> problem is IOPS when tens or hundreds of simultaneous accesses.

I have been hanging around the gluster mailing list recently and one of
the things you see with network filesystems is people not noticing how
much the latency is going to kill you. eg if your latency was 1ms to
the iSCSI server then under some circumstances you will max out at 1,000
IOs per second. Now depending how that translates to app performance
this can be a huge performance hit and presumably why Adrian commented
that even 10gbit FC with it's massively lower latency can still limit
bandwidth to each client to a fraction of the total server bandwidth.

I think your question is so academic as to be much better to simply
benchmark some tests and literally see what will work best for you? In
general an SSD will help most if you have io wait problems and it's less
beneficial for streaming reads/writes?

A datapoint though, but I recently upgraded my Macbook Pro to 6GB from
4GB ram and also bought a 256MB SSD to replace the spinning disk it had
before. Now the problem I was trying to solve was that I was short of
free memory while running some Windows 7 instances under parallels and
the machine was literally freezing completely for seconds at a time
while switching apps. Now the SSD arrived first and simply changing
this *completely* transformed the machine, literally night and day,
whole machine is lighting fast and zero slowdown booting some win7
instances even with low ram. I then added the extra ram a day later and
noticed no appreciable difference in performance. Now this is
COMPLETELY the opposite way around to what I expected, but interesting -
usually I would add ram first and faster disks second.

However, this is a desktop and here the speed of opening an application
(which only happens a dozen times a *day*) is the benchmark by which I
judge the speed of the computer! In contrast on a server we want to
optimise for steady state IOs happening throughout the day and it's less
clear how the SSD will benefit things here...

So I think you need to benchmark your setup and perhaps look instead at
something like a controller with writeback cache rather than an
SSD/iscsi? I see you can get the Dell PERC things quite cheap on ebay
for example?

Just a thought...

(P.S. Anyone with a laptop running slowly, the SSD blows the doors off
with perceived performance - seriously impressive...)

Ed W
Received on Thu Apr 1 17:38:50 2010

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