Re: [RFC PATCH] forget block layer request for FTLs

[Sean Young <sean@mess.org>]

On Fri, Nov 25, 2005 at 01:36:41PM +1300, Charles Manning wrote:
> Being a bit of a flash-head, I find this RPC quite interesting. I am 
> struggling though to see exactly where this is going to be used. 

I don't think I explained it very well. Allow my to try again.

A normal block device like a harddisk can overwrite (write-in-place)
sectors; however flash has larger erase units than the sector size (512)
which makes this impossible. This problem is handled by the flash
translation layer (FTL). It has a layer of indirection which stores
sectors a different location on the flash, along with a table of
sector number and an address on the flash memory chip. When a sector is
overwritten it saves it a different location and updates the table. When
no free space exists any more on the flash, it needs to make an entire
erase unit available for erase. To achieve this, it moves around sectors
while updating the table for consistency.

The problem I'm trying to solve is that the FTL is unaware of sectors
which are no longer relevant, sectors of deleted files for example. Those
sectors do not need to be moved, they can simply be "forgotten", causing
the flash memory to fill up less quickly before another erase is
necessary.

Also, by "forgetting" sectors an erase unit might become available for
erase without requiring moving of sectors, allowing the erase to happen
before the flash memory is full. So, it makes FTL faster and the flash
last longer.

I hope this clears things up.


Sean