Subject: Re: JFS
To: <>
From: David Laight <>
List: tech-kern
Date: 02/20/2003 17:15:40
On Thu, Feb 20, 2003 at 11:54:36AM -0500, Thor Lancelot Simon wrote:
> On Thu, Feb 20, 2003 at 07:48:31AM -0700, wrote:
> > Also NetBSD is weak on its flash support.  Embedded systems need file
> > systems that can tolerate being power off without warning and to have a file
> > system supporting compression.
> I build routers that run from flash for a living, and I strongly disagree
> with your conclusion, for several reasons:
> 1) If you want resilience when the machine's suddenly powered off, use
>    fsync() appropriately to get the commit semantics you require, or
>    simply mount the filesystem read-only.  It's trivial to do (in most
>    cases, one will want to do both of these: read-only for the system
>    image and a tiny read-write partition for the data).  FFS "tolerates 
>    being power off without warning" just fine and you can mount it "sync"
>    if you aren't sure you can manage to use fsync() correctly.

You definitely want to minimise writes to flash, so noatime and nodevmtime
are a must.  Also all flash memory has quite large physical sectors,
(they were 128kb on a board I used) so writes have to be large RMW
cycles - and the erase takes ~1 second (some modern parts may be faster).

Compact Flash cards aren't the answer either.  I've seen the effects
(I think) of the power being removed while the card was reorganising
the sectors.  Certainly I don't think there is anyway the system code
could have got the flash contents into the absolute mess they were in.


David Laight: