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Discard blocks when removing a thin device

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dmsetup does not discard blocks when removing a thin device. The unused blocks
are reused by the thin-pool, but will remain allocated in the underlying
device indefinitely. For loop device backed thin-pools, this results in
"lost" disk space in the underlying file system as the blocks remain allocated
in the loop device's backing file.

This change adds an option, discard_blocks, to the devmapper snapshotter which
causes the snapshotter to issue blkdiscard ioctls on the thin device before
removal. With this option enabled, loop device setups will see disk space
return to the underlying filesystem immediately on exiting a container.

Fixes #5691

Signed-off-by: Kern Walster <walster@amazon.com>
This commit is contained in:
Kern Walster
2021-07-19 16:37:46 +00:00
parent b88bf1eca7
commit f1d79d33b8
7 changed files with 114 additions and 4 deletions
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30
snapshots/devmapper/pool_device.go
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@@ -29,13 +29,15 @@ import (
"golang.org/x/sys/unix"
"github.com/containerd/containerd/log"
blkdiscard "github.com/containerd/containerd/snapshots/devmapper/blkdiscard"
"github.com/containerd/containerd/snapshots/devmapper/dmsetup"
)
// PoolDevice ties together data and metadata volumes, represents thin-pool and manages volumes, snapshots and device ids.
type PoolDevice struct {
poolName string
metadata *PoolMetadata
poolName string
metadata *PoolMetadata
discardBlocks bool
}
// NewPoolDevice creates new thin-pool from existing data and metadata volumes.
@@ -51,6 +53,15 @@ func NewPoolDevice(ctx context.Context, config *Config) (*PoolDevice, error) {
log.G(ctx).Infof("using dmsetup:\n%s", version)
if config.DiscardBlocks {
blkdiscardVersion, err := blkdiscard.Version()
if err != nil {
log.G(ctx).Error("blkdiscard is not available")
return nil, err
}
log.G(ctx).Infof("using blkdiscard:\n%s", blkdiscardVersion)
}
dbpath := filepath.Join(config.RootPath, config.PoolName+".db")
poolMetaStore, err := NewPoolMetadata(dbpath)
if err != nil {
@@ -64,8 +75,9 @@ func NewPoolDevice(ctx context.Context, config *Config) (*PoolDevice, error) {
}
poolDevice := &PoolDevice{
poolName: config.PoolName,
metadata: poolMetaStore,
poolName: config.PoolName,
metadata: poolMetaStore,
discardBlocks: config.DiscardBlocks,
}
if err := poolDevice.ensureDeviceStates(ctx); err != nil {
@@ -422,6 +434,16 @@ func (p *PoolDevice) DeactivateDevice(ctx context.Context, deviceName string, de
if err := p.transition(ctx, deviceName, Deactivating, Deactivated, func() error {
return retry(ctx, func() error {
if !deferred && p.discardBlocks {
err := dmsetup.DiscardBlocks(deviceName)
if err != nil {
if err == dmsetup.ErrInUse {
log.G(ctx).Warnf("device %q is in use, skipping blkdiscard", deviceName)
} else {
return err
}
}
}
if err := dmsetup.RemoveDevice(deviceName, opts...); err != nil {
return errors.Wrap(err, "failed to deactivate device")
}