rwessel (robertwessel@yahoo.com) on 1/6/09 wrote:
---------------------------
>Joern Engel (joern@logfs.org) on 1/6/09 wrote:
>---------------------------
>>And even in the good case, you usually put a filesystem on top of the flash,
>>which does logical->physical mapping, then pass the data to the controller which
>>does logical->physical mapping again. And unlike hard disks the mapping is not
>>n:n in the normal case with a fairly short list of exceptions, but essentially
>>random.
>>
>>So my gut feeling is that we can do a better job overall if the SSDs would get
>>dumber and the storage layer would get smarter. It requires work, no doubt.
>>But we can do better than the SSD can, because we can avoid double work and have
>>more information than some SATA device with a 32-entry window of outstanding
>>requests.
>
>
>An issue is that you don't want to tie yourself to *today’s* limitations of devices.
>Frankly, with a reasonably large number of writes (before wearout), and a reasonable
>number of spare sectors, it's hard to imagine what you really want the higher levels
>to have to deal with. And the sector vs. erase block size thing is likely going
>to be dealt with in the next few years (sectors are going 4KB anyway, and erase
>block sizes just have to get a bit smaller).
>

In other words exactly like current IDE error block remapping - I really dont know what all the fuss is about.

I think Linus is a bit off the pace in embedded systems - he forgets that to have a comtroller do that adds cost there when the 'main' cpu can do it easily enough - but in more normal systems, IDE sector remapping and flash write leveling are not dissimilar in how they should/should not be exposed.

Now, of course this is why there are 'RAID' type SATA drives, because they return faster and spend less time trying to remap (as that can KILL performance) and I see the same opportunity for flash - the performance cost of write leveling could be ugly in a few rare cases, so perhaps there should be an api to turn it off and then have to do it yourself, perhaps..