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authorEric Blake <eblake@redhat.com>2016-11-17 14:13:58 -0600
committerKevin Wolf <kwolf@redhat.com>2016-11-22 15:59:23 +0100
commit3482b9bc411a9a12b2efde1018e1ddc906cd817e (patch)
tree6b37b9e448550da0100e5d6315aef0f1dff1ef7e /balloon.c
parent49228d1e95e1be879c57f5dbccb44405670e343d (diff)
block: Pass unaligned discard requests to drivers
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. Rework the block layer discard algorithm to mirror the write zero algorithm: always peel off any unaligned head or tail and manage that in isolation, then do the bulk of the request on an aligned boundary. The fallback when the driver returns -ENOTSUP for an unaligned request is to silently ignore that portion of the discard request; but for devices that can pass the partial request all the way down to hardware, this can result in the hardware coalescing requests and discarding aligned pages after all. Reported by: Peter Lieven <pl@kamp.de> CC: qemu-stable@nongnu.org Signed-off-by: Eric Blake <eblake@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Diffstat (limited to 'balloon.c')
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