David Sterba wrote:
On Mon, May 06, 2019 at 05:37:40PM +0300, Timofey Titovets wrote:
From: Timofey Titovets <nefelim4ag@xxxxxxxxx>
Currently btrfs raid1/10 bаlance requests to mirrors,
based on pid % num of mirrors.
Regarding the patches to select mirror policy, that Anand sent, I think
we first should provide a sane default policy that addresses most
commong workloads before we offer an interface for users that see the
need to fiddle with it.
As just a regular btrfs user I would just like to add that I earlier
made a comment where I think that btrfs should have the ability to
assign certain DevID's to groups (storage device groups).
From there I think it would be a good idea to "assign" subvolumes to
either one (or more) group(s) so that btrfs would prefer (if free space
permits) to store data from that subvolume on a certain group of storage
devices.
If you could also set a weight value for read and write separately for a
group then you are from a humble users point of view good to go and any
PID% optimization (and management) while very interesting sounds less
important.
As BTRFS scales to more than 32 devices (I think there is a limit for 30
or 32????) device groups should really be in there from a management
point of view and mount options for readmirror policy does not sound
good the way I understand it as this would affect the fileystem globally.
Groups could also allow for useful features like making sure metadata
stays on fast devices, migrating hot data to faster groups automatically
on read, and when (if?) subvolumes support different storage profiles
"Raid1/10/5/6" it sounds like an even better idea to assign such
subvolumes to faster/slower groups depending on the storage profile.
Anyway... I just felt like airing some ideas since the readmirror topic
has come up a few times on the mailing list recently.
