Why does SL lag badly when the cache is on a flash drive.

[Jennifer Boyle]

I have run SL from a flash drive the last few years.  Since I do, it would be convenient to put the cache on the flash drive, which I have tried several times.  I consistantly experience SEVERE lag when I do, which is immediately resolved by moviing the cache to a hard drive.  Flash drives are supposed to be faster than hard drives, right?  After all, ReadyBoost would make no sense if they were not considerably faster.

Anyone have any ideas about what's going on?

[SaraCarena]

If you google "second life cache on flash drive" you should see a thread from slu about this. Not sure if it`s allowed to link to other forums here, so won`t.

The concensus was that cache on flash isn`t a good idea.

[ObviousAltIsObvious]

ReadyBoost can work well because flash has generally good read performance. Write performance is not so good, but it does not matter since SuperFetch is not in a hurry to make the file copies. the main copies of those fies are still available in their normal locations.

network apps like Second Life need faster write speeds than many flash drives can handle, because there are not those extra copies already on your disk like in the ReadyBoost situation.

[Coby Foden]

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Jennifer Boyle wrote:

Since I do, it would be convenient to put the cache on the flash drive, which I have tried several times.  I consistantly experience SEVERE lag when I do, which is immediately resolved by moviing the cache to a hard drive.

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The simple answer is that your flash drive has a much slower data transfer rate than what your hard disk has. The faster the disk where your cache is located the better your experience will be.

[Monti Messmer]

To add, it´s a matter of cache size too. When the cache of whatever drive is full the write rate slows down. Usually Flash drives are slower on write than read, the cache fills up and all slows down.

Anyway having the cache on same drive as the program caching isn´t the best idea ;-)

Monti

[Madelaine McMasters]

Jennifer, are you referring to a USB thumb drive, or SSD? Many (most?) thumb drives (and SD cards) are USB2 devices and will be much slower on read/write, but much faster on seek than a hard drive. But if you're talking about SSD, a well designed flash drive should be much faster than HDD in all regards.

An external USB3 thumbdrive/SD card of modest capacity may also perform poorly as it will contain only one Flash memory device. The speed of a single Flash ship is slower than a HDD. It's only when you aggregate a few chips that the memory controller can spread out read/write activity amongst the chips to increase overall speed. For this reason, larger SDDs are often faster than smaller ones, just as quad core processors are faster than single core (when all the cores can be kept busy).

My current iMac is all SSD (directly on PCI, so the controller is not the bottleneck), my previous was HDD. The processor in my current iMac is about 45% faster than that of the old one, but my SSD is nearly 10x faster. That's on both read and write. SL is quite zoomy now.

So, if you're using a USB2 thumbdrive/SD card, that's the problem. If you're using a SATA/PCI SSD, I can't explain your lag.

[Jennifer Boyle]

I was referring to a thumb drive.

[Jennifer Boyle]

Thanks to everyone who answered.

[Sassy Romano]

Jennifer, it's possible that you're using a USB2 thumb drive?

You may get much better performance if you use USB 3 drives as long as you use them in a USB 3 socket.  There's a huge difference available (as long as you don't get a USB 3 drive which just has a small cache itself).

If you really want to spend some money, I have some of these, they pretty much blow all other USB sticks to bits as they're basically an SSD in a USB form factor.

http://poweredbymushkin.com/index.php/catalog/item/33-ventura-ultra/752-ventura-ultra-240gb-flash-drive.html

(They're not cheap though!  The 60GB version is much cheaper, though obviously still more expensive than "normal" sticks)

 

[Nalates Urriah]

It has already been pointed out USB/Flash drives are slower than hard drives. I think you are confusing Solid State Drives (SSD) and USB drives. Both use a solid state drive technology. That part of both operates at similar speeds. BUT... and this is major, the USB interface between the USB drive and computer is a bottle neck.

There are USB, USB2, and USB3 interaces with each getting progressively fastter. But, even USB3 is slower than a convential spinning disk drive. 

SSD's use the same SATA interface the mechanical drives do, but without having to wait for the disk and read/write heads to move into place. This makes them much faster than mechanical spinning drives. Typically improving performance by 2 to 5 times.

The bottle neck with a flash drive is the USB connection process.

 

[Sassy Romano]

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Nalates Urriah wrote:

There are USB, USB2, and USB3 interaces with each getting progressively fastter. But, even USB3 is slower than a convential spinning disk drive. 

 

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You haven't tried a USB 3 SSD then.  I have some spinning disks that get slaughtered by my Mushkins.