Notice

This document is for a development version of Ceph.

Manifest

Introduction

As described in ../deduplication.rst, adding transparent redirect machinery to RADOS would enable a more capable tiering solution than RADOS currently has with “cache/tiering”.

See ../deduplication.rst

At a high level, each object has a piece of metadata embedded in the object_info_t which can map subsets of the object data payload to (refcounted) objects in other pools.

This document exists to detail:

  1. Manifest data structures

  2. Rados operations for manipulating manifests.

  3. Status and Plans

Intended Usage Model

RBD

For RBD, the primary goal is for either an OSD-internal agent or a cluster-external agent to be able to transparently shift portions of the consituent 4MB extents between a dedup pool and a hot base pool.

As such, RBD operations (including class operations and snapshots) must have the same observable results regardless of the current status of the object.

Moreover, tiering/dedup operations must interleave with RBD operations without changing the result.

Thus, here is a sketch of how I’d expect a tiering agent to perform basic operations:

  • Demote cold RBD chunk to slow pool:

    1. Read object, noting current user_version.

    2. In memory, run CDC implementation to fingerprint object.

    3. Write out each resulting extent to an object in the cold pool using the CAS class.

    4. Submit operation to base pool:

      • ASSERT_VER with the user version from the read to fail if the object has been mutated since the read.

      • SET_CHUNK for each of the extents to the corresponding object in the base pool.

      • EVICT_CHUNK for each extent to free up space in the base pool. Results in each chunk being marked MISSING.

    RBD users should then either see the state prior to the demotion or subsequent to it.

    Note that between 3 and 4, we potentially leak references, so a periodic scrub would be needed to validate refcounts.

  • Promote cold RBD chunk to fast pool.

    1. Submit TIER_PROMOTE

For clones, all of the above would be identical except that the initial read would need a LIST_SNAPS to determine which clones exist and the PROMOTE or SET_CHUNK/EVICT operations would need to include the cloneid.

RadosGW

For reads, RADOS Gateway (RGW) could operate as RBD does above relying on the manifest machinery in the OSD to hide the distinction between the object being dedup’d or present in the base pool

For writes, RGW could operate as RBD does above, but could optionally have the freedom to fingerprint prior to doing the write. In that case, it could immediately write out the target objects to the CAS pool and then atomically write an object with the corresponding chunks set.

Status and Future Work

At the moment, initial versions of a manifest data structure along with IO path support and rados control operations exist. This section is meant to outline next steps.

At a high level, our future work plan is:

  • Cleanups: Address immediate inconsistencies and shortcomings outlined in the next section.

  • Testing: Rados relies heavily on teuthology failure testing to validate features like cache/tiering. We’ll need corresponding tests for manifest operations.

  • Snapshots: We want to be able to deduplicate portions of clones below the level of the rados snapshot system. As such, the rados operations below need to be extended to work correctly on clones (e.g.: we should be able to call SET_CHUNK on a clone, clear the corresponding extent in the base pool, and correctly maintain OSD metadata).

  • Cache/tiering: Ultimately, we’d like to be able to deprecate the existing cache/tiering implementation, but to do that we need to ensure that we can address the same use cases.

Cleanups

The existing implementation has some things that need to be cleaned up:

  • SET_REDIRECT: Should create the object if it doesn’t exist, otherwise one couldn’t create an object atomically as a redirect.

  • SET_CHUNK:

    • Appears to trigger a new clone as user_modify gets set in do_osd_ops. This probably isn’t desirable, see Snapshots section below for some options on how generally to mix these operations with snapshots. At a minimum, SET_CHUNK probably shouldn’t set user_modify.

    • Appears to assume that the corresponding section of the object does not exist (sets FLAG_MISSING) but does not check whether the corresponding extent exists already in the object. Should always leave the extent clean.

    • Appears to clear the manifest unconditionally if not chunked, that’s probably wrong. We should return an error if it’s a REDIRECT

      case CEPH_OSD_OP_SET_CHUNK:
        if (oi.manifest.is_redirect()) {
          result = -EINVAL;
          goto fail;
        }
      
  • TIER_PROMOTE:

    • SET_REDIRECT clears the contents of the object. PROMOTE appears to copy them back in, but does not unset the redirect or clear the reference. This violates the invariant that a redirect object should be empty in the base pool. In particular, as long as the redirect is set, it appears that all operations will be proxied even after the promote defeating the purpose. We do want PROMOTE to be able to atomically replace a redirect with the actual object, so the solution is to clear the redirect at the end of the promote.

    • For a chunked manifest, we appear to flush prior to promoting. Promotion will often be used to prepare an object for low latency reads and writes, accordingly, the only effect should be to read any MISSING extents into the base pool. No flushing should be done.

  • High Level:

    • It appears that FLAG_DIRTY should never be used for an extent pointing at a dedup extent. Writing the mutated extent back to the dedup pool requires writing a new object since the previous one cannot be mutated, just as it would if it hadn’t been dedup’d yet. Thus, we should always drop the reference and remove the manifest pointer.

    • There isn’t currently a way to “evict” an object region. With the above change to SET_CHUNK to always retain the existing object region, we need an EVICT_CHUNK operation to then remove the extent.

Testing

We rely really heavily on randomized failure testing. As such, we need to extend that testing to include dedup/manifest support as well. Here’s a short list of the touchpoints:

  • Thrasher tests like qa/suites/rados/thrash/workloads/cache-snaps.yaml

    That test, of course, tests the existing cache/tiering machinery. Add additional files to that directory that instead setup a dedup pool. Add support to ceph_test_rados (src/test/osd/TestRados*).

  • RBD tests

    Add a test that runs an RBD workload concurrently with blind promote/evict operations.

  • RGW

    Add a test that runs a rgw workload concurrently with blind promote/evict operations.

Snapshots

Fundamentally we need to be able to manipulate the manifest status of clones because we want to be able to dynamically promote, flush (if the state was dirty when the clone was created), and evict extents from clones.

As such, the plan is to allow the object_manifest_t for each clone to be independent. Here’s an incomplete list of the high level tasks:

  • Modify the op processing pipeline to permit SET_CHUNK, EVICT_CHUNK to operation directly on clones.

  • Ensure that recovery checks the object_manifest prior to trying to use the overlaps in clone_range. ReplicatedBackend::calc_*_subsets are the two methods that would likely need to be modified.

See snaps.rst for a rundown of the librados snapshot system and OSD support details. I’d like to call out one particular data structure we may want to exploit.

The dedup-tool needs to be updated to use LIST_SNAPS to discover clones as part of leak detection.

An important question is how we deal with the fact that many clones will frequently have references to the same backing chunks at the same offset. In particular, make_writeable will generally create a clone that shares the same object_manifest_t references with the exception of any extents modified in that transaction. The metadata that commits as part of that transaction must therefore map onto the same refcount as before because otherwise we’d have to first increment refcounts on backing objects (or risk a reference to a dead object) Thus, we introduce a simple convention: consecutive clones which share a reference at the same offset share the same refcount. This means that a write that invokes make_writeable may decrease refcounts, but not increase them. This has some conquences for removing clones. Consider the following sequence

write foo [0, 1024)
flush foo ->
  head: [0, 512) aaa, [512, 1024) bbb
  refcount(aaa)=1, refcount(bbb)=1
snapshot 10
write foo [0, 512) ->
  head:               [512, 1024) bbb
  10  : [0, 512) aaa, [512, 1024) bbb
  refcount(aaa)=1, refcount(bbb)=1
flush foo ->
  head: [0, 512) ccc, [512, 1024) bbb
  10  : [0, 512) aaa, [512, 1024) bbb
  refcount(aaa)=1, refcount(bbb)=1, refcount(ccc)=1
snapshot 20
write foo [0, 512) (same contents as the original write)
  head:               [512, 1024) bbb
  20  : [0, 512) ccc, [512, 1024) bbb
  10  : [0, 512) aaa, [512, 1024) bbb
  refcount(aaa)=?, refcount(bbb)=1
flush foo
  head: [0, 512) aaa, [512, 1024) bbb
  20  : [0, 512) ccc, [512, 1024) bbb
  10  : [0, 512) aaa, [512, 1024) bbb
  refcount(aaa)=?, refcount(bbb)=1, refcount(ccc)=1

What should be the refcount for aaa be at the end? By our above rule, it should be 2 since the two `aaa` refs are not contiguous. However, consider removing clone 20

initial:
  head: [0, 512) aaa, [512, 1024) bbb
  20  : [0, 512) ccc, [512, 1024) bbb
  10  : [0, 512) aaa, [512, 1024) bbb
  refcount(aaa)=2, refcount(bbb)=1, refcount(ccc)=1
trim 20
  head: [0, 512) aaa, [512, 1024) bbb
  10  : [0, 512) aaa, [512, 1024) bbb
  refcount(aaa)=?, refcount(bbb)=1, refcount(ccc)=0

At this point, our rule dictates that refcount(aaa) is 1. This means that removing 20 needs to check for refs held by the clones on either side which will then match.

See osd_types.h:object_manifest_t::calc_refs_to_drop_on_removal for the logic implementing this rule.

This seems complicated, but it gets us two valuable properties:

  1. The refcount change from make_writeable will not block on incrementing a ref

  2. We don’t need to load the object_manifest_t for every clone to determine how to handle removing one – just the ones immediately preceding and succeeding it.

All clone operations will need to consider adjacent chunk_maps when adding or removing references.

Cache/Tiering

There already exists a cache/tiering mechanism based on whiteouts. One goal here should ultimately be for this manifest machinery to provide a complete replacement.

See cache-pool.rst

The manifest machinery already shares some code paths with the existing cache/tiering code, mainly stat_flush.

In no particular order, here’s in incomplete list of things that need to be wired up to provide feature parity:

  • Online object access information: The osd already has pool configs for maintaining bloom filters which provide estimates of access recency for objects. We probably need to modify this to permit hitset maintenance for a normal pool – there are already CEPH_OSD_OP_PG_HITSET* interfaces for querying them.

  • Tiering agent: The osd already has a background tiering agent which would need to be modified to instead flush and evict using manifests.

  • Use exiting existing features regarding the cache flush policy such as histset, age, ratio. - hitset - age, ratio, bytes

  • Add tiering-mode to manifest-tiering - Writeback - Read-only

Data Structures

Each RADOS object contains an object_manifest_t embedded within the object_info_t (see osd_types.h):

struct object_manifest_t {
        enum {
                TYPE_NONE = 0,
                TYPE_REDIRECT = 1,
                TYPE_CHUNKED = 2,
        };
        uint8_t type;  // redirect, chunked, ...
        hobject_t redirect_target;
        std::map<uint64_t, chunk_info_t> chunk_map;
}

The type enum reflects three possible states an object can be in:

  1. TYPE_NONE: normal RADOS object

  2. TYPE_REDIRECT: object payload is backed by a single object specified by redirect_target

  3. TYPE_CHUNKED: object payload is distributed among objects with size and offset specified by the ``chunk_map. chunk_map maps the offset of the chunk to a chunk_info_t as shown below, also specifying the length, target OID, and flags.

struct chunk_info_t {
  typedef enum {
    FLAG_DIRTY = 1,
    FLAG_MISSING = 2,
    FLAG_HAS_REFERENCE = 4,
    FLAG_HAS_FINGERPRINT = 8,
  } cflag_t;
  uint32_t offset;
  uint32_t length;
  hobject_t oid;
  cflag_t flags;   // FLAG_*

FLAG_DIRTY at this time can happen if an extent with a fingerprint is written. This should be changed to drop the fingerprint instead.

Request Handling

Similarly to cache/tiering, the initial touchpoint is maybe_handle_manifest_detail.

For manifest operations listed below, we return NOOP and continue onto dedicated handling within do_osd_ops.

For redirect objects which haven’t been promoted (apparently oi.size > 0 indicates that it’s present?) we proxy reads and writes.

For reads on TYPE_CHUNKED, if can_proxy_chunked_read (basically, all of the ops are reads of extents in the object_manifest_t chunk_map), we proxy requests to those objects.

RADOS Interface

To set up deduplication one must provision two pools. One will act as the base pool and the other will act as the chunk pool. The base pool need to be configured with the fingerprint_algorithm option as follows.

ceph osd pool set $BASE_POOL fingerprint_algorithm sha1|sha256|sha512
--yes-i-really-mean-it

Create objects

rados -p base_pool put foo ./foo
rados -p chunk_pool put foo-chunk ./foo-chunk

Make a manifest object

rados -p base_pool set-chunk foo $START_OFFSET $END_OFFSET --target-pool chunk_pool foo-chunk $START_OFFSET --with-reference

Operations:

  • set-redirect

    Set a redirection between a base_object in the base_pool and a target_object in the target_pool. A redirected object will forward all operations from the client to the target_object.

    void set_redirect(const std::string& tgt_obj, const IoCtx& tgt_ioctx,
                  uint64_t tgt_version, int flag = 0);
    
    rados -p base_pool set-redirect <base_object> --target-pool <target_pool>
     <target_object>
    

    Returns ENOENT if the object does not exist (TODO: why?) Returns EINVAL if the object already is a redirect.

    Takes a reference to target as part of operation, can possibly leak a ref if the acting set resets and the client dies between taking the ref and recording the redirect.

    Truncates object, clears omap, and clears xattrs as a side effect.

    At the top of do_osd_ops, does not set user_modify.

    This operation is not a user mutation and does not trigger a clone to be created.

    There are two purposes of set_redirect:

    1. Redirect all operation to the target object (like proxy)

    2. Cache when tier_promote is called (redirect will be cleared at this time).

  • set-chunk

    Set the chunk-offset in a source_object to make a link between it and a target_object.

    void set_chunk(uint64_t src_offset, uint64_t src_length, const IoCtx& tgt_ioctx,
               std::string tgt_oid, uint64_t tgt_offset, int flag = 0);
    
    rados -p base_pool set-chunk <source_object> <offset> <length> --target-pool
     <caspool> <target_object> <target-offset>
    

    Returns ENOENT if the object does not exist (TODO: why?) Returns EINVAL if the object already is a redirect. Returns EINVAL if on ill-formed parameter buffer. Returns ENOTSUPP if existing mapped chunks overlap with new chunk mapping.

    Takes references to targets as part of operation, can possibly leak refs if the acting set resets and the client dies between taking the ref and recording the redirect.

    Truncates object, clears omap, and clears xattrs as a side effect.

    This operation is not a user mutation and does not trigger a clone to be created.

    TODO: SET_CHUNK appears to clear the manifest unconditionally if it’s not chunked.

    if (!oi.manifest.is_chunked()) {
      oi.manifest.clear();
    }
    
  • evict-chunk

    Clears an extent from an object leaving only the manifest link between it and the target_object.

    void evict_chunk(
      uint64_t offset, uint64_t length, int flag = 0);
    
    rados -p base_pool evict-chunk <offset> <length> <object>
    

    Returns EINVAL if the extent is not present in the manifest.

    Note: this does not exist yet.

  • tier-promote

    Promotes the object ensuring that subsequent reads and writes will be local

    void tier_promote();
    
    rados -p base_pool tier-promote <obj-name>
    

    Returns ENOENT if the object does not exist

    For a redirect manifest, copies data to head.

    TODO: Promote on a redirect object needs to clear the redirect.

    For a chunked manifest, reads all MISSING extents into the base pool, subsequent reads and writes will be served from the base pool.

    Implementation Note: For a chunked manifest, calls start_copy on itself. The resulting copy_get operation will issue reads which will then be redirected by the normal manifest read machinery.

    Does not set the user_modify flag.

    Future work will involve adding support for specifying a clone_id.

  • unset-manifest

    Unset the manifest info in the object that has manifest.

    void unset_manifest();
    
    rados -p base_pool unset-manifest <obj-name>
    

    Clears manifest chunks or redirect. Lazily releases references, may leak.

    do_osd_ops seems not to include it in the user_modify=false ignorelist, and so will trigger a snapshot. Note, this will be true even for a redirect though SET_REDIRECT does not flip user_modify. This should be fixed – unset-manifest should not be a user_modify.

  • tier-flush

    Flush the object which has chunks to the chunk pool.

    void tier_flush();
    
    rados -p base_pool tier-flush <obj-name>
    

    Included in the user_modify=false ignorelist, does not trigger a clone.

    Does not evict the extents.

ceph-dedup-tool

ceph-dedup-tool has two features: finding an optimal chunk offset for dedup chunking and fixing the reference count (see ./refcount.rst).

  • Find an optimal chunk offset

    1. Fixed chunk

    To find out a fixed chunk length, you need to run the following command many times while changing the chunk_size.

    ceph-dedup-tool --op estimate --pool $POOL --chunk-size chunk_size
      --chunk-algorithm fixed --fingerprint-algorithm sha1|sha256|sha512
    
    1. Rabin chunk(Rabin-Karp algorithm)

    Rabin-Karp is a string-searching algorithm based on a rolling hash. But a rolling hash is not enough to do deduplication because we don’t know the chunk boundary. So, we need content-based slicing using a rolling hash for content-defined chunking. The current implementation uses the simplest approach: look for chunk boundaries by inspecting the rolling hash for pattern (like the lower N bits are all zeroes).

    Users who want to use deduplication need to find an ideal chunk offset. To find out ideal chunk offset, users should discover the optimal configuration for their data workload via ceph-dedup-tool. This information will then be used for object chunking through the set-chunk API.

    ceph-dedup-tool --op estimate --pool $POOL --min-chunk min_size
      --chunk-algorithm rabin --fingerprint-algorithm rabin
    

    ceph-dedup-tool has many options to utilize rabin chunk. These are options for rabin chunk.

               --mod-prime <uint64_t>
               --rabin-prime <uint64_t>
               --pow <uint64_t>
               --chunk-mask-bit <uint32_t>
               --window-size <uint32_t>
               --min-chunk <uint32_t>
               --max-chunk <uint64_t>
    
    Users need to refer following equation to use above options for ``rabin chunk``. ::
    
               rabin_hash =
                 (rabin_hash * rabin_prime + new_byte - old_byte * pow) % (mod_prime)
    
    1. Fixed chunk vs content-defined chunk

    Content-defined chunking may or not be optimal solution. For example,

    Data chunk A : abcdefgabcdefgabcdefg

    Let’s think about Data chunk A’s deduplication. The ideal chunk offset is from 1 to 7 (abcdefg). So, if we use fixed chunk, 7 is optimal chunk length. But, in the case of content-based slicing, the optimal chunk length could not be found (dedup ratio will not be 100%). Because we need to find optimal parameter such as boundary bit, window size and prime value. This is as easy as fixed chunk. But, content defined chunking is very effective in the following case.

    Data chunk B : abcdefgabcdefgabcdefg

    Data chunk C : Tabcdefgabcdefgabcdefg

  • Fix reference count

    The key idea behind of reference counting for dedup is false-positive, which means (manifest object (no ref),, chunk object(has ref)) happen instead of (manifest object (has ref), chunk 1(no ref)). To fix such inconsistencies, ceph-dedup-tool supports chunk_scrub.

    ceph-dedup-tool --op chunk_scrub --chunk_pool $CHUNK_POOL