Journal Config ReferenceΒΆ
Ceph OSDs use a journal for two reasons: speed and consistency.
- Speed: The journal enables the Ceph OSD Daemon to commit small writes quickly. Ceph writes small, random i/o to the journal sequentially, which tends to speed up bursty workloads by allowing the backing filesystem more time to coalesce writes. The Ceph OSD Daemon’s journal, however, can lead to spiky performance with short spurts of high-speed writes followed by periods without any write progress as the filesystem catches up to the journal.
- Consistency: Ceph OSD Daemons require a filesystem interface that guarantees atomic compound operations. Ceph OSD Daemons write a description of the operation to the journal and apply the operation to the filesystem. This enables atomic updates to an object (for example, placement group metadata). Every few seconds–between filestore max sync interval and filestore min sync interval–the Ceph OSD Daemon stops writes and synchronizes the journal with the filesystem, allowing Ceph OSD Daemons to trim operations from the journal and reuse the space. On failure, Ceph OSD Daemons replay the journal starting after the last synchronization operation.
Ceph OSD Daemons support the following journal settings:
journal dio
| Description: | Enables direct i/o to the journal. Requires journal block align set to true. |
|---|---|
| Type: | Boolean |
| Required: | Yes when using aio. |
| Default: | true |
journal aio
Changed in version 0.61: Cuttlefish
| Description: | Enables using libaio for asynchronous writes to the journal. Requires journal dio set to true. |
|---|---|
| Type: | Boolean |
| Required: | No. |
| Default: | Version 0.61 and later, true. Version 0.60 and earlier, false. |
journal block align
| Description: | Block aligns write operations. Required for dio and aio. |
|---|---|
| Type: | Boolean |
| Required: | Yes when using dio and aio. |
| Default: | true |
journal max write bytes
| Description: | The maximum number of bytes the journal will write at any one time. |
|---|---|
| Type: | Integer |
| Required: | No |
| Default: | 10 << 20 |
journal max write entries
| Description: | The maximum number of entries the journal will write at any one time. |
|---|---|
| Type: | Integer |
| Required: | No |
| Default: | 100 |
journal queue max ops
| Description: | The maximum number of operations allowed in the queue at any one time. |
|---|---|
| Type: | Integer |
| Required: | No |
| Default: | 500 |
journal queue max bytes
| Description: | The maximum number of bytes allowed in the queue at any one time. |
|---|---|
| Type: | Integer |
| Required: | No |
| Default: | 10 << 20 |
journal align min size
| Description: | Align data payloads greater than the specified minimum. |
|---|---|
| Type: | Integer |
| Required: | No |
| Default: | 64 << 10 |
journal zero on create
| Description: | Causes the file store to overwrite the entire journal with 0‘s during mkfs. |
|---|---|
| Type: | Boolean |
| Required: | No |
| Default: | false |
