Thread: [Under consideration] Remove 256MB limitation of the rootfs partition in the N900
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Matan is offline Matan
2010-07-08 , 18:14
Posts: 1,224 | Thanked: 1,763 times | Joined on Jul 2007
#95
Originally Posted by shadowjk View Post
What unit is your battery current figure in? How did you measure it?
We don't know the units, as they depend on the resistance of a specific resistor on the board. But the values are linear, so they are useful for such comparisons.

The big difference for nand vs emmc isn't in sequential read/write.. Whereas the emmc can do something like 4-8Megabyte/s write per second sequentially, the built-in flash translation layer can not at all deal with random write (such as swap, /opt and MyDocs), and the speed for 4K random writes degrades to a few hundred kilobytes.

The OneNand has no FTL, so we can manage it ourselves, and we have a 600MHz cpu and 256 megs of ram, plenty of room to do something intelligent. As a result, the onenand with ubifs is a few magnitudes faster at serving IOs from a multitasking demand-paging operating system.
But the swap is actually on the eMMC, so I don't really get your point. In normal activity there is very little write access to the root filesystem.

Do you have some numbers for "a few magnitudes faster ..."? It sounds too incredible to be true.

Originally Posted by egoshin View Post
So, the interpretation - the current usage of MTD5 as root file space is useless from many point of view - the Read performance is the same as internal 32GB flash BUT (!) - CPU usage is overtop!

Actually, I strongly suspect that MTD5 access performance is limited by CPU performance (due to compression in UBIFS?) - wall time == system time.
Using root file system on eMMC is slower, even though the hardware is better, as those benchmark show, due to concurrency with swap access also on eMMC.

The best options I can think of are:
  • Use a microsd card with root filesystem on eMMC and swap on uSD (or vice versa)
  • Use root on eMMC and swap on NAND.

Both options should give better performance than the current setup and give a larger (configurable) root file system size.

The downside to first method is probably increased power usage due to uSD card.

The downside of the second is limiting of swap to 250MB rather than the current 768MB.
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