On 3/20/25 12:27 PM, Matthew Wilcox wrote:
On Thu, Mar 20, 2025 at 09:15:23AM -0700, Bart Van Assche wrote:
The patch description mentions what has been changed but does not
mention why. Shouldn't the description of this patch explain why this
change has been made? Shouldn't the description of this patch explain
for which applications this change is useful?
The manufacturer chooses the block size. If they've made a bad decision,
their device will presumably not sell well. We don't need to justify
their decision in the commit message.
From a 2023 presentation by Luis
(https://lpc.events/event/17/contributions/1508/attachments/1298/2608/LBS_LPC2023.pdf):
- SSD manufacturers want to increase the indirection unit (IU) size.
- Increasing the IU size reduces SSD DRAM costs.
- LBS is not suitable for all workloads because smaller IOs with LBS can
cause write amplification (WAF) due to read modify writes.
- Some database software benefits of a 16 KiB logical block size.
If the goal is to reduce DRAM costs then I recommend SSD manufacturers
to implement zoned storage (ZNS) instead of only increasing the logical
block size. A big advantage of zoned storage is that the DRAM cost is
reduced significantly even if the block size is not increased.
Are there any applications that benefit from a block size larger than
64 KiB? If not, why to increase BLK_MAX_BLOCK_SIZE further? Do you agree
that this question should be answered in the patch description?
Thanks,
Bart.
