Glossary
The terms this documentation and the flexblock output use, in the sense flexBlock gives them.
A1in, Am, A1out
Section titled “A1in, Am, A1out”The three queues of the 2Q cache policy: probationary, protected, and the ghost list of recently evicted identities. See Cache Replacement Policy.
Attach
Section titled “Attach”Binding a volume to a cache device on this host and claiming ownership. Copies nothing. See flexblock attach.
Cache device
Section titled “Cache device”The local block device, partition or file that holds chunk slots, the write-ahead log and superblocks.
A fixed-size span of the device, 1, 2, 4 or 8 MiB, that is one object in the bucket and one slot in the cache.
Clean, dirty
Section titled “Clean, dirty”A slot is dirty when it holds data not yet uploaded to object storage, and clean once the upload has landed.
Detach
Section titled “Detach”Releasing ownership and clearing the local cache. The volume’s data in the bucket is left intact. See flexblock detach.
Uploading a dirty chunk to its key in object storage and marking its slot clean.
Grace period
Section titled “Grace period”The time after a grant’s expiry during which flexBlock only warns.
The signed license file that names the volume’s provider, bucket, prefix and connection details and carries its licensed size and dates. See Licensing.
The 45 seconds, counted from when each ownership check is sent, during which the daemon may write to the bucket once the check is confirmed. When checks cannot be confirmed, the lease lapses and bucket writes pause.
Logical block size
Section titled “Logical block size”The sector size the device advertises to the kernel, 512 or 4096 bytes.
Object index
Section titled “Object index”The daemon’s in-memory set of chunks that have an object, rebuilt by listing the prefix at every start.
Owner, ownership marker
Section titled “Owner, ownership marker”The host currently entitled to serve a volume, recorded in the __owner object. See Ownership.
Preflight
Section titled “Preflight”The checks start runs before serving: kernel features, device state, memory and clock.
Prefix
Section titled “Prefix”The key prefix inside the bucket under which a volume keeps all of its objects.
Removing a leaked or stale ublk device from the kernel. See flexblock reap.
A chunk-sized region of the cache device’s data area, plus its metadata record.
Superblock
Section titled “Superblock”The record of a volume’s geometry. One copy lives in the bucket as __superblock; two alternating copies live at the start of the cache device.
Throttled
Section titled “Throttled”The state in which guest I/O is rate-limited because the grant has expired, is pending, or does not cover the volume.
The Linux kernel driver that lets a userspace process serve a block device, exposed as /dev/ublkbN.
Volume
Section titled “Volume”A flexBlock block device as defined by its superblock and chunk objects in the bucket, independent of any host. See Volumes and the Cache.
Deleting a volume: every object under the prefix and the local cache. See flexblock wipe.
Write-ahead log (WAL)
Section titled “Write-ahead log (WAL)”The ring of dirty and discard intents on the cache device that recovery replays after an unclean stop.