Re: RAIDframe and alignment on 4K-sector disks?

["John D. Baker" <jdbaker%mylinuxisp.com@localhost>]

On Tue, 13 Jun 2017, Patrick Welche wrote:

> > Doesn't RAIDframe assume 512-byte sectors?  So, 64k stripe size would
> > be SecPerSU=128, yes?
> 
> I think it's SecPerSU=64 blocks = 32k. Two data disks in one write, so
> 2 * 32k = 64k = MAXPHYS written in one write.
> 
> (Please correct me if I'm wrong: I just make a 3 * 2T raid 5 this
> morning ;-) )

According to "raidctl(8)", in the "Performance tuning" section:

     Since the size of a `large IO' is often (currently) only 32K or 64K, on a
     5-drive RAID 5 set it may be desirable to select a SectPerSU value of 16
     blocks (8K) or 32 blocks (16K).  Since there are 4 data sectors per
     stripe, the maximum data per stripe is 64 blocks (32K) or 128 blocks
     (64K).  Again, empirical measurement will provide the best indicators of
     which values will yield better performance.

Which would seem to indicate "SectPerSU" refers to 512-byte sectors
across all components (less parity).  Mind you, this was written before
the advent of disks with 4K physical sectors, so the recommendations
may need to be adjusted.  Also adjust for 3-disk RAID-5 rather than
5-disk RAID-5 as described above.

The above recommendation would seem to suggest making frag=stripe so
that a full file system block (assuming -b 64k, or MAXPHYS) will occupy
multiple stripes.

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