diff --git a/tests/simplestorerepo.py b/tests/simplestorerepo.py
new file mode 100644
--- /dev/null
+++ b/tests/simplestorerepo.py
@@ -0,0 +1,589 @@
+# simplestorerepo.py - Extension that swaps in alternate repository storage.
+#
+# Copyright 2018 Gregory Szorc <gregory.szorc@gmail.com>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from __future__ import absolute_import
+
+from mercurial.i18n import _
+from mercurial.node import (
+    bin,
+    hex,
+    nullid,
+    nullrev,
+)
+from mercurial.thirdparty import (
+    cbor,
+)
+from mercurial import (
+    ancestor,
+    error,
+    filelog,
+    mdiff,
+    pycompat,
+    revlog,
+)
+
+# Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
+# extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
+# be specifying the version(s) of Mercurial they are tested with, or
+# leave the attribute unspecified.
+testedwith = 'ships-with-hg-core'
+
+def validatenode(node):
+    if isinstance(node, int):
+        raise ValueError('expected node; got int')
+
+    if len(node) != 20:
+        raise ValueError('expected 20 byte node')
+
+def validaterev(rev):
+    if not isinstance(rev, int):
+        raise ValueError('expected int')
+
+class filestorage(object):
+    """Implements storage for a tracked path.
+
+    Data is stored in the VFS in a directory corresponding to the tracked
+    path.
+
+    Index data is stored in an ``index`` file using CBOR.
+
+    Fulltext data is stored in files having names of the node.
+    """
+
+    def __init__(self, svfs, path):
+        self._svfs = svfs
+        self._path = path
+
+        self._storepath = b'/'.join([b'data', path])
+        self._indexpath = b'/'.join([self._storepath, b'index'])
+
+        indexdata = self._svfs.tryread(self._indexpath)
+        if indexdata:
+            indexdata = cbor.loads(indexdata)
+
+        self._indexdata = indexdata or []
+        self._indexbynode = {}
+        self._indexbyrev = {}
+        self.index = []
+        self._refreshindex()
+
+        # This is used by changegroup code :/
+        self._generaldelta = True
+        self.storedeltachains = False
+
+        self.version = 1
+
+    def _refreshindex(self):
+        self._indexbynode.clear()
+        self._indexbyrev.clear()
+        self.index = []
+
+        for i, entry in enumerate(self._indexdata):
+            self._indexbynode[entry[b'node']] = entry
+            self._indexbyrev[i] = entry
+
+        self._indexbynode[nullid] = {
+            b'node': nullid,
+            b'p1': nullid,
+            b'p2': nullid,
+            b'linkrev': nullrev,
+            b'flags': 0,
+        }
+
+        self._indexbyrev[nullrev] = {
+            b'node': nullid,
+            b'p1': nullid,
+            b'p2': nullid,
+            b'linkrev': nullrev,
+            b'flags': 0,
+        }
+
+        for i, entry in enumerate(self._indexdata):
+            p1rev, p2rev = self.parentrevs(self.rev(entry[b'node']))
+
+            # start, length, rawsize, chainbase, linkrev, p1, p2, node
+            self.index.append((0, 0, 0, -1, entry[b'linkrev'], p1rev, p2rev,
+                               entry[b'node']))
+
+        self.index.append((0, 0, 0, -1, -1, -1, -1, nullid))
+
+    def __len__(self):
+        return len(self._indexdata)
+
+    def __iter__(self):
+        return iter(range(len(self)))
+
+    def revs(self, start=0, stop=None):
+        step = 1
+        if stop is not None:
+            if start > stop:
+                step = -1
+
+            stop += step
+        else:
+            stop = len(self)
+
+        return range(start, stop, step)
+
+    def parents(self, node):
+        validatenode(node)
+
+        if node not in self._indexbynode:
+            raise KeyError('unknown node')
+
+        entry = self._indexbynode[node]
+
+        return entry[b'p1'], entry[b'p2']
+
+    def parentrevs(self, rev):
+        p1, p2 = self.parents(self._indexbyrev[rev][b'node'])
+        return self.rev(p1), self.rev(p2)
+
+    def rev(self, node):
+        validatenode(node)
+
+        # Will raise KeyError.
+        self._indexbynode[node]
+
+        for rev, entry in self._indexbyrev.items():
+            if entry[b'node'] == node:
+                return rev
+
+        raise error.ProgrammingError('this should not occur')
+
+    def node(self, rev):
+        validaterev(rev)
+
+        return self._indexbyrev[rev][b'node']
+
+    def lookup(self, node):
+        if isinstance(node, int):
+            return self.node(node)
+
+        if len(node) == 20:
+            try:
+                self.rev(node)
+                return node
+            except LookupError:
+                pass
+
+        try:
+            rev = int(node)
+            if '%d' % rev != node:
+                raise ValueError
+
+            if rev < 0:
+                rev = len(self) + rev
+            if rev < 0 or rev >= len(self):
+                raise ValueError
+
+            return self.node(rev)
+        except (ValueError, OverflowError):
+            pass
+
+        if len(node) == 40:
+            try:
+                rawnode = bin(node)
+                self.rev(rawnode)
+                return rawnode
+            except (TypeError, LookupError):
+                pass
+
+        raise LookupError(node, self._path, _('invalid lookup input'))
+
+    def linkrev(self, rev):
+        validaterev(rev)
+
+        return self._indexbyrev[rev][b'linkrev']
+
+    def flags(self, rev):
+        validaterev(rev)
+
+        return self._indexbyrev[rev][b'flags']
+
+    def deltaparent(self, rev):
+        validaterev(rev)
+
+        p1node = self.parents(self.node(rev))[0]
+        return self.rev(p1node)
+
+    def candelta(self, baserev, rev):
+        validaterev(rev)
+        validaterev(rev)
+
+        if ((self.flags(baserev) & revlog.REVIDX_RAWTEXT_CHANGING_FLAGS)
+            or (self.flags(rev) & revlog.REVIDX_RAWTEXT_CHANGING_FLAGS)):
+            return False
+
+        return True
+
+    def rawsize(self, rev):
+        validaterev(rev)
+        node = self.node(rev)
+        return len(self.revision(node, raw=True))
+
+    def _processflags(self, text, flags, operation, raw=False):
+        if flags == 0:
+            return text, True
+
+        validatehash = True
+        # Depending on the operation (read or write), the order might be
+        # reversed due to non-commutative transforms.
+        orderedflags = revlog.REVIDX_FLAGS_ORDER
+        if operation == 'write':
+            orderedflags = reversed(orderedflags)
+
+        for flag in orderedflags:
+            # If a flagprocessor has been registered for a known flag, apply the
+            # related operation transform and update result tuple.
+            if flag & flags:
+                vhash = True
+
+                if flag not in revlog._flagprocessors:
+                    message = _("missing processor for flag '%#x'") % (flag)
+                    raise revlog.RevlogError(message)
+
+                processor = revlog._flagprocessors[flag]
+                if processor is not None:
+                    readtransform, writetransform, rawtransform = processor
+
+                    if raw:
+                        vhash = rawtransform(self, text)
+                    elif operation == 'read':
+                        text, vhash = readtransform(self, text)
+                    else:  # write operation
+                        text, vhash = writetransform(self, text)
+                validatehash = validatehash and vhash
+
+        return text, validatehash
+
+    def checkhash(self, text, node, p1=None, p2=None, rev=None):
+        if p1 is None and p2 is None:
+            p1, p2 = self.parents(node)
+        if node != revlog.hash(text, p1, p2):
+            raise error.RevlogError(_("integrity check failed on %s") %
+                self._path)
+
+    def revision(self, node, raw=False):
+        validatenode(node)
+
+        if node == nullid:
+            return b''
+
+        self._indexbynode[node]
+
+        rev = self.rev(node)
+        flags = self.flags(rev)
+
+        path = b'/'.join([self._storepath, hex(node)])
+        rawtext = self._svfs.read(path)
+
+        text, validatehash = self._processflags(rawtext, flags, 'read', raw=raw)
+        if validatehash:
+            self.checkhash(text, node, rev=rev)
+
+        return text
+
+    def read(self, node):
+        validatenode(node)
+
+        revision = self.revision(node)
+
+        if not revision.startswith(b'\1\n'):
+            return revision
+
+        start = revision.index(b'\1\n', 2)
+        return revision[start + 2:]
+
+    def renamed(self, node):
+        validatenode(node)
+
+        if self.parents(node)[0] != nullid:
+            return False
+
+        fulltext = self.revision(node)
+        m = filelog.parsemeta(fulltext)[0]
+
+        if m and 'copy' in m:
+            return m['copy'], bin(m['copyrev'])
+
+        return False
+
+    def cmp(self, node, text):
+        validatenode(node)
+
+        t = text
+
+        if text.startswith(b'\1\n'):
+            t = b'\1\n\1\n' + text
+
+        p1, p2 = self.parents(node)
+
+        if revlog.hash(t, p1, p2) == node:
+            return False
+
+        if self.iscensored(self.rev(node)):
+            return text != b''
+
+        if self.renamed(node):
+            t2 = self.read(node)
+            return t2 != text
+
+        return True
+
+    def size(self, rev):
+        validaterev(rev)
+
+        node = self._indexbyrev[rev][b'node']
+
+        if self.renamed(node):
+            return len(self.read(node))
+
+        if self.iscensored(rev):
+            return 0
+
+        return len(self.revision(node))
+
+    def iscensored(self, rev):
+        validaterev(rev)
+
+        return self.flags(rev) & revlog.REVIDX_ISCENSORED
+
+    def commonancestorsheads(self, a, b):
+        validatenode(a)
+        validatenode(b)
+
+        a = self.rev(a)
+        b = self.rev(b)
+
+        ancestors = ancestor.commonancestorsheads(self.parentrevs, a, b)
+        return pycompat.maplist(self.node, ancestors)
+
+    def descendants(self, revs):
+        # This is a copy of revlog.descendants()
+        first = min(revs)
+        if first == nullrev:
+            for i in self:
+                yield i
+            return
+
+        seen = set(revs)
+        for i in self.revs(start=first + 1):
+            for x in self.parentrevs(i):
+                if x != nullrev and x in seen:
+                    seen.add(i)
+                    yield i
+                    break
+
+    # Required by verify.
+    def files(self):
+        entries = self._svfs.listdir(self._storepath)
+
+        # Strip out undo.backup.* files created as part of transaction
+        # recording.
+        entries = [f for f in entries if not f.startswith('undo.backup.')]
+
+        return [b'/'.join((self._storepath, f)) for f in entries]
+
+    # Required by verify.
+    def checksize(self):
+        return 0, 0
+
+    def add(self, text, meta, transaction, linkrev, p1, p2):
+        if meta or text.startswith(b'\1\n'):
+            text = filelog.packmeta(meta, text)
+
+        return self.addrevision(text, transaction, linkrev, p1, p2)
+
+    def addrevision(self, text, transaction, linkrev, p1, p2, node=None,
+                    flags=0):
+        validatenode(p1)
+        validatenode(p2)
+
+        if flags:
+            node = node or revlog.hash(text, p1, p2)
+
+        rawtext, validatehash = self._processflags(text, flags, 'write')
+
+        node = node or revlog.hash(text, p1, p2)
+
+        if node in self._indexbynode:
+            return node
+
+        if validatehash:
+            self.checkhash(rawtext, node, p1=p1, p2=p2)
+
+        path = b'/'.join([self._storepath, hex(node)])
+
+        self._svfs.write(path, text)
+
+        self._indexdata.append({
+            b'node': node,
+            b'p1': p1,
+            b'p2': p2,
+            b'linkrev': linkrev,
+            b'flags': flags,
+        })
+
+        self._reflectindexupdate()
+
+        return node
+
+    def _reflectindexupdate(self):
+        self._refreshindex()
+        self._svfs.write(self._indexpath, cbor.dumps(self._indexdata))
+
+    def addgroup(self, deltas, linkmapper, transaction, addrevisioncb=None):
+        nodes = []
+
+        transaction.addbackup(self._indexpath)
+
+        for node, p1, p2, linknode, deltabase, delta, flags in deltas:
+            linkrev = linkmapper(linknode)
+
+            nodes.append(node)
+
+            if node in self._indexbynode:
+                continue
+
+            # Need to resolve the fulltext from the delta base.
+            if deltabase == nullid:
+                text = mdiff.patch(b'', delta)
+            else:
+                text = mdiff.patch(self.revision(deltabase), delta)
+
+            self.addrevision(text, transaction, linkrev, p1, p2, flags)
+
+            if addrevisioncb:
+                addrevisioncb(self, node)
+
+        return nodes
+
+    def revdiff(self, rev1, rev2):
+        validaterev(rev1)
+        validaterev(rev2)
+
+        node1 = self.node(rev1)
+        node2 = self.node(rev2)
+
+        return mdiff.textdiff(self.revision(node1, raw=True),
+                              self.revision(node2, raw=True))
+
+    def headrevs(self):
+        # Assume all revisions are heads by default.
+        ishead = {rev: True for rev in self._indexbyrev}
+
+        for rev, entry in self._indexbyrev.items():
+            # Unset head flag for all seen parents.
+            ishead[self.rev(entry[b'p1'])] = False
+            ishead[self.rev(entry[b'p2'])] = False
+
+        return [rev for rev, ishead in sorted(ishead.items())
+                if ishead]
+
+    def heads(self, start=None, stop=None):
+        # This is copied from revlog.py.
+        if start is None and stop is None:
+            if not len(self):
+                return [nullid]
+            return [self.node(r) for r in self.headrevs()]
+
+        if start is None:
+            start = nullid
+        if stop is None:
+            stop = []
+        stoprevs = set([self.rev(n) for n in stop])
+        startrev = self.rev(start)
+        reachable = {startrev}
+        heads = {startrev}
+
+        parentrevs = self.parentrevs
+        for r in self.revs(start=startrev + 1):
+            for p in parentrevs(r):
+                if p in reachable:
+                    if r not in stoprevs:
+                        reachable.add(r)
+                    heads.add(r)
+                if p in heads and p not in stoprevs:
+                    heads.remove(p)
+
+        return [self.node(r) for r in heads]
+
+    def children(self, node):
+        validatenode(node)
+
+        # This is a copy of revlog.children().
+        c = []
+        p = self.rev(node)
+        for r in self.revs(start=p + 1):
+            prevs = [pr for pr in self.parentrevs(r) if pr != nullrev]
+            if prevs:
+                for pr in prevs:
+                    if pr == p:
+                        c.append(self.node(r))
+            elif p == nullrev:
+                c.append(self.node(r))
+        return c
+
+    def getstrippoint(self, minlink):
+
+        # This is largely a copy of revlog.getstrippoint().
+        brokenrevs = set()
+        strippoint = len(self)
+
+        heads = {}
+        futurelargelinkrevs = set()
+        for head in self.headrevs():
+            headlinkrev = self.linkrev(head)
+            heads[head] = headlinkrev
+            if headlinkrev >= minlink:
+                futurelargelinkrevs.add(headlinkrev)
+
+        # This algorithm involves walking down the rev graph, starting at the
+        # heads. Since the revs are topologically sorted according to linkrev,
+        # once all head linkrevs are below the minlink, we know there are
+        # no more revs that could have a linkrev greater than minlink.
+        # So we can stop walking.
+        while futurelargelinkrevs:
+            strippoint -= 1
+            linkrev = heads.pop(strippoint)
+
+            if linkrev < minlink:
+                brokenrevs.add(strippoint)
+            else:
+                futurelargelinkrevs.remove(linkrev)
+
+            for p in self.parentrevs(strippoint):
+                if p != nullrev:
+                    plinkrev = self.linkrev(p)
+                    heads[p] = plinkrev
+                    if plinkrev >= minlink:
+                        futurelargelinkrevs.add(plinkrev)
+
+        return strippoint, brokenrevs
+
+    def strip(self, minlink, transaction):
+        if not len(self):
+            return
+
+        rev, _ignored = self.getstrippoint(minlink)
+        if rev == len(self):
+            return
+
+        # Purge index data starting at the requested revision.
+        self._indexdata[rev:] = []
+        self._reflectindexupdate()
+
+def reposetup(ui, repo):
+    if not repo.local():
+        return
+
+    class simplestorerepo(repo.__class__):
+        def file(self, f):
+            return filestorage(self.svfs, f)
+
+    repo.__class__ = simplestorerepo