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    SubjectRe: Filesystem for block devices using flash storage?
    Jörn Engel wrote:
    > On Wed, 8 October 2008 16:51:46 -0400, Chris Snook wrote:
    >> Stefan Monnier wrote:
    >> Writes to magnetic disks are functionally atomic at the sector level. With
    >> SSDs, writing requires an erase followed by rewriting the sectors that
    >> aren't changing. This means that an ill-timed power loss can corrupt an
    >> entire erase block, which could be up to 256k on some MLC flash. Unless
    > What makes you think that? The standard mode of operation in El Cheapo
    > devices is to write to a new eraseblock first, then delete the old one.
    > An ill-timed power loss results in either the old or the new block being
    > valid as a whole. This has been the standard ever since you could buy
    > 4MB compactflash cards.
    >> logfs tries to solve the write amplification problem by forcing all write
    >> activity to be sequential. I'm not sure how mature it is.
    > Still under development. What exactly do you mean by the write
    > amplification problem?

    Write amplification is where a 512 byte write turns into a 128k write,
    due to erase block size.

    >>> Or is there some hope for SSDs to provide access to the MTD layer in the
    >>> not too distant future?
    >> I hope not. The proper fix is to have the devices report their physical
    >> topology via SCSI/ATA commands. This allows dumb software to function
    >> correctly, albeit inefficiently, and allows smart software to optimize
    >> itself. This technique also helps with RAID arrays, large-sector disks, etc.
    > Having access to the actual flash would provide a large number of
    > benefits. It just isn't a safe default choice at the moment.
    >> I suspect that in the long run, the problem will go away. Erase blocks are
    >> a relic of the days when flash was used primarily for low-power,
    >> read-mostly applications. As the SSD market heats up, the flash vendors
    >> will move to smaller erase blocks, possibly as small as the sector size.
    > Do you have any information to back this claim? AFAICT smaller erase
    > blocks would require more chip area per bit, making devices more
    > expensive. If anything, I can see a trend towards bigger erase blocks.

    Intel is claiming a write amplification factor of 1.1. Either they're
    using very small erase blocks, or doing something very smart in the

    -- Chris
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