Re: [PATCH v2] btrfs: drop the nossd flag when remounting with -o ssd

[Hans van Kranenburg <hans.van.kranenburg@mendix.com>]

On 03/31/2017 10:08 PM, Adam Borowski wrote:
> And when turning on nossd, drop ssd_spread.
> 
> Reported-by: Hans van Kranenburg <hans.van.kranenburg@mendix.com>
> Signed-off-by: Adam Borowski <kilobyte@angband.pl>
> ---
> On Fri, Mar 31, 2017 at 07:10:16PM +0200, David Sterba wrote:
>> On Fri, Mar 31, 2017 at 06:00:08PM +0200, Hans van Kranenburg wrote:
>>> On 03/31/2017 05:19 PM, Adam Borowski wrote:
>>>> Not sure if setting NOSSD should also disable SSD_SPREAD, there's currently
>>>> no way to disable that option once set.
>>
>> Missing inverse of ssd_spread is probably unintentional, as we once
>> added all complementary no* options, this one was forgotten.
>>
>> And yes, nossd should turn off ssd and ssd_spread, as ssd_spread without
>> ssd does not nothing anyway.
> 
> Added that.
> 
>>> How did you test this?
>>>
>>> This was also my first thought, but here's a weird thing:
>>>
>>> -# mount -o nossd /dev/sdx /mnt/btrfs/
>>>
>>> BTRFS info (device sdx): not using ssd allocation scheme
>>>
>>> -# mount -o remount,ssd /mnt/btrfs/
>>>
>>> BTRFS info (device sdx): use ssd allocation scheme
>>>
>>> -# mount -o remount,nossd /mnt/btrfs/
>>>
>>> BTRFS info (device sdx): use ssd allocation scheme
>>>
>>> That means that the case Opt_nossd: is never reached when doing this?
> 
> Seems to work for me:
> 
> [/tmp]# mount -onoatime foo /mnt/vol1 
> [  619.436745] BTRFS: device fsid 954fd6c3-b3ce-4355-b79a-60ece7a6a4e0 devid 1 transid 5 /dev/loop0
> [  619.438625] BTRFS info (device loop0): disk space caching is enabled
> [  619.438627] BTRFS info (device loop0): has skinny extents
> [  619.438629] BTRFS info (device loop0): flagging fs with big metadata feature
> [  619.441989] BTRFS info (device loop0): creating UUID tree
> [/tmp]# mount -oremount,ssd /mnt/vol1
> [  629.755584] BTRFS info (device loop0): use ssd allocation scheme
> [  629.755589] BTRFS info (device loop0): disk space caching is enabled
> [/tmp]# mount -oremount,nossd /mnt/vol1
> [  633.675867] BTRFS info (device loop0): not using ssd allocation scheme
> [  633.675872] BTRFS info (device loop0): disk space caching is enabled

Yes, but we're not doing the same thing here.

You have a file via a loop mount. If I do that, I get the same output as
you show, the right messages when I remount ssd and nossd.

My test was lvm based on an ssd. When I mount that, I get the "detected
SSD devices, enabling SSD mode", and everytime I remount, being it ssd
or nossd, it *always* says "use ssd allocation scheme".

So, this needs some more research I guess. It doesn't feel right.

>>> The fact that nossd,ssd,ssd_spread are different options complicates the
>>> whole thing, compared to e.g. autodefrag, noautodefrag.
>>
>> I think the the ssd flags reflect the autodetection of ssd, unlike
>> autodefrag and others.
> 
> The autodetection works for /dev/sd* and /dev/mmcblk*, but not for most
> other devices.
> 
> Two examples:
> nbd to a piece of rotating rust says:
> [45697.575192] BTRFS info (device nbd0): detected SSD devices, enabling SSD mode
> loop on tmpfs (and in case it spills, all swap is on ssd):
> claims it's rotational
> 
>> The ssd options says "enable the ssd mode", but it could be also
>> auto-detected if the non-rotational device is detected.
>>
>> nossd says, "do not do the autodetection, even if it's a non-rot
>> device, also disable all ssd modes".
> 
> These two options are nice whenever the autodetection goes wrong.
> 
>> So Adam's patch needs to be updated so NOSSD also disables SSD_SPREAD.

Ack.

> M'kay, updated this patch.
> 
>> Adding the 'nossd_spread' would be good to have, even if it might be
>> just a marginal usecase.

Please no, don't make it more complex if not needed.

> Not sure if there's much point.  In any case, that's a separate patch.
> Should I add one while we're here?

Since the whole ssd thing is a bit of a joke actually, I'd rather see it
replaces with an option to choose an extent allocator algorithm.

The amount of if statements using this SSD things in btrfs in the kernel
can be counted on one hand, and what they actually do is quite
questionable (food for another mail thread).

> 
> Meow!
> 
>  fs/btrfs/super.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 06bd9b332e18..ac1ca22d0c34 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -549,16 +549,19 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
>  		case Opt_ssd:
>  			btrfs_set_and_info(info, SSD,
>  					   "use ssd allocation scheme");
> +			btrfs_clear_opt(info->mount_opt, NOSSD);
>  			break;
>  		case Opt_ssd_spread:
>  			btrfs_set_and_info(info, SSD_SPREAD,
>  					   "use spread ssd allocation scheme");
>  			btrfs_set_opt(info->mount_opt, SSD);
> +			btrfs_clear_opt(info->mount_opt, NOSSD);
>  			break;
>  		case Opt_nossd:
>  			btrfs_set_and_info(info, NOSSD,
>  					     "not using ssd allocation scheme");
>  			btrfs_clear_opt(info->mount_opt, SSD);
> +			btrfs_clear_opt(info->mount_opt, SSD_SPREAD);
>  			break;
>  		case Opt_barrier:
>  			btrfs_clear_and_info(info, NOBARRIER,
> 


-- 
Hans van Kranenburg