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

[Charles Manning <manningc2@actrix.gen.nz>]

On Friday 25 November 2005 02:21, Sean Young wrote:
> Any flash device which appears as a normal block device to user space
> has a flash translation layer (FTL) which from time to time garbage
> collects, i.e. moves data around to make room for an erase. Data might
> be moved from one erase unit to another, so a full erase unit can be
> erased without losing data.
>
> The FTL is unaware of any "unused" sectors; e.g. if a file is
> unlinked the sectors where the data of that (former) file resides will
> still be considered for garbage collection, even though those sectors
> are no longer relevant.
>
> The purpose of this patch is to make it possible for file system drivers
> to inform the block device of "staleness" of no-longer-used sectors.  This
> patch introduces another barrier request, REQ_FORGET. The semantics
> of this requests are that no read() is expected before the next write(),
> i.e. read() may return garbage in the time between forget() and the
> next write().

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. 

>From what I understand, you're going to have file knowledge passed into the 
block driver and need a special fs to exploit this. This is starting to break 
down the whole point of a block driver isn't it?

Perhaps if you hacked a fs to use this that would show the benefits.

-- CHarles