From mboxrd@z Thu Jan 1 00:00:00 1970 From: Lennart Poettering Subject: Re: [PATCH] Btrfs: add a disk info ioctl to get the disks attached to a filesystem Date: Wed, 29 Sep 2010 13:59:04 +0200 Message-ID: <20100929115903.GA13882@tango.0pointer.de> References: <1285707196-16268-1-git-send-email-josef@redhat.com> <20100928232513.GA20629@infradead.org> <20100929000809.GC32420@dhcp231-156.rdu.redhat.com> <20100929001954.GA9182@tango.0pointer.de> <4CA2E9CD.5090700@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Josef Bacik , Christoph Hellwig , linux-btrfs@vger.kernel.org, Kay Sievers To: Ric Wheeler Return-path: In-Reply-To: <4CA2E9CD.5090700@redhat.com> List-ID: 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.