Re: [PATCH] loop: properly observe rotational flag of underlying device

[cc: linux-block]

On 05/12/16 22:28, gwendal grignou wrote:

Holger Hoffstätte <holger.hoffstaette <at> googlemail.com> writes:

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On 11/11/15 23:08, Holger Hoffstätte wrote:

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On 11/11/15 22:29, Jens Axboe wrote:

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On 11/11/2015 08:21 AM, Holger Hoffstätte wrote:

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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

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which are SSD-aware; in my case I frequently forget to tell 

mkfs.btrfs

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that my loop device on tmpfs is nonrotational, and that I really 

don't

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need any automatic metadata redundancy.

The attached patch fixes this by introspecting the rotational flag of 

the

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mapped file's underlying block device, if it exists. If the mapped 

file's

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filesystem has no associated block device - as is the case on e.g. 

tmpfs -

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we assume nonrotational storage. If there is a better way to identify 

such

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non-devices I'd love to hear them.

Signed-off-by: Holger Hoffstätte <holger.hoffstaette <at> 

googlemail.com>

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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

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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 <redacted>

I had completely forgotten about this, and apparently so had Jens. ;)
Thanks for the feedback, glad to hear it is useful.

Jens, any objections to merge this for 4.7? It should still apply
cleanly. The original patch was at: https://lkml.org/lkml/2015/11/11/288

Thanks,
Holger