From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ric Wheeler <rwheeler@redhat.com>
Subject: Re: [PATCH] Btrfs: add a disk info ioctl to get the disks attached
  to a filesystem
Date: Wed, 29 Sep 2010 21:08:09 +0900
Message-ID: <4CA32C29.6070404@redhat.com>
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> <20100929115903.GA13882@tango.0pointer.de>
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Cc: Josef Bacik <josef@redhat.com>,
	Christoph Hellwig <hch@infradead.org>,
	linux-btrfs@vger.kernel.org, Kay Sievers <kay.sievers@vrfy.org>
To: Lennart Poettering <lennart@poettering.net>
Return-path: <linux-btrfs-owner@vger.kernel.org>
In-Reply-To: <20100929115903.GA13882@tango.0pointer.de>
List-ID: <linux-btrfs.vger.kernel.org>

  On 09/29/2010 08:59 PM, Lennart Poettering wrote:
> 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
>

Makes sense to me,thanks!

Ric