Re: [PATCH 0/7][V11] btrfs: allocation_hint

[Josef Bacik <josef@toxicpanda.com>]

On Tue, Mar 01, 2022 at 07:55:07PM +0100, Goffredo Baroncelli wrote:
> On 01/03/2022 16.07, Josef Bacik wrote:
> > On Mon, Feb 28, 2022 at 10:01:35PM +0100, Goffredo Baroncelli wrote:
> > > Hi Josef,
> > > 
> > > On 28/02/2022 18.04, Josef Bacik wrote:
> > > > On Wed, Jan 26, 2022 at 09:32:07PM +0100, Goffredo Baroncelli wrote:
> > > > > From: Goffredo Baroncelli <kreijack@inwind.it>
> > > > > 
> > > > > Hi all,
> > > > > 
> > > [...
> > > 
> > > > > In V11 I added a new 'feature' file /sys/fs/btrfs/features/allocation_hint
> > > > > to help the detection of the allocation_hint feature.
> > > > > 
> > > > 
> > > > Sorry Goffredo I dropped the ball on this, lets try and get this shit nailed
> > > > down and done so we can get it merged.  The code overall looks good, I just have
> > > > two things I want changed
> > > > 
> > > > 1. The sysfs file should use a string, not a magic number.  Think how
> > > >      /sys/block/<dev>/queue/scheduler works.  You echo "metadata_only" >
> > > >      allocation_hint, you cat allocation_hint and it says "none" or
> > > >      "metadata_only".  If you want to be fancy you can do exactly like
> > > >      queue/scheduler and show the list of options with [] around the selected
> > > >      option.
> > > 
> > > Ok.
> > > > 
> > > > 2. I hate the major_minor thing, can you do similar to what we do for devices/
> > > >      and symlink the correct device sysfs directory under devid?
> > > > 
> > > Ok, do you have any suggestion for the name ? what about bdev ?
> > > 
> > 
> > You literally just add a link to the device kobj to the devid kobj.  If you look
> > at btrfs_sysfs_add_device(), you would do something like this (completely
> > untested and uncompiled)
> 
> I will give an eye to your code; thanks. However my question was more basic.
> 
> Now we have:
> 
> .../btrfs/<uuid>/devinfo/<dev-nr>/major_minor
> 
> with the link, as you suggested, I think that we will have:
> 
> .../btrfs/<uuid>/devinfo/<dev-nr>/bdev -> ../../../../devices/pci0000:00/0000:00:01.2/0000:01:00.1/ata6/host5/target5:0:0/5:0:0:0/block/sdg/sdg3
>                                   ^^^^
> 
> (notice 'bdev', which is the name that I asked)
> 
> looking at your patch, it seems to me that the link will be named like the device name:
> 
> .../btrfs/<uuid>/devinfo/<dev-nr>/sdg3 -> ../../../../devices/pci0000:00/0000:00:01.2/0000:01:00.1/ata6/host5/target5:0:0/5:0:0:0/block/sdg/sdg3
>                                   ^^^^
> 
> which is quite convoluted as approach, because the code has to find a name that matches a device (sdg3), instead to look for a fixed name (bdev).
> 
> Because I know that every people has a strong, valid (and above all different !) opinion about the name, I want to ask it before issue another patches set.
> For the record, I like 'bdev' only because I saw used (by bcache)
> 
> IMHO, the btrfs world had been simpler if devices/ sysfs directory was populated by the btrfs-dev-nr instead the device name
>

Ahh ok I see, you make a good point.  I agree it would have been better to have
the dev nrs in devices and then links in there, but here we are.

I think for now drop this patch from this series, since it's another bike
shedding opportunity and I'd rather get the core functionality in.  Do what I
asked in #1 and drop this patch from this series, follow up with a different
series if you feel strongly enough about it and that way we can have that
discussion in that thread and not hold up your feature.  Thanks,

Josef