Re: Great jffs2 speedup

[Sergei Sharonov <sergei.sharonov@halliburton.com>]

Peter,

thanks for taking time to do the tests.

> Hope that covers what you wanted to see. For the record, I am using the
> head of mtd cvs with the centralized summary patch applied. EBS and CS
> are both enabled.

Aha!!! Of course, CS reduced your time to first write down to seconds.
Could you repeat the test after the un-clean unmount (e.g. power off without
sync)? I bet the numbers will be very different. Probably you also need to 
be writing to flash during the power off to force full scan on startup.

> > Good point.. Can you estimate speedup due to the hw controller?
> 
> We can get about 15 MB/s for raw reads and about 6 MB/s for raw writes.

Hmm.. I was getting a read throughput jffs2 -> samba network fs of about 
1.1 MB/s (180 MHz ARM9, linux 2.6.10, mtd 3/28/2005). I believe jffs2/mtd 
was the bottleneck since the throughput was 7.25x higher when files were 
comming from ext2/RAM. Looks like your hw controller is doing a good job.
Not sure how overhead is split between mtd and jffs2.

> By raw, I mean reading and writing via an mtd character device to/from a
> raw partition. I do not know how these numbers compare to other flash
> access methods; but bit-banging a flash part directly would have to be
> considerably slower and probably very dependent on the CPU speed. My
> device has a 384 MHz PowerPC 405 CPU, by the way.

That helps too ;-)

> > Most embedded systems cannot rely on proper shutdown. So, IMHO CS is of
> > limited use in this respect.
> 
> My device is usb powered, but has a small battery so that when the
> device looses bus power, it can shutdown cleanly. 

Tricky, tricky, tricky ;-)
Not everybody is that lucky to have a battery. Reminds me of a conversation
with one "high reliability RTOS" vendor, you guess who.
- Is your file system power fail safe?
- Yes, of course. We do require that you use UPS though.

> However, the thing to
> remember about centralized summary is that in the unclean shutdown
> cases, the mount time is no worse than jffs2 without CS. So even if you
> only expect the filesystem to be cleanly unmounted sometimes, CS can be
> worth it.

As long as specification allows for an "occasional" 30 minutes startup time.

Your hw controller does sound very interesting.
What is the hardware implementation : asic, fpga? Is it available as a 
separate product (chip)?

Sergei