Re: [PATCH] Btrfs: add a disk info ioctl to get the disks attached to a filesystem

[Lennart Poettering <lennart@poettering.net>]

On Wed, 29.09.10 16:25, Ric Wheeler (rwheeler@redhat.com) wrote:

> >This in fact is how all current readahead implementations work, be it
> >the fedora, the suse or ubuntu's readahead or Arjan's sreadahead. What's
> >new is that in the systemd case we try to test for ssd/rotating
> >properly, instead of just hardcoding a check for
> >/sys/class/block/sda/queue/rotational.
> >
> 
> A couple of questions pop into mind - is systemd the right place to
> automatically tune readahead?  If this is a generic feature for the
> type of device, it sounds like something that we should be doing
> somewhere else in the stack (not relying on tuning from user space).

Note that this is not the kind of readahead that is controllable via 
/sys/class/block/sda/queue/read_ahead_kb, this is about detecting "hot"
files at boot, and then preloading them on the next boot. i.e. the
problem Jens once proposed fcache for.

> Second question is why is checking in /sys a big deal, would  you
> prefer an interface like we did for alignment in libblkid?

Well, currently there's no way to discover the underlying block devices
if you have a btrfs mount point. This is what Josef's patch added for
us.

Lennart

-- 
Lennart Poettering - Red Hat, Inc.