2 HAMMER2 DESIGN DOCUMENT
9 * These features have been speced in the media structures.
11 * Implementation work has begun.
13 * Filesytem core is now operational, cluster messaging links are primitive
14 but work (and are fully encrypted). Work continues on the block allocator
15 and work has not yet begun on copies, block-encryption, block-compression,
16 mirroring, or quorum/cluster ops.
18 * Obviously a fully functional filesystem is not yet ready but once the
19 freemap and the backend garbage collector is implemented the HAMMER2
20 filesystem will be usable. Missing many features, but usable.
22 * Design of all media elements is complete.
26 * Multiple roots (allowing snapshots to be mounted). This is implemented
27 via the super-root concept. When mounting a HAMMER2 filesystem you specify
28 a device path and a directory name in the super-root. (HAMMER1 had only
31 * Roots are really no different from snapshots (HAMMER1 distinguished between
32 its root mount and its PFS's. HAMMER2 does not).
34 * Snapshots are writable (in HAMMER1 snapshots were read-only).
36 * Snapshots are explicit but trivial to create. In HAMMER1 snapshots were
37 both explicit and fine-grained/automatic. HAMMER2 does not implement
38 automatic fine-grained snapshots. H2 snapshots are cheap enough that you
39 can create fine-grained snapshots if you desire.
41 * HAMMER2 flushes formalized a synchronization point for the flush, wait
42 for all running modifying operations to complete to memory (not to disk)
43 while temporarily stalling new modifying operation initiations. The
44 flush is then able to proceed concurrent with unstalling and allowing
45 new modifying operations to run.
47 * The flush is fully meta-data-synchronized in HAMMER2. In HAMMER1 it was
48 possible for flushes to bisect inode creation vs directory entry creation
49 and to create problems with directory renames. HAMMER2 has no issues with
50 any of these. Dealing with data synchronization is another matter but
51 it should be possible to address explcit write()'s properly. mmap()'d
52 R+W data... not so easy.
54 * Directory sub-hierarchy-based quotas for space and inode usage tracking.
55 Any directory can be used.
57 * Low memory footprint. Except for the volume header, the buffer cache
58 is completely asynchronous and dirty buffers can be retired by the OS
59 directly to backing store with no further interactions with the filesystem.
61 * Incremental queueless mirroring / mirroring-streams. Because HAMMER2 is
62 block-oriented and copy-on-write each blockref tracks both direct
63 modifications to the referenced data via (modify_tid) and indirect
64 modifications to the referenced data or any sub-tree via (mirror_tid).
65 This makes it possible to do an incremental scan of meta-data that covers
66 only changes made since the mirror_tid recorded in a prior-run.
68 This feature is also intended to be used to locate recently allocated
69 blocks and thus be able to fixup the freemap after a crash.
71 HAMMER2 mirroring works a bit differently than HAMMER1 mirroring in
72 that HAMMER2 does not keep track of 'deleted' records. Instead any
73 recursion by the mirroring code which finds that (modify_tid) has
74 been updated must also send the direct block table or indirect block
75 table state it winds up recursing through so the target can check
76 similar key ranges and locate elements to be deleted. This can be
77 avoided if the mirroring stream is mostly caught up in that very recent
78 deletions will be cached in memory and can be queried, allowing shorter
79 record deletions to be passed in the stream instead.
81 * Will support multiple compression algorithms configured on subdirectory
82 tree basis and on a file basis. Up to 64K block compression will be used.
83 Only compression ratios near powers of 2 that are at least 2:1 (e.g. 2:1,
84 4:1, 8:1, etc) will work in this scheme because physical block allocations
85 in HAMMER2 are always power-of-2.
87 Compression algorithm #0 will mean no compression and no zero-checking.
88 Compression algorithm #1 will mean zero-checking but no other compression.
89 Real compression will be supported starting with algorithm 2.
91 * Zero detection on write (writing all-zeros), which requires the data
92 buffer to be scanned, will be supported as compression algorithm #1.
93 This allows the writing of 0's to create holes and will be the default
94 compression algorithm for HAMMER2.
96 * Copies support for redundancy. Each copy has its own blockref. The
97 blockrefs representing the copies must exist within the same blockset
98 (set of 8 blockrefs), though I may relax this requirement in the
101 The design is such that the filesystem should be able to function at
102 full speed even if disks are pulled or inserted, as long as at least one
103 good copy is present. A background task will be needed to resynchronize
104 missing copies (or remove excessive copies in the case where the copies
105 value is reduced on a live filesystem).
107 Copies are specified using the same copyinfo[] array that is used to
108 specify cluster interconnections for PFS's.
110 * Clusterable with MESI cache coherency and dynamic granularity.
111 The media format for HAMMER1 was less condusive to logical clustering
112 than I had hoped so I was never able to get that aspect of my personal goals
113 working with HAMMER1. HAMMER2 effectively solves the issues that cropped
114 up with HAMMER1 (mainly that HAMMER1's B-Tree did not reflect the logical
115 file/directory hierarchy, making cache coherency very difficult).
117 * Hardlinks will be supported. All other standard features will be supported
118 too of course. Hardlinks in this sort of filesystem require significant
121 * The media blockref structure is now large enough to support up to a 192-bit
122 check value, which would typically be a cryptographic hash of some sort.
123 Multiple check value algorithms will be supported with the default being
124 a simple 32-bit iSCSI CRC.
126 * Fully verified deduplication will be supported and automatic (and
127 necessary in many respects).
129 * Non-verified de-duplication will be supported as a configurable option on
130 a file or subdirectory tree. Non-verified deduplication would use the
131 largest available check code (192 bits) and not bother to verify data
132 matches during the dedup pass, which is necessary on extremely large
133 filesystems with a great deal of deduplicable data (as otherwise a large
134 chunk of the media would have to be read to implement the dedup).
136 This feature is intended only for those files where occassional corruption
137 is ok, such as in a large data store of farmed web content.
141 HAMMER2 generally implements a copy-on-write block design for the filesystem,
142 which is very different from HAMMER1's B-Tree design. Because the design
143 is copy-on-write it can be trivially snapshotted simply by referencing an
144 existing block, and because the media structures logically match a standard
145 filesystem directory/file hierarchy snapshots and other similar operations
146 can be trivially performed on an entire subdirectory tree at any level in
149 The copy-on-write nature of the filesystem implies that any modification
150 whatsoever will have to eventually synchronize new disk blocks all the way
151 to the super-root of the filesystem and the volume header itself. This forms
152 the basis for crash recovery. All disk writes are to new blocks except for
153 the volume header, thus allowing all writes to run concurrently except for
154 the volume header update at the end.
156 Clearly this method requires intermediate modifications to the chain to be
157 cached so multiple modifications can be aggregated prior to being
158 synchronized. One advantage, however, is that the cache can be flushed at
159 any time WITHOUT having to allocate yet another new block when further
160 modifications are made as long as the volume header has not yet been flushed.
161 This means that buffer cache overhead is very well bounded and can handle
162 filesystem operations of any complexity even on boxes with very small amounts
165 I intend to implement a shortcut to make fsync()'s run fast, and that is to
166 allow deep updates to blockrefs to shortcut to auxillary space in the
167 volume header to satisfy the fsync requirement. The related blockref is
168 then recorded when the filesystem is mounted after a crash and the update
169 chain is reconstituted when a matching blockref is encountered again during
170 normal operation of the filesystem.
172 Basically this means that no real work needs to be done at mount-time
175 Directories are hashed, and another major design element is that directory
176 entries ARE INODES. They are one and the same. In addition to directory
177 entries being inodes the data for very small files (512 bytes or smaller)
178 can be directly embedded in the inode (overloaded onto the same space that
179 the direct blockref array uses). This should result in very high
182 Inode numbers are not spatially referenced, which complicates NFS servers
183 but doesn't complicate anything else. The inode number is stored in the
184 inode itself, an absolutely necessary feature in order to support the
185 hugely flexible snapshots that we want to have in HAMMER2.
187 DISK I/O OPTIMIZATIONS
189 The freemap implements a 1KB allocation resolution. The minimum I/O size
190 is 16KB. HAMMER2 typically implements 16KB and 64KB physical I/O sizes
191 and will cluster larger I/O's.
193 Each 2MB segment managed by the freemap handles just one particular
194 physical I/O size. Typically this means that inodes, small data, and
195 initial (small) indirect blocks get clustered together. Also large 64KB
196 file-data and indirect blocks get clustered together.
200 Hardlinks are a particularly sticky problem for HAMMER2 due to the lack of
201 a spatial reference to the inode number. We do not want to have to have
202 an index of inode numbers for any basic HAMMER2 feature if we can help it.
204 Hardlinks are handled by placing the inode for a multiply-hardlinked file
205 in the closest common parent directory. If "a/x" and "a/y" are hardlinked
206 the inode for the hardlinked file will be placed in directory "a", e.g.
207 "a/3239944", but it will be invisible and will be in an out-of-band namespace.
208 The directory entries "a/x" and "a/y" will be given the same inode number
209 but in fact just be placemarks that cause HAMMER2 to recurse upwards through
210 the directory tree to find the invisible inode number.
212 Because directories are hashed and a different namespace (hash key range)
213 is used for hardlinked inodes, standard directory scans are able to trivially
214 skip this invisible namespace and inode-specific lookups can restrict their
215 lookup to within this space.
217 The nature of snapshotting makes handling link-count 2->1 and 1->2 cases
218 trivial. Basically the inode media structure is copied as needed to break-up
219 or re-form the standard directory entry/inode. There are no backpointers in
220 HAMMER2 and no reference counts on the blocks (see FREEMAP NOTES below), so
221 it is an utterly trivial operation.
225 In order to implement fast snapshots (and writable snapshots for that
226 matter), HAMMER2 does NOT ref-count allocations. The freemap which
227 is still under design just won't do that. All the freemap does is
228 keep track of 100% free blocks.
230 This not only trivializes all the snapshot features it also trivializes
231 hardlink handling and solves the problem of keeping the freemap sychronized
232 in the event of a crash. Now all we have to do after a crash is make
233 sure blocks allocated before the freemap was flushed are properly
234 marked as allocated in the allocmap. This is a trivial exercise using the
235 same algorithm the mirror streaming code uses (which is very similar to
236 HAMMER1)... an incremental meta-data scan that covers only the blocks that
237 might have been allocated between the last allocation map sync and now.
239 Thus the freemap does not have to be synchronized during a fsync().
241 The complexity is in figuring out what can be freed... that is, when one
242 can mark blocks in the freemap as being free. HAMMER2 implements this as
243 a background task which essentially must scan available meta-data to
244 determine which blocks are not being referenced.
246 Part of the ongoing design work is finding ways to reduce the scope of this
247 meta-data scan so the entire filesystem's meta-data does not need to be
248 scanned (though in tests with HAMMER1, even full meta-data scans have
249 turned out to be fairly low cost). In other words, its an area that we
250 can continue to improve on as the filesystem matures. Not only that, but
251 we can completely change the freemap algorithms without creating
252 incompatibilities (at worse simply having to require that a R+W mount do
253 a full meta-data scan when upgrading or downgrading the freemap algorithm).
257 Clustering, as always, is the most difficult bit but we have some advantages
258 with HAMMER2 that we did not have with HAMMER1. First, HAMMER2's media
259 structures generally follow the kernel's filesystem hiearchy. Second,
260 HAMMER2's writable snapshots make it possible to implement several forms
261 of multi-master clustering.
263 The mount device path you specify serves to bootstrap your entry into
264 the cluster. This can be local media or directly specify a network
265 cluster connection (or several). When a local media mount is used the
266 volume header is scanned for local copies and the best volume header is
267 selected from all available copies. Multiple devices may be specified for
270 The volume header on local media also contains cluster connection
271 specifications keyed by super-root pfsid. Network connections are
272 maintained to all targets. ALL ELEMENTS ARE TREATED ACCORDING TO TYPE
273 NO MATTER WHICH ONE YOU MOUNT FROM.
275 The actual networked cluster may be far larger than the elements you list
276 in the hammer2_copy_data[] array, but your machine will only make direct
277 connections as specified by the array.
279 In the simplest case you simply network a few machines together as ring 0
280 masters and each client connects directly to all the masters (and/or are
281 the masters themselves). Thus any quorum operation is straight-forward.
282 These master nodes are labeled 'ring 0'.
284 If you have too many clients to reasonably connect directly you set up
285 sub-clusters as satellites. This is called 'ring 1'. Ring 1 may contain
286 several sub-clusters. A client then connects to all the nodes in a
287 particular sub-cluster (typically 3). The quorum protocol runs as per
288 normal except that once the operation is resolved against the sub-cluster
289 an aggregation must be resolved against the master nodes (ring 0). The
290 sub-cluster does this for the client... all the client sees is the normal
291 quorum operation against the sub-cluster.
293 Since each node in the sub-cluster connects to all master nodes we get
294 a multiplication. If we set a reasonable upper limit of, say, 256
295 connections at each master node then ring 1 may contain 85 sub-clusters x 3
296 nodes in each sub-cluster.
298 In the most complex case when one wishes to support potentially millions
299 of clients then further fan-out is required into ring 2, ring 3, and
300 so forth. However, each sub-cluster in ring 2 must only connect to
301 1 sub-cluster in ring 1 (otherwise the cache state will become mightily
302 confused). Using reasonable metrics this will allow ring 2 to contain
303 85 * 85 = 7225 sub-clusters. At this point you could have 1000 clients
304 connect to each sub-cluster and support 7.2 million clients, but if that
305 isn't enough going to another ring will support 61M clients, and so forth.
307 Each ring imposes additional latencies for cache operations but the key
308 to making this work efficiently is that the satellite clusters can negotiate
309 coarse-grained cache coherency locks with the next lower ring and then
310 fan-out finer-grained locks to the next higher ring. Since caching can
311 occur anywhere (including on the connecting client), it is the cache
312 coherency lock that ultimately dictates efficiency and allows a client
313 (or satellite) to access large amoutns of data from local storage.
315 Modifying operations, particularly commits, also have higher latencies
316 when multiple rings are in use. In this situation it is possible to
317 short-cut localized operations by having competing clients connect to
318 to sub-clusters which are near each other topologically... having the
319 competing clients connect to the same sub-cluster would be the most optimal.
321 In addition, sub-clusters (typically in ring 1) can act in SOFT_MASTER mode
322 which allows the sub-cluster to acknowledge a full commit within its own
323 quorum only, and then resolve asynchronously to the masters in ring 0.
325 The nodes in these intermediate rings can be pure proxies with only memory
326 caches, use local media for persistent cache, or use local media to
327 completely slave the filesystem.
329 ADMIN - Media does not participate, administrative proxy only
330 CLIENT - Media does not participate, client only
331 CACHE - Media only acts as a persistent cache
332 COPY - Media only acts as a local copy
333 SLAVE - Media is a RO slave that can be mounted RW
335 SOFT_SLAVE - This is a SLAVE which can become writable when
336 the quorum is not available, but is not guaranteed
337 to be able to be merged back when the quorum becomes
338 available again. Elements which cannot be merged
339 back remain localized and writable until manual
340 or scripted intervention recombines them.
342 SOFT_MASTER - Similar to the above but can form a sub-cluster
343 and run the quorum protocol within the sub-cluster
344 to serve machines that connect to the sub-cluster
345 when the master cluster is not available.
347 The SOFT_MASTER nodes in a sub-cluster must be
348 fully interconnected with each other.
350 MASTER - This is a MASTER node in the quorum protocol.
352 The MASTER nodes in a cluster must be fully
353 interconnected with each other.
355 There are four major protocols:
359 This protocol is used between MASTER nodes to vote on operations
360 and resolve deadlocks.
362 This protocol is used between SOFT_MASTER nodes in a sub-cluster
363 to vote on operations, resolve deadlocks, determine what the latest
364 transaction id for an element is, and to perform commits.
368 This is the MESI sub-protocol which runs under the Quorum
369 protocol. This protocol is used to maintain cache state for
370 sub-trees to ensure that operations remain cache coherent.
372 Depending on administrative rights this protocol may or may
373 not allow a leaf node in the cluster to hold a cache element
374 indefinitely. The administrative controller may preemptively
375 downgrade a leaf with insufficient administrative rights
376 without giving it a chance to synchronize any modified state
381 The Quorum and Cache protocols only operate between MASTER
382 and SOFT_MASTER nodes. All other node types must use the
383 Proxy protocol to perform similar actions. This protocol
384 differs in that proxy requests are typically sent to just
385 one adjacent node and that node then maintains state and
386 forwards the request or performs the required operation.
387 When the link is lost to the proxy, the proxy automatically
388 forwards a deletion of the state to the other nodes based on
389 what it has recorded.
391 If a leaf has insufficient administrative rights it may not
392 be allowed to actually initiate a quorum operation and may only
393 be allowed to maintain partial MESI cache state or perhaps none
394 at all (since cache state can block other machines in the
395 cluster). Instead a leaf with insufficient rights will have to
396 make due with a preemptive loss of cache state and any allowed
397 modifying operations will have to be forwarded to the proxy which
398 continues forwarding it until a node with sufficient administrative
399 rights is encountered.
401 To reduce issues and give the cluster more breath, sub-clusters
402 made up of SOFT_MASTERs can be formed in order to provide full
403 cache coherent within a subset of machines and yet still tie them
404 into a greater cluster that they normally would not have such
405 access to. This effectively makes it possible to create a two
406 or three-tier fan-out of groups of machines which are cache-coherent
407 within the group, but perhaps not between groups, and use other
408 means to synchronize between the groups.
412 This is basically the physical media protocol.
414 There are lots of ways to implement multi-master environments using the
415 above core features but the implementation is going to be fairly complex
416 even with HAMMER2's feature set.
418 Keep in mind that modifications propagate all the way to the super-root
419 and volume header, so in any clustered arrangement the use of (modify_tid)
420 and (mirror_tid) is critical in determining the synchronization state of
421 portion(s) of the filesystem.
423 Specifically, since any modification propagates to the root the (mirror_tid)
424 in higher level directories is going to be in a constant state of flux. This
425 state of flux DOES NOT invalidate the cache state for these higher levels
426 of directories. Instead, the (modify_tid) is used on a node-by-node basis
427 to determine cache state at any given level, and (mirror_tid) is used to
428 determine whether any recursively underlying state is desynchronized.
429 The inode structure also has two additional transaction ids used to optimize
430 path lookups, stat, and directory lookup/scan operations.
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