96cc493c74
resolves: RHEL-88543 Remove usage of nbdkit-cacheextents-filter resolves: RHEL-88857 Print better mountpoint stats in debug output resolves: RHEL-88861 Add virt-v2v -io vddk-noextents=true so we can test noextents resolves: RHEL-88863 Remove several ancient, deprecated options resolves: RHEL-88866
56 lines
2.4 KiB
Diff
56 lines
2.4 KiB
Diff
From 65a9c8ed09f4cd04ae2176b48c4c9c1f69b08399 Mon Sep 17 00:00:00 2001
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From: "Richard W.M. Jones" <rjones@redhat.com>
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Date: Tue, 29 Apr 2025 10:42:46 +0100
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Subject: [PATCH] input: Document my findings with nbdkit-noextents-filter
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This just adds a comment, so makes no change.
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(cherry picked from commit 29fae7985eda1d1cf3e176f123a16b60cac2db53)
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---
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input/nbdkit_vddk.ml | 32 ++++++++++++++++++++++++++++++++
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1 file changed, 32 insertions(+)
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diff --git a/input/nbdkit_vddk.ml b/input/nbdkit_vddk.ml
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index b79c28cc..3ba00d55 100644
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--- a/input/nbdkit_vddk.ml
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+++ b/input/nbdkit_vddk.ml
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@@ -140,6 +140,38 @@ See also the virt-v2v-input-vmware(1) manual.") libNN
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*)
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Nbdkit.add_filter_if_available cmd "retry";
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+ (* VDDK's QueryAllocatedBlocks API is infamously slow. It appears
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+ * to block all other requests while it is running. This API is
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+ * also only called during the copy phase, not during conversion
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+ * (or if it is, extremely rarely).
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+ *
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+ * If fstrim was successful, then trimmed blocks are stored in
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+ * the COW filter (see below), and so requests for extents stop
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+ * at that layer. However for areas of the disk that fstrim
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+ * thinks contain data, we still have to go through to VDDK to
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+ * fetch extents.
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+ *
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+ * We could therefore add nbdkit-noextents-filter here (below COW,
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+ * above VDDK plugin) which stops extents requests from going
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+ * to VDDK, which would stop QueryAllocatedBlocks ever being
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+ * called. In my testing this is a moderate performance win.
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+ *
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+ * However ... in the case where fstrim failed, or for filesystems
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+ * or partitions on the disk that we don't understand, doing this
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+ * would mean that those are copied completely, as there would be
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+ * no extent data (nbdcopy will still sparsify them on the target,
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+ * but we'd have to copy all the bits from VMware). Because
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+ * here we don't know if this is the case, be conservative and
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+ * actually don't use this filter.
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+ *
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+ * If used, this filter should be close to the plugin and MUST
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+ * be below the COW filter.
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+ *
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+ * XXX Add some kind of debugging flag so we can test how this
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+ * works in production.
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+ *)
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+ (*Nbdkit.add_filter_if_available cmd "noextents";*)
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+
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(* Split very large requests to avoid out of memory errors on the
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* server. Since we're using this filter, also add minblock=512
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* although it will make no difference.
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