https://bugzilla.kernel.org/show_bug.cgi?id=195233
Bug ID: 195233
Summary: sd: discard_granularity set smaller than
minimum_io_size when LBPRZ=1
Product: IO/Storage
Version: 2.5
Kernel Version: 4.4.0
Hardware: All
OS: Linux
Tree: Mainline
Status: NEW
Severity: normal
Priority: P1
Component: SCSI
Assignee: linux-scsi@vger.kernel.org
Reporter: dbuck...@oreilly.com
Regression: No
Between 3.10 and 4.4 kernels this patch:
https://patchwork.kernel.org/patch/6995131/ was merged to ensure
discard_granularity is set to logical_block_size when a disk reports LBPRZ=1.
In a case where a disk reports LBPRZ=1 with a 512 logical_block_size and 4096
physical_block_size and minimum_io_size, this results in discard_granularity
incorrectly being set to 512 which at least in my case results in discard
requests being ignored.
I believe the easiest fix would be changing:
if (sdkp->lbprz) {
q->limits.discard_alignment = 0;
q->limits.discard_granularity = logical_block_size;
} else {
to:
if (sdkp->lbprz) {
q->limits.discard_alignment = 0;
q->limits.discard_granularity = max(logical_block_size,
minimum_io_size);
} else {
but any change which results in discard_granularity being set properly would
solve this.
I have verified that with a 3.10 kernel discard_granularity is set to 4096 and
works as expected, but with 4.4 it is set to 512 and does not work.
This is a fairly severe bug for my use case, as I rely on discard to free
unused blocks from the storage backing LUNs mounted by Linux clients and this
effectively makes critical functions like thin-provisioning and snapshots
infeasible for 4.x clients.
I have only tested this with the Ubuntu-packaged kernels, but the problematic
code still is present in the current source. I'm happy to provide any
additional information that might be helpful.
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