ZFS vs UFS2 overhead and may be a bug?

[Bakul Shah]

> >> Interesting. There are two problems. First is that cat(1) uses
> >> st_blksize to find out best size of I/O request and we force it to
> >> PAGE_SIZE, which is very, very wrong for ZFS - it should be equal to
> >> recordsize. I need to find discussion about this:
> 
> >> ...
> >> 	sb->st_blksize = PAGE_SIZE;
> >
> > This does seem suboptimal.  Almost always one reads an entire
> 
> It's just broken.

What should it be?

> > file and the overhead of going to the disk is high enough
> > that one may as well read small files in one syscall.  Apps
> > that want to keep lots and lots of files open can always
> > adjust the buffer size.
> >
> > Since disk seek access time is the largest cost component,
> > ideally contiguously allocated data should be read in one
> > access in order to avoid any extra seeks.  At the very least
> 
> Buffering makes the userland i/o size have almost no effect on
> physical disk accesses.  Perhaps even for zfs, since IIRC your
> benchmark showed anamolies for the case of sparse files where
> no disk accesses are involved.

This is perhaps a separate problem from that of sparse file
access.  In my tests on regular files (not sparse) ZFS took
8% more time to read a 10G file when 4K buffer was used and
90% more time for 1K buffer.  May be it is simply the ZFS
overhead but as size of read() buffer has an non-negligible
effect, something needs to be done.