add snap man page from brian

Reference: /n/atom/patch/applied/snapman
Date: Sat Mar 22 21:19:21 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/man/3/snap	Thu Jan  1 00:00:00 1970
+++ /sys/man/3/snap	Sat Mar 22 21:19:17 2014
@@ -0,0 +1,137 @@
+.TH SNAP 3
+.SH NAME
+snap \- Snaphot Device
+.SH SYNOPSIS
+.nf
+.B bind #ℙ /n/snap
+
+.B /n/snap/ctl
+
+.B /n/snap/...
+.SH DESCRIPTION
+.PP
+This device provides snapshot functionality layered on
+top of a file (normally the data file of another device).
+The active region of the storage space is presented as
+a data file named by a base name specified when the
+snapshot space is formatted.
+Individual snapshots are named by
+.BR basename.suffix .
+.PP
+When the system first comes up, devsnap presents a single file
+in its name space.
+The file,
+.BR ctl ,
+serves the conventional role of providing a control and status
+interface for the snap device.
+There are five commands that can be written to the control file:
+.TP
+.B bind:
+The command,
+.BR bind ,
+is used to attach an existing snapshotted store to the snap device.
+It takes a single argument which is the data file representing the
+storage space where snapshots are stored.
+E.g.
+.IP
+.B bind '#ℙ' /n/snap
+.IP
+.B echo bind /dev/sdC1/data > /n/snap/ctl
+.IP
+Devsnap scans the data file and creates a data file for each
+snapshot (including the active store) it finds there.
+.TP
+.B ream:
+The
+.B ream
+command is used to initially format a storage space where
+snapshots are to be managed.
+It takes two arguments, a name given to the active store and
+the path name of the data file where the snapshots are to
+be managed.
+E.g.
+.IP
+.B echo ream fs /dev/sdC1/data > /n/snap/ctl
+.TP
+.B revert:
+Reverting takes two arguments, the
+name of the active region and the name of the snapshot to which
+we should revert.
+E.g.
+.IP
+.B echo revert fs fs.20131119 > /n/snap/ctl
+.IP
+The effect of this command is to roll back the store to the same
+state it was in at the time that Snapshot
+.B fs.20131119
+was taken.
+.TP
+.B snap:
+Taking a snapshot is triggered by the
+.B snap
+command which takes two arguments.
+The first argument is the name of the active store to be
+snapshotted, and the second argument is the name to be
+given to the newly created snapshot.
+E.g.
+.IP
+.B echo snap fs fs.20131119 > /n/snap/ctl
+.TP
+.B unbind:
+The
+.B unbind
+command takes a single argument which is the path name
+for the storage space data file previously connected with
+a
+.B ream
+or
+.B bind
+command.
+E.g.
+.IP
+.B echo unbind /dev/sdC1/data > /n/snap/ctl
+.IP
+The effect of an
+.B unbind
+is the closing of the connection to the underlying device and the
+removal of the associated files from devsnap's name space.
+.PP
+Reads of the
+.B ctl
+file return a list of the snapshots and active stores currently known
+to devsnap.
+E.g.
+.IP
+.B 0 fs /dev/sdC1/data 1
+.B 10720 10849 10863
+.IP
+.B 1 fs.20131119 /dev/sdC1/data 3
+.B 10683 0 10683
+.IP
+.B 2 fs.201311191 /dev/sdC1/data 3
+.B 10720 10683 10849
+.PP
+The items on each line are: the internal slot number, the
+name of the snapshot, the path name to the underlying data
+store, the internal flags, the number of blocks in use, and the
+starting and ending block numbers for that snapshot region.
+.PP
+For each snapshot, devsnap provides a data file in its name
+space.
+E.g.
+.IP
+.B --rw-rw-r-- ℙ ... "            0" ... ctl
+.IP
+.B --rw-rw-r-- ℙ ... 1000204886016 ... fs
+.IP
+.B --r--r--r-- ℙ ...  "  11201937408" ... fs.20131119
+.IP
+.B --r--r--r-- ℙ ...  "  11240734720" ... fs.201311191
+.PP
+Note that the snapshots are given read-only permissions, and
+the active store is given read-write permissions.
+Also the sizes differ among the data files.
+The size of the active store is the size of the underlying data file.
+The size of a snapshot is the number of bytes used by that snapshot.
+.SH SOURCE
+.B /sys/src/9/port/devsnap.c