Re: [PATCH] block: Increase BLK_DEF_MAX_SECTORS_CAP

[Damien Le Moal <dlemoal@kernel.org>]

On 6/17/25 23:52, Ming Lei wrote:
> On Tue, Jun 17, 2025 at 03:34:30PM +0900, Damien Le Moal wrote:
>> Back in 2015, commit d2be537c3ba3 ("block: bump BLK_DEF_MAX_SECTORS to
>> 2560") increased the default maximum size of a block device I/O to 2560
>> sectors (1280 KiB) to "accommodate a 10-data-disk stripe write with
>> chunk size 128k". This choice is rather arbitrary and since then,
>> improvements to the block layer have software RAID drivers correctly
>> advertize their stripe width through chunk_sectors and abuses of
>> BLK_DEF_MAX_SECTORS_CAP by drivers (to set the HW limit rather than the
>> default user controlled maximum I/O size) have been fixed.
>>
>> Since many block devices can benefit from a larger value of
>> BLK_DEF_MAX_SECTORS_CAP, and in particular HDDs, increase this value to
>> be 4MiB, or 8192 sectors.
>>
>> Suggested-by: Martin K . Petersen <martin.petersen@oracle.com>
>> Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
>> ---
>>  include/linux/blkdev.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>> index 85aab8bc96e7..7c35b2462048 100644
>> --- a/include/linux/blkdev.h
>> +++ b/include/linux/blkdev.h
>> @@ -1238,7 +1238,7 @@ enum blk_default_limits {
>>   * Not to be confused with the max_hw_sector limit that is entirely
>>   * controlled by the driver, usually based on hardware limits.
>>   */
>> -#define BLK_DEF_MAX_SECTORS_CAP	2560u
>> +#define BLK_DEF_MAX_SECTORS_CAP	8192u
> 
> The change itself looks good, but the definition should belong to block
> layer internal, so why not move it into internal header?

Good point. Now that no driver uses this macro directly, we can do that. I will
send a V2.

-- 
Damien Le Moal
Western Digital Research