Re: [LSF/MM/BPF TOPIC] Cloud storage optimizations

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Wed, 2023-03-08 at 17:53 +0000, Matthew Wilcox wrote:
> On Mon, Mar 06, 2023 at 11:12:14AM -0500, Theodore Ts'o wrote:
> > What HDD vendors want is to be able to have 32k or even 64k
> > *physical* sector sizes.  This allows for much more efficient
> > erasure codes, so it will increase their byte capacity now that
> > it's no longer easier to get capacity boosts by squeezing the
> > tracks closer and closer, and their have been various engineering
> > tradeoffs with SMR, HAMR, and MAMR.  HDD vendors have been asking
> > for this at LSF/MM, and in othervenues for ***years***.
> 
> I've been reminded by a friend who works on the drive side that a
> motivation for the SSD vendors is (essentially) the size of sector_t.
> Once the drive needs to support more than 2/4 billion sectors, they
> need to move to a 64-bit sector size, so the amount of memory
> consumed by the FTL doubles, the CPU data cache becomes half as
> effective, etc. That significantly increases the BOM for the drive,
> and so they have to charge more.  With a 512-byte LBA, that's 2TB;
> with a 4096-byte LBA, it's at 16TB and with a 64k LBA, they can keep
> using 32-bit LBA numbers all the way up to 256TB.

I thought the FTL operated on physical sectors and the logical to
physical was done as a RMW through the FTL?  In which case sector_t
shouldn't matter to the SSD vendors for FTL management because they can
keep the logical sector size while increasing the physical one. 
Obviously if physical size goes above the FS block size, the drives
will behave suboptimally with RMWs, which is why 4k physical is the max
currently.

James