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

From: Paul Jones <paul@pauljones.id.au>
To: Josef Bacik <josef@toxicpanda.com>,
	"kreijack@inwind.it" <kreijack@inwind.it>
Cc: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>,
	David Sterba <dsterba@suse.cz>,
	Sinnamohideen Shafeeq <shafeeqs@panasas.com>,
	"linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>
Subject: RE: [RFC][V8][PATCH 0/5] btrfs: allocation_hint mode
Date: Thu, 16 Dec 2021 02:30:50 +0000	[thread overview]
Message-ID: <SYXPR01MB191899ADCADD056A4FEB74A39E779@SYXPR01MB1918.ausprd01.prod.outlook.com> (raw)
In-Reply-To: <Ybn0aq548kQsQu+z@localhost.localdomain>

> -----Original Message-----
> From: Josef Bacik <josef@toxicpanda.com>
> Sent: Thursday, 16 December 2021 12:58 AM
> To: kreijack@inwind.it
> Cc: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>; David Sterba
> <dsterba@suse.cz>; Sinnamohideen Shafeeq <shafeeqs@panasas.com>;
> Paul Jones <paul@pauljones.id.au>; linux-btrfs@vger.kernel.org
> Subject: Re: [RFC][V8][PATCH 0/5] btrfs: allocation_hint mode
> 
> 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?
> 
> 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.

I don't think that is necessary - I've remounted my filesystems a few times with an unpatched kernel, and all that happens is new blocks get allocated without any hints - I like that simplicity and compatibility. As long as it's mentioned in the docs I think it's fine.

Having a balance filter that only rebalances wrongly allocated blocks would be a nice addition though.

Paul.