Re: [PATCH] scsi: sd: Contribute to randomness when running rotational device

[Xuewei Zhang]

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