On Thu, Sep 6, 2018 at 3:42 PM Theodore Y. Ts'o <tytso@xxxxxxx> wrote: > > On Thu, Sep 06, 2018 at 03:27:53PM -0700, Bart Van Assche wrote: > > > > Although this patch looks fine to me, seeing this patch makes me wonder > > whether the default should be changed (QUEUE_FLAG_MQ_DEFAULT) instead of > > modifying the sd driver. Can anyone remind me why QUEUE_FLAG_MQ_DEFAULT does > > not include QUEUE_FLAG_ADD_RANDOM? Besides Ted's point of "SSD users have no entropy", I think there are two more reasons: 1. setting QUEUE_FLAG_ADD_RANDOM has a more visible performance hit on SSD disks than rotational disks. 2. SSD disks provide less entropy than rotational disks. I actually experimented on Container-Optimized OS, running on Google Compute Engine with QUEUE_FLAG_ADD_RANDOM set. Turns out the VM will have ~800 bit of entropy provided on boot on rotational disk; and will only have ~70 bit of entropy if running on SSD (and remember there are ~50 bit contributed from other sources). (in the above experiment, both disks were virtualized disks) > > There was a discussion about a number of *years* ago; blk-mq has been > baking for a very long time. In the early days of block_mq, the > overwhelming percentage of the users of blk-mq where those who were > using PCIe attached flash. So when, I raised this question, the > argument was that SSD users have no entropy. Which I agree with; but > now that blk-mq is the default, and hard drives are using blk-mq, it's > time for a patch like Xuewei's. > > Cheers, > > - Ted
