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

On 12/15/21 14:58, Josef Bacik wrote:

On Tue, Dec 14, 2021 at 09:41:21PM +0100, Goffredo Baroncelli wrote:

quoted

On 12/14/21 21:34, Josef Bacik wrote:

quoted

On Tue, Dec 14, 2021 at 03:04:32PM -0500, Zygo Blaxell wrote:

quoted

On Tue, Dec 14, 2021 at 08:03:45PM +0100, Goffredo Baroncelli wrote:

quoted

quoted

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.

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)
  

You'll also need to modify the tree checker to check the dev item allocation
hints to make sure they're valid, via check_dev_item.  Thanks,

Ok

Josef

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