David Shwatrz wrote:
My question is this:
I know that the ONLP project uses FLASH drives. This probably
gave some push for developing FLASH filesystems for Linux.
what is the state of Linux regarding SSD ?
From some talk to someone working at SanDisk, I gathered the following:
They don't like jffs. The guy claims that jffs is suited for explicit
flash devices (where the flash device appears as a character device),
but not for an SSD, where the machine-device interface is that of a
standard disk (block level access). I'm not sure I understood this part
of his statement correctly, and in any case, it may be due to the
technical reasons that follow, rather than any actual interface
differences between standard flash memory and SSDs.
The push for lowering of the manufacturing costs means that the actual
flash memory inside is able to sustain less and less erase-write cycles.
To compensate for that, the SSD controller uses dynamic mapping between
block number and actual storage region. This means, if my understanding
is correct, that a block deemed rarely updated (say, the master boot
record of the disk) may get copied over to a flash area that is near its
erase limit, so that the area it used to occupy (which has a low erase
count precisely due to the reason stated above) can be used for more
frequently updated data, extending the overall lifetime of the device.
What the SSD manufacturers would really like is, perhaps, not so much a
flash aware filesystem, but an implementation of bus extensions that
allow the OS to report to the SSD that a certain block is no longer in
use (which would allow the flash device to stop copying it around all
the time), or, according to a more advance proposal, even list typical
uses, which would, again, allow the SSD to better place the blocks (i.e.
- less copies, resulting in longer lifetime).
If I understand this correctly, adding such support requires changes
throughout all of the layers, from bus communication all the way up to
user space reporting. As far as I know, Linux has no such support at the
moment (with the possible exception of the bus level as third party
patches). Windows neither.
Shachar
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