Ubuntu - a Speedup guide

Posted Oct 6, 2007 9:05 UTC (Sat) by drag (guest, #31333) [Link]

Ya, but seek times are very very fast (compared to harddrives). If your dealing with large amounts of small files or cache'd bits and peices of large files or something like that then flash will help out a lot.

In a modern system most time waiting on the harddrive, I figure, is waiting on seek times. For example during boot up I would expect that flash would have considurable performance advantages.

This is the classic performance killer for application start-up for Linux. Often apps will poll all sorts of different files all over your directory system.. all of these are different sizes and made at different times so you can end up with considurable performance badness as your harddrive is spinning around seeking.

> * flash wears out with writes (but the real question is, does it wear out faster than the mechanical parts)

The answer is 'It Depends'.

In Unix there are traditionally been things like 'Block devices' and 'Character Devices'. Characters are things like keyboard and mice, Block devices are storage, right? (well something like that).

Well true flash-based drives are neither. They are a third thing called MTD for memory technology devices.....

HOWEVER for consumer technology for things like Compact Flash, USB Mass storage Flash, SD Flash cards and so on and so forth use hardware to emulation to be block devices. So even though flash is technically MTD the OS can only see them as block devices, generally.

So what does that matter?

Well it's all about the 'load balancing'. That's how flash devices can outlive their mechanical counterparts. If you have efficient load balancing AND don't use the full capacity of the flash drive you can have a flash drive that will easily outlive a harddrive even under 24/7 loads. Modern high-quality flash drives could probably last 5-10 years even under heavy loads if everything is cool.

But if you have inefficient load balancing AND/OR are continously using the device at the full capacity then that will cause parts of the flash drive to wear out much quicker and then you'll have problems.

So idealy Linux should access these things as MTD and use intellegent load-balancing file systems and work with the hardware to ensure the highest performance and long-life possible. Linux has a couple FS's that can do this sorta well, but you run into two major problems. A) because of the Windows consumer market nobody makes commodity MTD except for embedded markets, B) currently flash file systems for MTD are limited in scalability. These FSs are things like JFFS/JFFS2 and such.

People do use Flash file systems on consumer 'flash' devices though.. but that's through MTD emulation on top of a block device. So you have MTD software emulation on Block hardware emulation on MTD devices, which is hardly ideal.

So for the most part your utterly dependant on the hardware built-in to your device for any loadbalancing features. This is critical that it's high quality.

Plus I beleive the second part if your using 50% the capacity of a device it'll last a lot longer then if your using 90%, given the same load. A lot more to balance.

And then it also depends on the application... if your using a harddrive in a mobile device you can understand that flash-based drives have considerable advantages... ie if you drop them they won't break.

Plus they use a lot less electricity to maintain and run and if you shut them off then they are non-volitile memory.

So they are ideal for situations were you can heavily leverage file system cache, swap file, pre-loading applications, and in rugged environments. Traditional harddrive are good for large amounts of storage and streaming media. The less you use the harddrive the more reliable your mobile device will be, the faster it'll be, and the less battery it'll use, but less suitable for large amounts of storage and media applications.

So by combining the two your hopefully going to see significant advantages then compared to using just one or the other.

Now it would be nice to have 16 gigs of RAM cache with battery backup. That would be like Flash++, but batteries wear out over time and the memory is going to be fairly expensive. 16gigs of Flash is pretty cheap in comparision, in terms of the actual device and the sort of hardware you need to support it.