[Btrfs-devel] btrfs and Solid State Disks (SSD).

[Dongjun Shin]

On Feb 12, 2008 7:40 PM, Miguel Figueiredo Mascarenhas Sousa Filipe
<miguel.filipe at gmail.com> wrote:
> Hi to All,
>
> On Feb 11, 2008 8:34 PM, Claudio Martins <ctpm at ist.utl.pt> wrote:
> > On Monday 11 February 2008, Miguel Figueiredo Mascarenhas Sousa Filipe wrote:
> > >
> > > >From what I read, current SSD are characterized by:
> > >
> > > - poor performance in random writes (because of block erasure)
> > > - require wear leveling, even those that emulate sata/ide/scsi disk
> > > with onboard wear leveling logic.
> >
> >  I thought that SSDs that emulate scsi disks (like USB pen drives, etc) and
> > also CompactFlash and SD cards, had no need of additional wear leveling since
> > they have, as you say, onboard wear leveling logic because they were
> > basically designed to work well with FAT16/32.
> > Though I'd also really like to hear if anyone on the list has more information
> > about this problem.
>
> I've read this from the author of LogFS (Joern Engel).
> In this post http://linux.derkeiler.com/Mailing-Lists/Kernel/2007-06/msg05668.html
> he mentions that wear leveling is done by splitting the device in
> zones (about 1024 blocks each zone)
> and that all wear leveling is done inside of that zone. This happens
> of course on all zones.
> That works fine for FAT, since the ondisk layout of it is quite
> widespread, not that well on ext family of filesystems.
>
> That post is also a good read for learning bit more about the
> internals of nowadays "pendrives".
>

I think it's not a good idea to generalize the smartmedia as flash-based storage
because smartmedia does not have embedded controller to run the,
what is called, FTL (Flash Translation Layer) that handles the wear leveling.

I can't guarantee if all FTL perfom wear leveling so well. But, in general,
FTL cares about wear leveling regardless of the filesystem that runs
on top of it.

If filesystem does the wear leveling by itself, it can deal with the
corner cases
like smartmedia or flash storage with bad FTL. I'm not sure if these cases
should be taken into consideration with higher priority.

--
Dongjun