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Re: [Qemu-devel] [PATCH v2 5/9] block: Pass unaligned discard requests t

From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH v2 5/9] block: Pass unaligned discard requests to drivers
Date: Fri, 18 Nov 2016 00:44:03 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

On 17.11.2016 21:13, Eric Blake wrote:
> Discard is advisory, so rounding the requests to alignment
> boundaries is never semantically wrong from the data that
> the guest sees.  But at least the Dell Equallogic iSCSI SANs
> has an interesting property that its advertised discard
> alignment is 15M, yet documents that discarding a sequence
> of 1M slices will eventually result in the 15M page being
> marked as discarded, and it is possible to observe which
> pages have been discarded.
> Between commits 9f1963b and b8d0a980, we converted the block
> layer to a byte-based interface that ultimately ignores any
> unaligned head or tail based on the driver's advertised
> discard granularity, which means that qemu 2.7 refuses to
> pass any discard request smaller than 15M down to the Dell
> Equallogic hardware.  This is a slight regression in behavior
> compared to earlier qemu, where a guest executing discards
> in power-of-2 chunks used to be able to get every page
> discarded, but is now left with various pages still allocated
> because the guest requests did not align with the hardware's
> 15M pages.
> Since the SCSI specification says nothing about a minimum
> discard granularity, and only documents the preferred
> alignment, it is best if the block layer gives the driver
> every bit of information about discard requests, rather than
> rounding it to alignment boundaries early.

Is this series supposed to address this issue? Because if so, I fail to
see where it does. If the device advertises 15 MB as the discard
granularity, then the iscsi driver will still drop all discard requests
that are not aligned to 15 MB boundaries, no?

The only difference is that it's now the iscsi driver that drops the
request instead of the generic block layer.


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