Re: [RFC] LFTL: a FTL for large parallel IO flash cards

[Artem Bityutskiy <dedekind1@gmail.com>]

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On Sat, 2012-11-17 at 01:04 +0530, srimugunthan dhandapani wrote:
> Hi all,
> 
>  Due to fundamental limits like size-per-chip and interface speed
>  limits all  large capacity Flash  are made of multiple chips or banks.
>  The presence of multiple chips readily offers parallel read or write support.
>  Unlike an SSD, for a raw flash card , this parallelism is visible to
>  the software layer and there are many opportunities
>  for exploiting this parallelism.
> 
>  The presented LFTL is meant for flash cards with multiple banks and
>  larger minimum write sizes.
>  LFTL mostly reuses code from mtd_blkdevs.c and mtdblock.c.
>  The LFTL was tested on  a 512GB raw flash card which has no firmware
>  for wearlevelling or garbage collection.
> 
>  The following are the important points regarding the LFTL:
> 
>  1. multiqueued/multithreaded design:(Thanks  to Joern engel for a
>  mail-discussion)
>  The mtd_blkdevs.c dequeues block I/O requests from the block layer
>  provided request queue from a single kthread.
>  This design of IO requests dequeued from a single queue by a single
>  thread is a bottleneck for flash cards that supports hundreds of MB/sec.
>  We use a multiqueued and multithreaded design.
>  We bypass the block layer by registering a new make_request and
>  the LFTL maintains several queues of its own and the block IO requests are
>  put in one of these queues. For every queue there is an associated kthread
>  that processes requests from that queue. The number of "FTL IO kthreads"
>  is #defined as 64 currently.

Hmm, should this be done in MTD layer, not hacked in in LFTL, so that
every MTD user could benefit?

Long time ago Intel guys implemented "striping" in MTD, sent out, but it
did not make it to upstream. This is probably something your need.

With striping support in MTD, you will end up with a 'virtual' MTD
device with larger eraseblock and minimum I/O unit. MTD would split all
the I/O requests and work with all the chips in parallel.

This would be a big work, but everyone would benefit.

-- 
Best Regards,
Artem Bityutskiy

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