why use 2 drives for LR3 and PS?

Hey everyone! I have been reading many of the threads about building a new computer for photo editing and while I have learned a lot I still have one question that I have not been able to clear up. It appears that it is best to be running the OS on 1 drive and use another for running LR3 and PS from what I have gathered and to use that second non-OS disc for caching to speed things up also. To make it even faster , using a SSD is even better.
I guess what I am confused about is do you install the OS on the one drive, and then when you go to install LR3 on the 'puter, do you put LR3 install on the second drive? So, when you open LR3, it will boot and run from the second drive? Also, does caching on same drive as LR3 best way to go?One last thing is where to save your files at? Second drive and not the OS drive?
I know it sounds super remedial and while I'm not a computer nerd, I am not totally clueless. I am just trying to better understand how to build and use a new computer while learning about new technology. A lot has changed since my last computer!!
Thanks for the everyone's help. This is the best forum on the web by far. So much info.

OS and programs typically go on your primary drive, whether a hard disk or SS drive. Some have a 3rd drive reserved JUST for page file (OS virtual memory)/scratch disk (PS working "memory") the former accessed when "out of" physical memory, the latter accessed when opening very large image files/multiple layers etc....at least that's how I THINK it works
...However with current 64bit OS and 6gb or more of RAM, page file/virtual memory is used much less frequently by "average" non-heavy PS users opening 1-4 files at a time, with minimal layer processing...
With 2 drives, the page file and scratch disk can be put on the 2ndary disk drive and I've found this to be fine...though I've read that this isn't good for the drive, I've not experienced any problems with having 2 separate partitions for each them on the 2nd drive, the reason being, I suspect, that the PS scratch disk is accessed while the page file remains dormant with 8gb of memory being plenty to run OS and PS...
...the 2nd disk is also used as file saving/storage away from any corruptions to the OS drive in case THAT drive needs to be reformatted....
...As for SSD vs faster HDDs (7200 RPM and faster) opening and reopening programs, the difference is best timed by a stop-watch for most "light" users...

it all boils down to hard drive speed, when you open up an application like lightroom or photoshop it has to read the hard drive and the more things its doing the slower it is which means the longer it takes for the app to open

I messed around a lot trying to get good performance vs costs and what I ended up doing was putting a SSD for the OS & also the apps on it, then putting the cache files from LR / photoshop on it and then for my pictures I put 2 disks in raid0 to get max speed.... and keep in mind all HD's are not the same !

in a perfect world having a SSD to boot + apps, then another for the cache files + another for the pics but that would be crazy expensive

when you import a photo into LR for example it copies the file to wherever you choose, it renders a copy based on your setting and by default its not 1:1 and puts it in the cache folder, then when you go into a 100% zoom of the photo it has to render it

for the copy its really a mute point the memory card is the slowest thing so it doesn't matter if you have a SSD or a slow HD its not going to impact it

when it does the import rendering on the default settings its reading the file from wherever you store it so the drive is the better, and then of course its writing the cached file to the cache location so again the faster the better

then for the 100% zoom(maybe by default when you enter develop also actually) it does a complete render so again the faster the HD is that its reading the better, and of course then it writes that render to the cached folder ... so this step whole is really where you want the disk IO

As an example I initially had a single HD with OS / apps / cache on it and then had an external disk with my photos, it was so slow it drove me crazy ! I swapped the HD for a SSD and LR / Photoshop opened fast but it was still fairly slow rendering and I figured out the external disk IO was the problem. Then I put in 2 x WD black HD's in raid 0 and put the photos on there and holy crap what a difference that made.

One of the most annoying things for me was when you were in library and you pulled the slider to the left so it displayed as many photos as possible and then you would scroll and it would show blank photos and then they would slowly appear that drove me nuts. Now it rebuilds them so much faster.

so if your looking to build a machine my recommendation for cost is to do what i did 1 x SSD for apps + cache + OS and then where your storing your photos get whatever you can afford, at a minimum get 1 disk that is fast like the western digital black but if you can swing extras and put them in raid 0 you will love it .... of course all of this is great BUT you need backups as raid 0 has no redundancy and SSD's are not the most reliable ! if your using MAC its simple just use time machine, on PC use crashplan its free and works well, I use both and back everything up daily to an external raid

one last note is on processor, definitely that helps during the rendering so the faster the better, get an i7 if you can

sorry its so long

Good point about the external drive...it IS much slower if USB2...the new USB3 is reportedly MUCH improved in that department...

even a single usb3 drive is slow and will impact using LR, keep in mind a single USB3 external drive won't be any faster than a single drive inside the machine, I haven't looked at external raid usb3 drives but I imagine there are some out there that are decent but I am sure they are expensive

I am waiting for reasonable thunderbolt options because that is going to be where the real speed is

Thanks everyone. I am still confused still however. Not that difficult to do, really. Would you say put LR3 and PS on the SAME drive as the OS? I thought that by having LR3 on a second drive, it would speed things up, as well as rendering 1:1 before you do any work on your photos will help too though you pay for the rendering time on the front end.

for LR you have a few things

1) the application
2) the LR database
3) the LR cache files
4) the photos

for the application once its open and running its not looking at the disk often if at all

the database is pretty small but it should be on the fastest disk you have, usually on the SSD but I am not sure how much it really impacts even if it was on a slower disk

if your importing photos and doing 1:1 rending to make it fast you need CPU, fast read /write access to the photos and fast write access to the cache / database. If you just put LR on its own drive along with the database / cache it will make it faster but one thing many people overlook is placing the photos on a slow drive

depending on how many pics you have keeping 1:1 rendering could take a lot of storage but yeah if you can keep them all it will make it faster, by default it only keeps them for 30 days and its only set to 1GB which doesn't last long

Disks are the slowest part of a PC. Spinning disks are especially slow, and SSDs are reasonably quickly. I have five drives in my PC, but only three are relevant in this context:
- OS is on a 60GB SSD, and I put swap and scratch on here too, since once the OS and programs have loaded it's otherwise idle
- Cache is on a 120GB SSD - even SSDs can slow down if you put too much through them. You'd get away with 60GB easy.
- Images are on a spinning disk. I'll experiment with putting images on the SSD some time.

Get an SSD big enough to hold it all - OS, Apps, LR Catalog, Cache, Working Images, etc. Have a big HDD to data that doesn't need quick access (music, videos, images that you've finished with). Having a separate SSD for cache, etc will make virtually zero difference. It was always the access times that made it a good idea to have spearate drives in the past. But SSD access times are so fast that the difference is negligible.

I doubt to many people can afford having a SSD that holds everything, having a separate SSD for just the cache will make a big difference for sure, especially if you change LR to keep all 1:1 previews and not just 30days

tekkie wrote in post #12897162
I doubt to many people can afford having a SSD that holds everything

Hmm, 120GB drives are getting quite cheap nowadays.

tekkie wrote in post #12897162
especially if you change LR to keep all 1:1 previews and not just 30days

How many people go back and look at stuff in Lightroom a couple of months after they've processed?

hollis_f wrote in post #12897353
Hmm, 120GB drives are getting quite cheap nowadays

My MBP, which is not my primary machine, has 200Gig+ used on the HDD…
On my MacPro, OS and applications alone probably barely fit on anything less then a 100GB drive.

hollis_f wrote in post #12897353
Hmm, 120GB drives are getting quite cheap nowadays.

How many people go back and look at stuff in Lightroom a couple of months after they've processed?

what are you going to do with 120GB? most people have more photos than that is my guess after a few years

I go back and look at old photos all the time, but for me its not a big problem because my photos are on a decent disk system

hollis_f wrote in post #12896185
Have a big HDD to data that doesn't need quick access (music, videos, images that you've finished with).

tekkie wrote in post #12899133
what are you going to do with 120GB? most people have more photos than that is my guess after a few years

Yes, and they go on the HDD. Just like I said in my post!

tekkie wrote in post #12899133
I go back and look at old photos all the time, but for me its not a big problem because my photos are on a decent disk system

The discussion was about 1:1 previews in Lightroom. Not many people are going to be using LR to view already processed images at 100%.

Having everything on one disk means backup images of the drive are larger than necessary. I've started using the Windows 7 built in backup, so that's not really an issue any more, I don't think.