MariaDB 10.4.3 will introduce a MLOG_INIT_FREE_PAGE record for this purpose. This allows this bug to be fixed in MariaDB 10.4 later without breaking crash-downgrade to earlier MariaDB 10.4 versions.

In MDEV-12699, MLOG_INIT_FREE_PAGE should be handled in a similar way as the existing record MLOG_INIT_FILE_PAGE2.

I think that this is realistic to do in the 10.4 time frame. Assigning to myself, because this is closely related to MDEV-12699, which I am working on.

When scrubbing is not enabled and the page is not on SSD and the file is not page_compressed, we can discard the page from the buf_pool->flush_list once the MLOG_INIT_FREE_PAGE record has been written. For scrubbing, it is kind of mandatory to initialize the page.

For non-compressed tables on SSD, we will have to evaluate whether punching holes is going to improve or hurt performance on the average.

As part of this, we should simplify the freeing of B-tree root pages:

diff --git a/storage/innobase/btr/btr0btr.cc b/storage/innobase/btr/btr0btr.cc

index 753ed9b077c..21609edb7f3 100644

--- a/storage/innobase/btr/btr0btr.cc

+++ b/storage/innobase/btr/btr0btr.cc

@@ -961,12 +961,9 @@ constexpr index_id_t	BTR_FREED_INDEX_ID = 0;

 /** Free a B-tree root page. btr_free_but_not_root() must already

 have been called.

-In a persistent tablespace, the caller must invoke fsp_init_file_page()

-before mtr.commit().

 @param[in,out]	block		index root page

-@param[in,out]	mtr		mini-transaction

-@param[in]	invalidate	whether to invalidate PAGE_INDEX_ID */

-static void btr_free_root(buf_block_t *block, mtr_t *mtr, bool invalidate)

+@param[in,out]	mtr		mini-transaction */

+static void btr_free_root(buf_block_t *block, mtr_t *mtr)

 {

   ut_ad(mtr_memo_contains_flagged(mtr, block,

 				  MTR_MEMO_PAGE_X_FIX | MTR_MEMO_PAGE_SX_FIX));

(Obviously, also the code inside if (invalidate) should be removed and the callers be adjusted.)
We should ensure that MLOG_INIT_FREE_PAGE records will be emitted by fseg_free_step(), and thus no log needs to be written in this function.

This looks OK to me, after addressing my feedback for commit 1 of 2. We will address the following later, ideally before the 10.5 GA release:

• clean up btr_free_root() as I suggested in my previous comment
• address the FIXME comments in buf_page_free() by introducing a data structure that covers freed page ranges also for pages that are not in the buffer pool, and enable the scrubbing tests (MDEV-8139)

commit a5584b13d1e04f38b843602413669591aa65c359 (HEAD -> 10.5, origin/10.5, 10.5-work)

Author: Thirunarayanan Balathandayuthapani <thiru@mariadb.com>

Date:   Tue Mar 10 10:47:24 2020 +0530

commit a35b4ae89871d8184f04abb112c385481d557dbb

Author: Thirunarayanan Balathandayuthapani <thiru@mariadb.com>

Date:   Mon Mar 9 13:25:33 2020 +0530

    MDEV-15528 Punch holes when pages are freed

Patch has been pushed to 10.5. Thanks to marko and @matthias.leich for testing abd reviewing it