Re: [RFC ONLY 0/8] btrfs: introduce raid-stripe-tree

[Johannes Thumshirn <Johannes.Thumshirn@wdc.com>]

On 16/05/2022 16:58, Josef Bacik wrote:
> On Mon, May 16, 2022 at 07:31:35AM -0700, Johannes Thumshirn wrote:
>> Introduce a raid-stripe-tree to record writes in a RAID environment.
>>
>> In essence this adds another address translation layer between the logical
>> and the physical addresses in btrfs and is designed to close two gaps. The
>> first is the ominous RAID-write-hole we suffer from with RAID5/6 and the
>> second one is the inability of doing RAID with zoned block devices due to the
>> constraints we have with REQ_OP_ZONE_APPEND writes.
>>
>> Thsi is an RFC/PoC only which just shows how the code will look like for a
>> zoned RAID1. Its sole purpose is to facilitate design reviews and is not
>> intended to be merged yet. Or if merged to be used on an actual file-system.
>>
> 
> This is hard to talk about without seeing the code to add the raid extents and
> such.  Reading it makes sense, but I don't know how often the stripes are meant
> to change.  Are they static once they're allocated, like dev extents?  I can't
> quite fit in my head the relationship with the rest of the allocation system.
> Are they coupled with the logical extent that gets allocated?  Or are they
> coupled with the dev extent?  Are they somewhere in between?

The stripe extents have a 1:1 relationship file the file-extents, i.e:

stripe_extent_key.objectid = btrfs_file_extent_item.disk_bytenr;
stripe_extent_type = BTRFS_RAID_STRIPE_EXTENT;
stripe_extent_offset = btrfs_file_extent_item.disk_num_bytes;


> Also I realize this is an RFC, but we're going to need some caching for reads so
> we're not having to do a tree search on every IO with the RAID stripe tree in
> place.

Do we really need to do caching of stripe tree entries? They're read, once the
corresponding btrfs_file_extent_item is read from disk, which then gets cached
in the page cache. Every override is cached in the page cache as well.

If we're flushing the cache, we need to re-read both, the file_extent_item and 
the stripe extents.