Re: [RFC][V8][PATCH 0/5] btrfs: allocation_hint mode

[Josef Bacik <josef@toxicpanda.com>]

On Wed, Dec 15, 2021 at 07:53:40PM +0100, Goffredo Baroncelli wrote:
> On 12/15/21 14:58, Josef Bacik wrote:
> > On Tue, Dec 14, 2021 at 09:41:21PM +0100, Goffredo Baroncelli wrote:
> > > On 12/14/21 21:34, Josef Bacik wrote:
> > > > On Tue, Dec 14, 2021 at 03:04:32PM -0500, Zygo Blaxell wrote:
> > > > > On Tue, Dec 14, 2021 at 08:03:45PM +0100, Goffredo Baroncelli wrote:
> > > 
> > > > > 
> > > > > I don't have a strong preference for either sysfs or ioctl, nor am I
> > > > > opposed to simply implementing both.  I'll let someone who does have
> > > > > such a preference make their case.
> > > > 
> > > > I think echo'ing a name into sysfs is better than bits for sure.  However I want
> > > > the ability to set the device properties via a btrfs-progs command offline so I
> > > > can setup the storage and then mount the file system.  I want
> > > > 
> > > > 1) The sysfs interface so you can change things on the fly.  This stays
> > > >      persistent of course, so the way it works is perfect.
> > > > 
> > > > 2) The btrfs-progs command sets it on offline devices.  If you point it at a
> > > >      live mounted fs it can simply use the sysfs thing to do it live.
> > > 
> > > #2 is currently not implemented. However I think that we should do.
> > > 
> > > The problem is that we need to update both:
> > > 
> > > - the superblock		(simple)
> > > - the dev_item item		(not so simple...)
> > > 
> > > What about using only bits from the superblock to store this property ?
> > 
> > I'm looking at the patches and you only are updating the dev_item, am I missing
> > something for the super block?
> 
> When btrfs write the superblocks (see write_all_supers() in disk-io.c), it copies
> the dev_item fields (contained in fs_info->fs_devices->devices lists) in each
> superblock before updating it.
> 

Oh right.  Still, I hope we're doing this correctly in btrfs-progs, if not
that's a problem.

> > 
> > For offline all you would need to do is do the normal open_ctree,
> > btrfs_search_slot to the item and update the device item type, that's
> > straightforward.
> > 
> > For online if you use btrfs prop you can see if the fs is mounted and just find
> > the sysfs file to modify and do it that way.
> > 
> > But this also brings up another point, we're going to want a compat bit for
> > this.  It doesn't make the fs unusable for old kernels, so just a normal
> > BTRFS_FS_COMPAT_<whatever> flag is fine.  If the setting gets set you set the
> > compat flag.
> 
> Why we need a "compact" bit ? The new kernels know how treat the dev_item_type field.
> The old kernels ignore it. The worst thing is that a filesystem may require a balance
> before reaching a good shape (i.e. the metadata on ssd and the data on a spinning disk)
> 

So you can do the validation below, tho I'm thinking I care about it less, if we
just make sure that type is correct regardless of the compat bit then that's
fine.  Thanks,

Josef