HAMMER filesystem update - design document
dillon at apollo.backplane.com
Wed Oct 10 12:36:44 PDT 2007
Ok, here's the final design document that I am now implementing.
Again, I expect most or all of these features to be ready and the
filesystem to be beta-quality by the December release.
(I) General Storage Abstraction
HAMMER uses a basic 16K filesystem buffer for all I/O. Buffers are
collected into clusters, cluster are collected into volumes, and a
single HAMMER filesystem may span multiple volumes.
HAMMER maintains a small hinted radix tree for block management in
each layer. A small radix tree in the volume header manages cluster
allocations within a volume, one in the cluster header manages buffer
allocations within a cluster, and most buffers (pure data buffers
excepted) will embed a small tree to manage item allocations within
Volumes are typically specified as disk partitions, with one volume
designated as the root volume containing the root cluster. The root
cluster does not need to be contained in volume 0 nor does it have to
be located at any particular offset.
Data can be migrated on a cluster-by-cluster or volume-by-volume basis
and any given volume may be expanded or contracted while the filesystem
is live. Whole volumes can be added and (with appropriate data
HAMMER's storage management limits it to 32768 volumes, 32768 clusters
per volume, and 32768 16K filesystem buffers per cluster. A volume
is thus limited to 16TB and a HAMMER filesystem as a whole is limited
to 524288TB. HAMMER's on-disk structures are designed to allow future
expansion through expansion of these limits. In particular, the volume
id is intended to be expanded to a full 32 bits in the future and using
a larger buffer size will also greatly increase the cluster and volume
size limitations by increasing the number of elements the buffer-
restricted radix trees can manage.
HAMMER breaks all of its information down into objects and records.
Records have a creation and deletion transaction id which allows HAMMER
to maintain a historical store. Information is only physically deleted
based on the data retention policy. Those portions of the data retention
policy affecting near-term modifications may be acted upon by the live
filesystem but all historical vacuuming is handled by a helper process.
All information in a HAMMER filesystem is CRCd to detect corruption.
(II) Filesystem Object Topology
The objects and records making up a HAMMER filesystem is organized into
a single, unified B-Tree. Each cluster maintains a B-Tree of the
records contained in that cluster and a unified B-Tree is constructed by
linking clusters together. HAMMER issues PUSH and PULL operations
internally to open up space for new records and to balance the global
B-Tree. These operations may have the side effect of allocating
new clusters or freeing clusters which become unused.
B-Tree operations tend to be limited to a single cluster. That is,
the B-Tree insertion and deletion algorithm is not extended to the
whole unified tree. If insufficient space exists in a cluster HAMMER
will allocate a new cluster, PUSH a portion of the existing
cluster's record store to the new cluster, and link the existing
cluster's B-Tree to the new one.
Because B-Tree operations tend to be restricted and because HAMMER tries
to avoid balancing clusters in the critical path, HAMMER employs a
background process to keep the topology as a whole in balance. One
side effect of this is that HAMMER is fairly loose when it comes to
inserting new clusters into the topology.
HAMMER objects revolve around the concept of an object identifier.
The obj_id is a 64 bit quantity which uniquely identifies a filesystem
object for the entire life of the filesystem. This uniqueness allows
backups and mirrors to retain varying amounts of filesystem history by
removing any possibility of conflict through identifier reuse. HAMMER
typically iterates object identifiers sequentially and expects to never
run out. At a creation rate of 100,000 objects per second it would
take HAMMER around 6 million years to run out of identifier space.
The characteristics of the HAMMER obj_id also allow HAMMER to operate
in a multi-master clustered environment.
A filesystem object is made up of records. Each record references a
variable-length store of related data, a 64 bit key, and a creation
and deletion transaction id which is indexed along with the key.
HAMMER utilizes a 64 bit key to index all records. Regular files use
the base data offset of the record as the key while directories use a
namekey hash as the key and store one directory entry per record. For
all intents and purposes a directory can store an unlimited number of
HAMMER is also capable of associating any number of out-of-band
attributes with a filesystem object using a separate key space. This
key space may be used for extended attributes, ACLs, and anything else
the user desires.
(III) Access to historical information
A HAMMER filesystem can be mounted with an as-of date to access a
snapshot of the system. Snapshots do not have to be explicitly taken
but are instead based on the retention policy you specify for any
given HAMMER filesystem. It is also possible to access individual files
or directories (and their contents) using an as-of extension on the
HAMMER uses the transaction ids stored in records to present a snapshot
view of the filesystem as-of any time in the past, with a granularity
based on the retention policy chosen by the system administrator.
feature also effectively implements file versioning.
(IV) Mirrors and Backups
HAMMER is organized in a way that allows an information stream to be
generated for mirroring and backup purposes. This stream includes all
historical information available in the source. No queueing is required
so there is no limit to the number of mirrors or backups you can have
and no limit to how long any given mirror or backup can be taken offline.
Resynchronization of the stream is not considered to be an expensive
Mirrors and backups are maintained logically, not physically, and may
have their own, independant retention polcies. For example, your live
filesystem could have a fairly rough retention policy, even none at all,
then be streamed to an on-site backup and from there to an off-site
backup, each with different retention policies.
(V) Transactions and Recovery
HAMMER implement an instant-mount capability and will recover information
on a cluster-by-cluster basis as it is being accessed.
HAMMER numbers each record it lays down and stores a synchronization
point in the cluster header. Clusters are synchronously marked 'open'
when undergoing modification. If HAMMER encounters a cluster which is
unexpectedly marked open it will perform a recovery operation on the
cluster and throw away any records beyond the synchronization point.
HAMMER supports a userland transactional facility. Userland can query
the current (filesystem wide) transaction id, issue numerous operations
and on recovery can tell HAMMER to revert all records with a greater
transaction id for any particular set of files. Multiple userland
applications can use this feature simultaniously as long as the files
they are accessing do not overlap. It is also possible for userland
to set up an ordering dependancy and maintain completely asynchronous
operation while still being able to guarentee recovery to a fairly
recent transaction id.
(VI) Database files
HAMMER uses 64 bit keys internally and makes key-based files directly
available to userland. Key-based files are not regular files and do not
operate using a normal data offset space.
You cannot copy a database file using a regular file copier. The
file type will not be S_IFREG but instead will be S_IFDB. The file
must be opened with O_DATABASE. Reads which normally seek the file
forward will instead iterate through the records and lseek/qseek can
be used to acquire or set the key prior to the read/write operation.
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