Re: [9fans] Throwing in the Towel

[Dave Eckhardt <davide+p9@cs.cmu.edu>]

> For the napkin calculation: On disk, the IEntry is 38Bytes.  Alas,
> writes occur always in (the ssd internal) blocksize.  So, essentially
> (assuming 4096 byte blocksize, which is quite optimistic), we have
> a write efficiency of less than 1 percent.

While I see how such a model can predict disaster, I don't think that
model matches how FTLs work, because it can't.

Many file systems (FAT, ext2/3/4) write the same logical block over
and over and over and over and over.  I think the default interval
for ext4 to synch the superblock and the journal is five seconds,
which if true is more than 15,000 times every *day* for a busy
file system (and I think lots of Linux systems are busy in that
sense).

> A good firmware in the ssd could avoid needing a new block for the
> write, if all bits are changed in teh same direction by the new
> data.

Again, I believe this model predicts that no regular Linux file
system can be used on any SSD, thus I believe this model is not
accurate.

To quote Wikipedia:

  https://en.wikipedia.org/wiki/Flash_memory_controller

> The mapping units of an FTL can differ so that LBAs are mapped
> block-, page- or even sub-page-based.  Depending on the usage
> pattern, a finer mapping granularity can significantly reduce
> the flash wear out and maximize the endurance of a flash based
> storage media.

Also, I feel as if this point is several assumption layers deep.
I think one user reported an unknown number of failures in two
sets of SSDs of unknown brand and model.  I don't think we know
that it was venti SSDs that went bad as opposed to fossil SSDs,
let alone knowing it was index SSDs for venti.

> It seems, venti in its current form is a ssd killer, if they
> are used for the isects.

I don't think this claim is yet supported well.

Dave Eckhardt

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