Re: DMMP request-queue vs. BiO

[Christoph Hellwig <hch@lst.de>]

On Fri, Nov 15, 2024 at 03:05:21PM +0100, Mikulas Patocka wrote:
> Note, that if a database uses buffered block device, performance will be 
> suboptimal, because the buffering mechanism can't create large bios, it 
> only sends page-sized bios. But that is expected to not be used - the 
> database should either use a block device with direct I/O or a filesystem 
> with or without direct I/O.

And, as pointed out in the private mail that John forwarded to the list
without my permission if we really have a workload that cares md could
implement the plugging callback as done in md to operate on a batch
of bios.

Also not building large bios is not a fundamental property of block
device writes but because it uses the legacy buffered head helpers.
That means:

  a) the same is applicable to file systems using them as well
  b) can be fixed if someone cares enough, but apparently no one does