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Re: [Qemu-block] [PATCH 4/4] block: Cater to iscsi with non-power-of-2 d

From: Peter Lieven
Subject: Re: [Qemu-block] [PATCH 4/4] block: Cater to iscsi with non-power-of-2 discard
Date: Tue, 25 Oct 2016 16:20:08 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0

Am 25.10.2016 um 15:59 schrieb Eric Blake:
On 10/25/2016 07:42 AM, Peter Lieven wrote:
But hey, that firmware is seriously weird. :)
Yes, so you would not change the new implementation?

Even if the discard is e.g. 1MB it could theretically be that internally
the device has a finer granularity. Its an optimal discard alignment
not the minimum required discard size. I think thats a difference.
It does not say I can't handle smaller discards.
The firmware is probably technically buggy for advertising too large of
a minimum granularity, if it can piece together smaller requests into a
larger discard.  If discards need to happen at a smaller granularity,
the firmware (or kernel quirk system) should fix the advertisement to
the actual granularity that it will honor.  I don't see a reason to
change qemu's current behavior.

The issue is that the optimal unmap granularity is only a hint.
There is no evidence what happens with unaligned requests or requests
that are smaller. They could still lead to a discard operation in the
storage device. It just says if you can send me discards of that size
thats optimal for me. Thats not said that smaller or unaligned requests
have no effect.


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