[Discussion] Extent Block Group allocations

[ashford@whisperpc.com]

Hi all,

I searched the archives, and didn't find any answers to my questions, so I
think it's time to ask.

From:  http://btrfs.wiki.kernel.org/index.php/Btrfs_design#Extent_Block_Groups

        Block groups have a flag that indicate if they are preferred for data
        or metadata allocations, and at mkfs time the disk is broken up into
        alternating metadata (33% of the disk) and data groups (66% of the
        disk). As the disk fills, a group's preference may change back and
        forth, but Btrfs always tries to avoid intermixing data and metadata
        extents in the same group. This substantially improves fsck throughput,
        and reduces seeks during writeback while the FS is mounted. It does
        slightly increase the seeks while reading.

Based on this, it appears that there is a semi-fixed allocation of 33% of the
disk to metadata, but that this allocation can change dynamically as the disk
fills.  It would appear that if the metadata approaches/exceeds its
allocation, a data group will be reallocated to it, and the same with the data
(an extent group would be reallocated).

At the present, there is only one logical device per file-system (single,
RAID-0, RAID-1 or RAID-10 - each is one logical device).  Based on the
documentation, there appears to be an intent to support RAID-6 (and optionally
RAID-5 - I believe this would be good) as logical devices.

>From what I see in the Multiple Device Support page
(http://btrfs.wiki.kernel.org/index.php/Multiple_Device_Support), it appears
that the intent in the future is to allow a BTRFS file-system to reside on
multiple logical devices.  This is the starting point for my questions.

In an installation where a large number of physical devices are available for
use (something like a Sun Thumper - 48 total disks, or a server connected to a
SAN), the optimum configuration might be to dedicate certain logical devices
(small/fast disks in RAID-1) to metadata, and other devices (large/slow disks
in RAID-5 or RAID-6) to data.  To perform this, the metadata allocation
percentage would need to be tunable (0% for data-only and 100% for
metadata-only), and it would have to be able to be locked, so that the block
group reallocation between metadata and data would be disabled (another option
might be to allow metadata to reallocate data block groups, but not the other
way around).

I believe that a configuration like this would be more flexible than having
the metadata block groups interleaved with the data block groups.  I also
believe that this should be able to provide better overall response and
throughput on a large multi-user server.

Is something like this intended to be possible?

Thank you.

Peter Ashford