Proposal: Avoid writing layer blobs to disk when discard_unpacked_layers is enabled.

[omerXfaruq]

Pain point

When discard_unpacked_layers = true, layer blobs are downloaded, unpacked, and deleted within seconds. But plugins/content/local/writer.go still fsyncs every blob on commit:

containerd/plugins/content/local/writer.go

Line 97 in f9372ec

if err := fp.Sync(); err != nil {

This is an unnecessary write and creates a bottleneck on low bandwidth disks.

Proposal

Add content.WithDiscardable(). When set, the local content writer:

1. Opens the ingest file with O_TMPFILE on the content root.
2. Skips fp.Sync() on commit and close.
3. Links the file into place with linkat after digest verification.

Impact

• Reduce disk writes up to 50%
• Increase disk lifespan
• No change for callers that don't opt in.

[omerXfaruq]

Hi @samuelkarp @dmcgowan do you have any thoughts about this proposal, happy to open a PR if it sounds right.