Re: ext2/3: document conditions when reliable operation is possible

[Kyle Moffett <kyle@moffetthome.net>]

On Mon, Mar 16, 2009 at 5:43 PM, Rob Landley <rob@landley.net> wrote:
> Flash gets into trouble when it presents the _interface_ of rotational media
> (a USB block device with normal 512 byte read/write sectors, which never wear
> out) which doesn't match what the hardware's actually doing (erase block sizes
> of up to several megabytes at a time, hidden behind a block remapping layer
> for wear leveling).
>
> For devices that have built in flash that DON'T pretend to be a conventional
> block device, but instead expose their flash erase granularity and let the OS
> do the wear levelling itself, we have special flash filesystems that can be
> reasonably reliable.  It's just that ext3 isn't one of them, jffs2 and ubifs
> and logfs are.  The problem with these flash filesystems is they ONLY work on
> flash, if you want to mount them on something other than flash you need
> something like a loopback interface to make a normal block device pretend to
> be flash.  (We've got a ramdisk driver called "mtdram" that does this, but
> nobody's bothered to write a generic wrapper for a normal block device you can
> wrap over the loopback driver.)

The really nice SSDs actually reserve ~15-30% of their internal
block-level storage and actually run their own log-structured virtual
disk in hardware.  From what I understand the Intel SSDs are that way.
 Real-time garbage collection is tricky, but if you require (for
example) a max of ~80% utilization then you can provide good latency
and bandwidth guarantees.  There's usually something like a
log-structured virtual-to-physical sector map as well.  If designed
properly with automatic hardware checksumming, such a system can
actually provide atomic writes and barriers with virtually no impact
on performance.

With firmware-level hardware knowledge and the ability to perform
extremely efficient parallel reads of flash blocks, such a
log-structured virtual block device can be many times more efficient
than a general purpose OS running a log-structured filesystem.  The
result is that for an ordinary ext3-esque filesystem with 4k blocks
you can treat the SSD as though it is an atomic-write seek-less block
device.

Now if only I had the spare cash to go out and buy one of the shiny
Intel ones for my laptop... :-)

Cheers,
Kyle Moffett