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32 .\" $DragonFly: src/share/man/man5/hammer.5,v 1.15 2008/11/02 18:56:47 swildner Exp $
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34 .Dd November 2, 2008
35 .Os
36 .Dt HAMMER 5
37 .Sh NAME
38 .Nm HAMMER
39 .Nd HAMMER file system
40 .Sh SYNOPSIS
41 To compile this driver into the kernel,
42 place the following line in your
43 kernel configuration file:
44 .Bd -ragged -offset indent
45 .Cd options HAMMER
46 .Ed
47 .Pp
48 Alternatively, to load the driver as a
49 module at boot time, place the following line in
50 .Xr loader.conf 5 :
51 .Bd -literal -offset indent
52 hammer_load="YES"
53 .Ed
54 .Pp
55 To mount via
56 .Xr fstab 5 :
57 .Bd -literal -offset indent
58 /dev/ad0s1d[:/dev/ad1s1d:...] /mnt hammer rw 2 0
59 .Ed
60 .Sh DESCRIPTION
61 The
62 .Nm
63 file system provides facilities to store file system data onto disk devices
64 and is intended to replace
65 .Xr ffs 5
66 as the default file system for
67 .Dx .
68 Among its features are instant crash recovery,
69 large file systems spanning multiple volumes,
70 data integrity checking,
71 fine grained history retention,
72 mirroring capability, and pseudo file systems.
73 .Pp
74 All functions related to managing
75 .Nm
76 file systems are provided by the
77 .Xr newfs_hammer 8 ,
78 .Xr mount_hammer 8 ,
79 .Xr hammer 8 ,
80 and
81 .Xr undo 1
82 utilities.
83 .Pp
84 For a more detailed introduction refer to the paper and slides listed in the
85 .Sx SEE ALSO
86 section.
87 For some common usages of
88 .Nm
89 see the
90 .Sx EXAMPLES
91 section below.
92 .Ss Instant Crash Recovery
93 After a non-graceful system shutdown,
94 .Nm
95 file systems will be brought back into a fully coherent state
96 when mounting the file system, usually within a few seconds.
97 .Ss Large File Systems & Multi Volume
98 A
99 .Nm
100 file system can span up to 256 volumes.
101 Each volume occupies a
102 .Dx
103 disk slice or partition, or another special file,
104 and can be up to 4096 TB in size.
105 For volumes over 2 TB in size
106 .Xr gpt 8
107 and
108 .Xr disklabel64 8
109 normally need to be used.
110 .Ss Data Integrity Checking
111 .Nm
112 has high focus on data integrity,
113 CRC checks are made for all major structures and data.
114 .Nm
115 snapshots implements features to make data integrity checking easier:
116 The atime and mtime fields are locked to the ctime for files accessed via a snapshot.
117 The
118 .Fa st_dev
119 field is based on the PFS
120 .Ar shared-uuid
121 and not on any real device.
122 This means that archiving the contents of a snaphot with e.g.\&
123 .Xr tar 1
124 and piping it to something like
125 .Xr md5 1
126 will yield a consistent result.
127 The consistency is also retained on mirroring targets.
128 .Ss Transaction IDs
129 The
130 .Nm
131 file system uses 64 bit, hexadecimal transaction IDs to refer to historical
132 file or directory data.
133 An ID has the
134 .Xr printf 3
135 format
136 .Li %#016llx ,
137 such as
138 .Li 0x00000001061a8ba6 .
139 .Pp
140 Related
141 .Xr hammer 8
142 commands:
143 .Ar synctid
144 .Ss History & Snapshots
145 History metadata on the media is written with every sync operation, so that
146 by default the resolution of a file's history is 30-60 seconds until the next
147 prune operation.
148 Prior versions of files or directories are generally accessible by appending
149 .Li @@
150 and a transaction ID to the name.
151 The common way of accessing history, however, is by taking snapshots.
152 .Pp
153 Snapshots are softlinks to prior versions of directories and their files.
154 Their data will be retained across prune operations for as long as the
155 softlink exists.
156 Removing the softlink enables the file system to reclaim the space
157 again upon the next prune & reblock operations.
158 .Pp
159 Related
160 .Xr hammer 8
161 commands:
162 .Ar cleanup ,
163 .Ar history ,
164 .Ar snapshot ;
165 see also
166 .Xr undo 1
167 .Ss Pruning & Reblocking
168 Pruning is the act of deleting file system history.
169 Only history used by the given snapshots and history from after the latest
170 snapshot will be retained.
171 All other history is deleted.
172 Reblocking will reorder all elements and thus defragment the file system and
173 free space for reuse.
174 After pruning a file system must be reblocked to recover all available space.
175 Reblocking is needed even when using the
176 .Ar nohistory
177 .Xr mount_hammer 8
178 option.
179 .Pp
180 Related
181 .Xr hammer 8
182 commands:
183 .Ar cleanup ,
184 .Ar prune ,
185 .Ar prune-everything ,
186 .Ar reblock ,
187 .Ar reblock-btree ,
188 .Ar reblock-inodes ,
189 .Ar reblock-dirs ,
190 .Ar reblock-data
191 .Ss Mirroring & Pseudo File Systems
192 In order to allow inode numbers to be duplicated on the slaves
193 .Nm Ap s
194 mirroring feature uses
195 .Dq Pseudo File Systems
196 (PFSs).
197 A
198 .Nm
199 file system supports up to 65535 PFSs.
200 Multiple slaves per master are supported, but multiple masters per slave
201 are not.
202 Slaves are always read-only.
203 Upgrading slaves to masters and downgrading masters to slaves are supported.
204 .Pp
205 It is recommended to use a
206 .Nm null
207 mount to access a PFS;
208 this way no tools are confused by the PFS root being a symlink
209 and inodes not being unique across a
210 .Nm
211 file system.
212 .Pp
213 Related
214 .Xr hammer 8
215 commands:
216 .Ar pfs-master ,
217 .Ar pfs-slave ,
218 .Ar pfs-cleanup ,
219 .Ar pfs-status ,
220 .Ar pfs-update ,
221 .Ar pfs-destroy ,
222 .Ar pfs-upgrade ,
223 .Ar pfs-downgrade ,
224 .Ar mirror-copy ,
225 .Ar mirror-stream ,
226 .Ar mirror-read ,
227 .Ar mirror-read-stream ,
228 .Ar mirror-write ,
229 .Ar mirror-dump
230 .Ss NFS Export
231 .Nm
232 file systems support NFS export.
233 NFS export of PFSs is done using
234 .Nm null
235 mounts.
236 For example, to export the PFS
237 .Pa /hammer/pfs/data ,
238 create a
239 .Nm null
240 mount, e.g.\& to
241 .Pa /hammer/data
242 and export the latter path.
243 .Pp
244 Don't export a directory containing a PFS (e.g.\&
245 .Pa /hammer/pfs
246 above).
247 Only
248 .Nm null
249 mount for PFS root
250 (e.g.\&
251 .Pa /hammer/data
252 above)
253 should be exported
254 (subdirectory may be escaped if exported).
255 .Sh EXAMPLES
256 .Ss Preparing the File System
257 To create and mount a
258 .Nm
259 file system use the
260 .Xr newfs_hammer 8
261 and
262 .Xr mount_hammer 8
263 commands.
264 Note that all
265 .Nm
266 file systems must have a unique name on a per-machine basis.
267 .Bd -literal -offset indent
268 newfs_hammer -L HOME /dev/ad0s1d
269 mount_hammer /dev/ad0s1d /home
270 .Ed
271 .Pp
272 Similarly, multi volume file systems can be created and mounted by
273 specifying additional arguments.
274 .Bd -literal -offset indent
275 newfs_hammer -L MULTIHOME /dev/ad0s1d /dev/ad1s1d
276 mount_hammer /dev/ad0s1d /dev/ad1s1d /home
277 .Ed
278 .Pp
279 Once created and mounted,
280 .Nm
281 file systems need periodic clean up making snapshots, pruning and reblocking,
282 in order to have access to history and file system not to fill up.
283 For this it is recommended to use the
284 .Xr hammer 8
285 .Ar cleanup
286 metacommand.
287 .Pp
288 By default,
289 .Dx
290 is set up to run
291 .Nm hammer Ar cleanup
292 nightly via
293 .Xr periodic 8 .
294 .Pp
295 It is also possible to perform these operations individually via
296 .Xr crontab 5 .
297 For example, to reblock the
298 .Pa /home
299 file system every night at 2:15 for up to 5 minutes:
300 .Bd -literal -offset indent
301 15 2 * * * hammer -c /var/run/HOME.reblock -t 300 reblock /home \e
302 >/dev/null 2>&1
303 .Ed
304 .Ss Snapshots
305 The
306 .Xr hammer 8
307 utility's
308 .Ar snapshot
309 command provides several ways of taking snapshots.
310 They all assume a directory where snapshots are kept.
311 .Bd -literal -offset indent
312 mkdir /snaps
313 hammer snapshot /home /snaps/snap1
314 (...after some changes in /home...)
315 hammer snapshot /home /snaps/snap2
316 .Ed
317 .Pp
318 The softlinks in
319 .Pa /snaps
320 point to the state of the
321 .Pa /home
322 directory at the time each snapshot was taken, and could now be used to copy
323 the data somewhere else for backup purposes.
324 .Pp
325 By default,
326 .Dx
327 is set up to create nightly snapshots of all
328 .Nm
329 file systems via
330 .Xr periodic 8
331 and to keep them for 60 days.
332 .Ss Pruning
333 A snapshot directory is also the argument to the
334 .Xr hammer 8 Ap s
335 .Ar prune
336 command which frees historical data from the file system that is not
337 pointed to by any snapshot link and is not from after the latest snapshot.
338 .Bd -literal -offset indent
339 rm /snaps/snap1
340 hammer prune /snaps
341 .Ed
342 .Ss Mirroring
343 Mirroring can be set up using
344 .Nm Ap s
345 pseudo file systems.
346 To associate the slave with the master its shared UUID should be set to
347 the master's shared UUID as output by the
348 .Nm hammer Ar pfs-master
349 command.
350 .Bd -literal -offset indent
351 hammer pfs-master /home/pfs/master
352 hammer pfs-slave /home/pfs/slave shared-uuid=<master's shared uuid>
353 .Ed
354 .Pp
355 The
356 .Pa /home/pfs/slave
357 link is unusable for as long as no mirroring operation has taken place.
358 .Pp
359 To mirror the master's data, either pipe a
360 .Fa mirror-read
361 command into a
362 .Fa mirror-write
363 or, as a short-cut, use the
364 .Fa mirror-copy
365 command (which works across a
366 .Xr ssh 1
367 connection as well).
368 Initial mirroring operation has to be done to the PFS path (as
369 .Xr mount_null 8
370 can't access it yet).
371 .Bd -literal -offset indent
372 hammer mirror-copy /home/pfs/master /home/pfs/slave
373 .Ed
374 .Pp
375 After this initial step
376 .Nm null
377 mount can be setup for
378 .Pa /home/pfs/slave .
379 Further operations can use
380 .Nm null
381 mounts.
382 .Bd -literal -offset indent
383 mount_null /home/pfs/master /home/master
384 mount_null /home/pfs/slave /home/slave
386 hammer mirror-copy /home/master /home/slave
387 .Ed
388 .Ss NFS Export
389 To NFS export from the
390 .Nm
391 file system
392 .Pa /hammer
393 the directory
394 .Pa /hammer/non-pfs
395 without PFSs, and the PFS
396 .Pa /hammer/pfs/data ,
397 the latter is null mounted to
398 .Pa /hammer/data .
399 .Pp
400 Add to
401 .Pa /etc/fstab
402 (see
403 .Xr fstab 5 ) :
404 .Bd -literal -offset indent
405 /hammer/pfs/data /hammer/data null rw
406 .Ed
407 .Pp
408 Add to
409 .Pa /etc/exports
410 (see
411 .Xr exports 5 ) :
412 .Bd -literal -offset indent
413 /hammer/non-pfs
414 /hammer/data
415 .Ed
416 .Sh SEE ALSO
417 .Xr md5 1 ,
418 .Xr tar 1 ,
419 .Xr undo 1 ,
420 .Xr ffs 5 ,
421 .Xr disklabel64 8 ,
422 .Xr gpt 8 ,
423 .Xr hammer 8 ,
424 .Xr mount_hammer 8 ,
425 .Xr mount_null 8 ,
426 .Xr newfs_hammer 8
427 .Rs
428 .%A Matthew Dillon
429 .%D June 2008
430 .%O http://www.dragonflybsd.org/hammer/hammer.pdf
431 .%T "The HAMMER Filesystem"
432 .Re
433 .Rs
434 .%A Matthew Dillon
435 .%D October 2008
436 .%O http://www.dragonflybsd.org/hammer/nycbsdcon/
437 .%T "Slideshow from NYCBSDCon 2008"
438 .Re
439 .Sh FILESYSTEM PERFORMANCE
440 The
441 .Nm
442 file system has a front-end which processes VNOPS and issues necessary
443 block reads from disk, and a back-end which handles meta-data updates
444 on-media and performs all meta-data write operations. Bulk file write
445 operations are handled by the front-end.
446 Because
447 .Nm
448 defers meta-data updates virtually no meta-data read operations will be
449 issued by the frontend while writing large amounts of data to the filesystem
450 or even when creating new files or directories, and even though the
451 kernel prioritizes reads over writes the fact that writes are cached by
452 the drive itself tends to lead to excessive priority given to writes.
453 .Pp
454 There are four bioq sysctls which can be adjusted to give reads a higher
455 priority:
456 .Bd -literal -offset indent
457 kern.bioq_reorder_minor_bytes: 262144
458 kern.bioq_reorder_burst_bytes: 3000000
459 kern.bioq_reorder_minor_interval: 5
460 kern.bioq_reorder_burst_interval: 60
461 .Ed
462 .Pp
463 If a higher read priority is desired it is recommended that the
464 .Fa kern.bioq_reorder_minor_interval
465 be increased to 15, 30, or even 60, and the
466 .Fa kern.bioq_reorder_burst_bytes
467 be decreased to 262144 or 524288.
468 .Sh HISTORY
469 The
470 .Nm
471 file system first appeared in
472 .Dx 1.11 .
473 .Sh AUTHORS
474 .An -nosplit
475 The
476 .Nm
477 file system was designed and implemented by
478 .An Matthew Dillon Aq dillon@backplane.com .
479 This manual page was written by
480 .An Sascha Wildner .