Varnish as cache for image servers

[Anders Berg]

And the slides :)

Doh.

Anders Berg
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On Nov 20, 2006, at 0:45, Anders Berg wrote:

>
> On Nov 19, 2006, at 18:23, Andrew Miehs wrote:
>
>> Dear List,
>>
>> I only yesterday discovered Varnish. After spending most of today  
>> reading and trawling the web on information concerning it, I am  
>> dying to try it out on our web farm tomorrow.
>
> Hi Andrew, and welcome to Varnish.
>
>>
>> My current problem is that I have 3 web servers, each with the  
>> same 32GB of static image files. I am currently using Lighttpd  
>> running on Linux 2.6 to deliver this static content to our  
>> customers. The web servers are 32bit machines with 4GB of RAM.  
>> (Yes 64bit would be better) :-). We currently have have approx  
>> 1000req/s per server (files about 30k each). An F5 Loadbalancer is  
>> splitting the load between these machines.
>>
>> # free
>>              total       used       free     shared    buffers      
>> cached
>> Mem:       3930100    1917892    2012208          0     517032      
>> 971216
>> -/+ buffers/cache:     429644    3500456
>> Swap:      1028152          0    1028152
>> #
>>
>> The original idea was to let the linux kernel do all the caching  
>> with its filesystem cache. Unfortunately, it does not seem to want  
>> to use all the RAM available for this purpose. I was hoping that  
>> by making more effective use of the available RAM, I would be able  
>> to reduce IO Wait.
>>
>> I am aware that I can help do this with faster disks, bigger RAID,  
>> etc, but I was hoping to try and improve the situation by using  
>> the onboard RAM.
>>
>> Now for my questions:
>>
>
> I'll have to answer these questions based on my experience and  
> maybe in a generic way.
>
>> 1) Would you recommend using Varnish for this situation, or would  
>> you suggest something like Apache with Mod_cache_memory or Zeus  
>> with memory caching?
>
> I would say that since your content is static I would in either go  
> 2 ways based on the traffic pattern and what your data is "doing".  
> For example, where I work we have a rig that serv flash ads, and  
> since some tags for ads today make a uniqe URL for each serv (even  
> if content is satic). Since caching works bad in situations like  
> this, I have ended up using a FreeBSD with accel filtering and  
> apache, and that works fine. Mind that is with a small "often used"  
> dataset.
> But when it comes to "pure" static files I would say that Varnish  
> in theory should be a perfect match for what you are trying to do  
> (I say in theory since I have not used Varnish like that in a prod.  
> evironment). The reason I say so is because Varnish uses some  
> "state of the art" features in the OS (Even if they have been  
> around a long time :) Like you say, let the OS do the stuff it  
> knows how to do best.). Right now I use Varnish in a prod.  
> environment with mixed content static/dynamic, and it works  
> amazing. Read the attached PDF for a presentation and some stats.
>
> I wanna add stuff here about 32 bit VS 64 bit, and talk some more  
> about you dataset instead of answering those questions later. Since  
> the 32 bit is slowly dying out we did not put much effort into it  
> and you will see "stupid" limits like max size on filesize (cache  
> size). We mmap the a file and let the OS do its work, and a 32 bit  
> machine would, as you say, have a 3 GB limit (I have not tried  
> Varnish on a 32 bit machine, so I can't say where the limit is.).  
> If that is "enuff" to make Varnish work at it's best for you  
> depends on your dataset.  Lets say you only "often" serv 3 GB of  
> that static content, well, then that is not a big setback, since  
> Varnish will slowly change data out with the default LRU algorithm  
> (we have planned more algorithms, and Varnish is made modular so  
> it's easy to change to one you'd like/make). If you can cache the  
> static content for lets say a week and new static content is added  
> at a slow pace and not 30 GB a day, not much disk IO will get done  
> I guess. I know people wanna use their hardware to the end, but  
> Varnish running on 64 bit shines so good, in many cases it's an  
> immediate return of investment buying new machines and running  
> Varnish. I changed out 12 machines running Squid with 2 64 bit  
> machines with Varnish :)
>
> As I mentioned we wanna add more algorithms in the future, if there  
> is need/requests for it, but right now we are gonna focues our work  
> on fixing bugs, getting things to run rock-hard stable, and adding  
> fetaures in VCL. We are also starting work with a spec for Varnish  
> 2.0 (ESI/ESI'ish behaviour beeing the most popular request feature  
> it seems), and getting the governing board down. Our list of things  
> we wanna see in Varnish is long and uber-cool. Stay tuned.
>
>> 2) The ALPHA notes suggested that Varnish did not check for  
>> backend errors - has this been fixed in the 1.0.2 version? IE: Can  
>> I use it in a live situation and sleep at night?
>
> We cut some corners before launch, and VCL (thats where backend  
> loadbalancing and some other "save-backend" features are  
> implemented) did not get as feature-rich as we hoped. But I am  
> happy to inform that the break we took after the 1.0 launch, is now  
> over and features like backend-loadbalancing and checking are high  
> on the list and "quick" to implement I am told. So again, stay  
> tuned. Also note that I use Varnish in prod., but note that I am  
> using FreeBSD STABLE and Varnish works great with a sometimes  
> overloaded backend.
>
>>
>> 3) Do you think running web server and Varnish on the same box  
>> would cause a problem?
>
> Have not tried it. But should work fine depending on RAM and  
> loadtypes again. :)
>
>> 4) How do you limit the size of Varnish? Is this limited by the  
>> size of the -s file? or size that you set with -s? I realise that  
>> this will be a max of 3GB with a 32bit kernel....
>
> -s uses or makes a file depending on if there was a file there in  
> the first place.
>
>> 5) Is there a suggested 'startup command line' that I should use  
>> in such a situation - like the one used for for your test run?
>>
>> and last but not least
>>
>> 6) As I am quite happy with the modified dates, etc that my web  
>> server is delivering, do I need to use VCL scripts?
>
> No, the default VCL will honor your own settings in most cases.
>
>
>
> Hope that helps some.
>
> I guess I should add that Varnish was my initative and I provided  
> the funding for Varnish 1.0 and therefore I would have to be biased :)
>
>
> YS
> Anders Berg
>
>>
>>
>> Thanks for getting through this mail!
>>
>>
>> Cheers
>>
>>
>> Andrew
>
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