On Mon, Mar 16, 2009 at 5:43 PM, Rob Landley <rob@xxxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
