Re: [PATCH] btrfs: add ssd_metadata mode

[Adam Borowski <kilobyte@angband.pl>]

On Fri, Oct 23, 2020 at 03:23:30PM +0800, Wang Yugui wrote:
> Hi, Goffredo Baroncelli 
> 
> We can move 'rotational of struct btrfs_device_info' to  'bool rotating
> of struct btrfs_device'.
> 
> 1, it will be more close to 'bool rotating of struct btrfs_fs_devices'.
> 
> 2, it maybe used to enhance the path of '[PATCH] btrfs: balance RAID1/RAID10 mirror selection'.
> https://lore.kernel.org/linux-btrfs/3bddd73e-cb60-b716-4e98-61ff24beb570@oracle.com/T/#t

I don't think it should be a bool -- or at least, turned into a bool
late in the processing.

There are many storage tiers; rotational applies only to one of the
coldest.  In my use case, at least, I've added the following patchlet:

-               devices_info[ndevs].rotational = !test_bit(QUEUE_FLAG_NONROT,
+               devices_info[ndevs].rotational = !test_bit(QUEUE_FLAG_DAX,

Or, you may want Optane NVMe vs legacy (ie, NAND) NVMe.

The tiers look like:
* DIMM-connected Optane (dax=1)
* NVMe-connected Optane
* NVMe-connected flash
* SATA-connected flash
* SATA-connected spinning rust (rotational=1)
* IDE-connected spinning rust (rotational=1)
* SD cards
* floppies?

And even that is just for local storage only.

Thus, please don't hardcode the notion of "rotational", what we want is
"faster but smaller" vs "slower but bigger".

> > From: Goffredo Baroncelli <kreijack@inwind.it>
> > 
> > When this mode is enabled, the allocation policy of the chunk
> > is so modified:
> > - allocation of metadata chunk: priority is given to ssd disk.
> > - allocation of data chunk: priority is given to a rotational disk.
> > 
> > When a striped profile is involved (like RAID0,5,6), the logic
> > is a bit more complex. If there are enough disks, the data profiles
> > are stored on the rotational disks only; instead the metadata profiles
> > are stored on the non rotational disk only.
> > If the disks are not enough, then the profiles is stored on all
> > the disks.

And, a newer version of Goffredo's patchset already had
"preferred_metadata".  It did not assign the preference automatically,
but if we want god defaults, they should be smarter than just rotationality.


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