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Subject: Re: [PATCH] scsi: sd: Contribute to randomness when running
 rotational device
To:     Xuewei Zhang <xueweiz@google.com>, Theodore Ts'o <tytso@mit.edu>,
        "James E.J. Bottomley" <jejb@linux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        ming.lei@redhat.com, hare@suse.de, hch@lst.de, pbonzini@redhat.com,
        linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org,
        Aditya Kali <adityakali@google.com>, groeck@chromium.org,
        =?UTF-8?Q?Maciej_=c5=bbenczykowski?= <maze@google.com>
References: <20180906203719.209399-1-xueweiz@google.com>
 <1536272873.26747.11.camel@acm.org> <20180906224210.GK5098@thunk.org>
 <CAPtwhKoFE-eZcRkvhpJgyutGK91ZouwSjTRb-cwHLQjBkriksw@mail.gmail.com>
From:   Bart Van Assche <bvanassche@acm.org>
Message-ID: <26273801-8dd5-983a-ef09-f8eef21749ac@acm.org>
Date:   Fri, 7 Sep 2018 21:06:57 -0700
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On 09/06/18 16:03, Xuewei Zhang wrote:
> On Thu, Sep 6, 2018 at 3:42 PM Theodore Y. Ts'o <tytso@mit.edu> 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@acm.org>