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To: linux-kernel@vger.kernel.org
From: gwendal grignou <gwendal@chromium.org>
Subject: Re: [PATCH] loop: properly observe rotational flag of underlying device
Date: Thu, 12 May 2016 20:28:34 +0000 (UTC)
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References: <56435D0F.80006@googlemail.com> <5643B341.9010600@fb.com> <5643BC5E.8060701@googlemail.com> <569438BB.6050009@googlemail.com>
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Holger Hoffstätte <holger.hoffstaette <at> googlemail.com> writes:

> 
> On 11/11/15 23:08, Holger Hoffstätte wrote:
> > On 11/11/15 22:29, Jens Axboe wrote:
> >> On 11/11/2015 08:21 AM, Holger Hoffstätte wrote:
> >>>
> >>> The loop driver always declares the rotational flag of its device as
> >>> rotational, even when the device of the mapped file is nonrotational,
> >>> as is the case with SSDs or on tmpfs. This can confuse filesystem 
tools
> >>> which are SSD-aware; in my case I frequently forget to tell 
mkfs.btrfs
> >>> that my loop device on tmpfs is nonrotational, and that I really 
don't
> >>> need any automatic metadata redundancy.
> >>>
> >>> The attached patch fixes this by introspecting the rotational flag of 
the
> >>> mapped file's underlying block device, if it exists. If the mapped 
file's
> >>> filesystem has no associated block device - as is the case on e.g. 
tmpfs -
> >>> we assume nonrotational storage. If there is a better way to identify 
such
> >>> non-devices I'd love to hear them.
> >>>
> >>> Signed-off-by: Holger Hoffstätte <holger.hoffstaette <at> 
googlemail.com>

> 
> Jens,
> 
> I haven't seen this merged in any trees yet and was wondering if there's
> any chance to get this into 4.5? If there's something left to fix up 
please
> let me know.
> 
> Thanks,
> Holger
> 
> 
This patch proved useful for ureadahead: when we use it on a loop device, 
it would use the HDD method to place the data in cache using the pack 
information instead of the SSD method.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>