Re: impact of 4k sector size on the IO & FS stack

[Andreas Dilger <adilger@clusterfs.com>]

On Mar 12, 2007  10:26 -0400, Jeff Garzik wrote:
> In my own experiments on my own Fedora workstation, ~66% of IOs in Linux 
> start on an odd sector, and ~33% started on even-numbered sectors.  For 
> a 1K-sector drive with 'odd' alignment, the configuration Microsoft will 
> likely want, that means the majority of disk transactions will avoid a 
> RMW cycle, but a still-numerous minority will not.

Isn't that purely an artifact of the DOS partition table alignment, possibly
skewed by the fact that most of your IO is on partition 1 & 3?  Hard to
believe this because of the nice even numbers though.

Since ext3 has at least 1kB blocksize and defaults to 4kB blocksize with
most modern disks because they are > 500MB in size, you should never
have misaligned writes generated by the filesystem itself.

> I did not test 
> transfer length, to see how many transfers /ended/ on an odd sector, 
> thus determining how many RMW cycles the tail of an average I/O requires.

I'd guess a vast majority of IO will have the end similarly misaligned as
the start.  Very little filesystem IO is 512 bytes, possibly excluding XFS
in an unusual mode.

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.