On 09/06/18 16:03, Xuewei Zhang wrote: > On Thu, Sep 6, 2018 at 3:42 PM Theodore Y. Ts'o <tytso@xxxxxxx> wrote: >> 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. > > 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) All of the above makes sense to me and is much less risky than changing QUEUE_FLAG_MQ_DEFAULT. Hence: Reviewed-by: Bart Van Assche <bvanassche@xxxxxxx>
