Write-only Caching Without Backing

[Gordan Bobic <gordan-UpbECiGlrmGsTnJN9+BGXg@public.gmane.org>]

Hi,

I'm looking at whether Bcache can be used (or easily modified to be 
used) for rectifying the poor random write performance of cheap flash media.

I am aware of the following projects that provide similar functionality 
to what I require:

Managed Flash (MFT) (not OSS, purely commercial, requires a substantial 
amount of RAM for mapping)
ReSSD (research paper, requires less RAM than MFT, no code available)

Unfortunately, I am specifically interested in this for ARM machines 
that frequently don't have SATA ports but only SD card ports. Most SD 
cards have exceptionally poor random-write performance, and since the 
only semi-solution (MFT requires a substantial amount of RAM which is 
also an issue on a 256MB ARM machine) doesn't support ARM, I'm looking 
for alternatives.

So what I envisage (hope for?) is using Bcache in a way that does 
random-write caching by linearizing writes into sequential writes. My 
performance tests indicate that even with small blocks (4KB) and even on 
some of the best SD cards, sequential writes are still at least 3-4x 
faster than random writes. On the worst SD cards, the difference can be 
as much as 100x.

Note that this would involve having a "cache" partition on the same SD card.

However, the random read performance on these is very good (as good as 
any flash), and typical measurements are 500-1500 IOPS on random reads. 
Therefore, there is no point in caching data for reading, since the read 
performance from the cache partition would be the same as the read 
performance off the real data source.

The other use case I envisage is for things like swap that don't need to 
be persisted across reboots, and thus don't need to be committed back to 
backing storage from cache (i.e. it is OK for them to disappear on 
reboot). In this use-case, I would like to use Bcache without the 
backing storage - simply use it in write-back caching mode without 
committing anything back, and re-using blocks as and when they get 
over-written (or perhaps use a discard feature for early pruning?).

So to summarize:
1) Can Bcache be configured or relatively easily modified to only do 
random-write caching with no read caching (except in cases where there 
is a request for written data that hasn't yet been committed to the 
backing storage)?
2) Can Bcache be used in full write-back-only mode without backing data 
store for non-persistent data?

Thanks.

Gordan