On 10/30/20 9:51 AM, Naohiro Aota wrote:
This is the 1/3 patch to enable tree log on ZONED mode. The tree-log feature does not work on ZONED mode as is. Blocks for a tree-log tree are allocated mixed with other metadata blocks, and btrfs writes and syncs the tree-log blocks to devices at the time of fsync(), which is different timing from a global transaction commit. As a result, both writing tree-log blocks and writing other metadata blocks become non-sequential writes that ZONED mode must avoid. We can introduce a dedicated block group for tree-log blocks so that tree-log blocks and other metadata blocks can be separated write streams. As a result, each write stream can now be written to devices separately. "fs_info->treelog_bg" tracks the dedicated block group and btrfs assign "treelog_bg" on-demand on tree-log block allocation time. This commit extends the zoned block allocator to use the block group. Signed-off-by: Johannes Thumshirn <johannes.thumshirn@xxxxxxx> Signed-off-by: Naohiro Aota <naohiro.aota@xxxxxxx>
If you're going to remove an entire block group from being allowed to be used for metadata you are going to need to account for it in the space_info, otherwise we're going to end up with nasty ENOSPC corners here.
But this begs the question, do we want the tree log for zoned? We could just commit the transaction and call it good enough. We lose performance, but zoned isn't necessarily about performance.
If we do then at a minimum we're going to need to remove this block group from the space info counters for metadata. Thanks,
Josef
