1 ==================================
2 Design for import/export version 2
3 ==================================
5 .. contents:: :depth: 4
7 Current state and shortcomings
8 ------------------------------
10 Ganeti 2.2 introduced :doc:`inter-cluster instance moves <design-2.2>`
11 and replaced the import/export mechanism with the same technology. It's
12 since shown that the chosen implementation was too complicated and and
13 can be difficult to debug.
15 The old implementation is henceforth called "version 1". It used
16 ``socat`` in combination with a rather complex tree of ``bash`` and
17 Python utilities to move instances between clusters and import/export
18 them inside the cluster. Due to protocol limitations, the master daemon
19 starts a daemon on the involved nodes and then keeps polling a status
20 file for updates. A non-trivial number of timeouts ensures that jobs
23 In version 1, the destination node would start a daemon listening on a
24 random TCP port. Upon receiving the destination information, the source
25 node would temporarily stop the instance, create snapshots, and start
26 exporting the data by connecting to the destination. The random TCP port
27 is chosen by the operating system by binding the socket to port 0.
28 While this is a somewhat elegant solution, it causes problems in setups
29 with restricted connectivity (e.g. iptables).
31 Another issue encountered was with dual-stack IPv6 setups. ``socat`` can
32 only listen on one protocol, IPv4 or IPv6, at a time. The connecting
33 node can not simply resolve the DNS name, but it must be told the exact
36 Instance OS definitions can provide custom import/export scripts. They
37 were working well in the early days when a filesystem was usually
38 created directly on the block device. Around Ganeti 2.0 there was a
39 transition to using partitions on the block devices. Import/export
40 scripts could no longer use simple ``dump`` and ``restore`` commands,
41 but usually ended up doing raw data dumps.
47 Unlike in version 1, in version 2 the destination node will connect to
48 the source. The active side is swapped. This design assumes the
49 following design documents have been implemented:
51 - :doc:`design-x509-ca`
52 - :doc:`design-http-server`
54 The following design is mostly targetted at inter-cluster instance
55 moves. Intra-cluster import and export use the same technology, but do
56 so in a less complicated way (e.g. reusing the node daemon certificate
59 Support for instance OS import/export scripts, which have been in Ganeti
60 since the beginning, will be dropped with this design. Should the need
61 arise, they can be re-added later.
67 - HTTP client: cURL/pycURL (already used for inter-node RPC and RAPI
69 - Authentication: X509 certificates (server and client)
75 Instead of a home-grown, mostly raw protocol the widely used HTTP
76 protocol will be used. Ganeti already uses HTTP for its :doc:`Remote API
77 <rapi>` and inter-node communication. Encryption and authentication will
78 be implemented using SSL and X509 certificates.
84 The source machine will identify connecting clients by their SSL
85 certificate. Unknown certificates will be refused.
87 Version 1 created a new self-signed certificate per instance
88 import/export, allowing the certificate to be used as a Certificate
89 Authority (CA). This worked by means of starting a new ``socat``
90 instance per instance import/export.
92 Under the version 2 model, a continously running HTTP server will be
93 used. This disallows the use of self-signed certificates for
94 authentication as the CA needs to be the same for all issued
97 See the :doc:`separate design document for more details on how the
98 certificate authority will be implemented <design-x509-ca>`.
100 Local imports/exports will, like version 1, use the node daemon's
101 certificate/key. Doing so allows the verification of local connections.
102 The client's certificate can be exported to the CGI/FastCGI handler
103 using lighttpd's ``ssl.verifyclient.exportcert`` setting. If a
104 cluster-local import/export is being done, the handler verifies if the
105 used certificate matches with the local node daemon key.
111 The source can be the same physical machine as the destination, another
112 node in the same cluster, or a node in another cluster. A
113 physical-to-virtual migration mechanism could be implemented as an
116 In the case of a traditional import, the source is usually a file on the
117 source machine. For exports and remote imports, the source is an
118 instance's raw disk data. In all cases the transported data is opaque to
121 All nodes of a cluster will run an instance of Lighttpd. The
122 configuration is automatically generated when starting Ganeti. The HTTP
123 server is configured to listen on IPv4 and IPv6 simultaneously.
124 Imports/exports will use a dedicated TCP port, similar to the Remote
127 See the separate :ref:`HTTP server design document
128 <http-srv-shortcomings>` for why Ganeti's existing, built-in HTTP server
129 is not a good choice.
131 The source cluster is provided with a X509 Certificate Signing Request
132 (CSR) for a key private to the destination cluster.
134 After shutting down the instance, creating snapshots and restarting the
135 instance the master will sign the destination's X509 certificate using
136 the :doc:`X509 CA <design-x509-ca>` once per instance disk. Instead of
137 using another identifier, the certificate's serial number (:ref:`never
138 reused <x509-ca-serial>`) and fingerprint are used to identify incoming
139 requests. Once ready, the master will call an RPC method on the source
140 node and provide it with the input information (e.g. file paths or block
141 devices) and the certificate identities.
143 The RPC method will write the identities to a place accessible by the
144 HTTP request handler, generate unique transfer IDs and return them to
145 the master. The transfer ID could be a filename containing the
146 certificate's serial number, fingerprint and some disk information. The
147 file containing the per-transfer information is signed using the node
148 daemon key and the signature written to a separate file.
150 Once everything is in place, the master sends the certificates, the data
151 and notification URLs (which include the transfer IDs) and the public
152 part of the source's CA to the job submitter. Like in version 1,
153 everything will be signed using the cluster domain secret.
155 Upon receiving a request, the handler verifies the identity and
156 continues to stream the instance data. The serial number and fingerprint
157 contained in the transfer ID should be matched with the certificate
158 used. If a cluster-local import/export was requested, the remote's
159 certificate is verified with the local node daemon key. The signature of
160 the information file from which the handler takes the path of the block
161 device (and more) is verified using the local node daemon certificate.
162 There are two options for handling requests, :ref:`CGI
163 <lighttpd-cgi-opt>` and :ref:`FastCGI <lighttpd-fastcgi-opt>`.
165 To wait for all requests to finish, the master calls another RPC method.
166 The destination should notify the source once it's done with downloading
167 the data. Since this notification may never arrive (e.g. network
168 issues), an additional timeout needs to be used.
170 There is no good way to avoid polling as the HTTP requests will be
171 handled asynchronously in another process. Once, and if, implemented
172 :ref:`RPC feedback <rpc-feedback>` could be used to combine the two RPC
175 Upon completion of the transfer requests, the instance is removed if
179 .. _lighttpd-cgi-opt:
184 While easier to implement, this option requires the HTTP server to
185 either run as "root" or a so-called SUID binary to elevate the started
186 process to run as "root".
188 The export data can be sent directly to the HTTP server without any
192 .. _lighttpd-fastcgi-opt:
197 Unlike plain CGI, FastCGI scripts are run separately from the webserver.
198 The webserver talks to them via a Unix socket. Webserver and scripts can
199 run as separate users. Unlike for CGI, there are almost no bootstrap
200 costs attached to each request.
202 The FastCGI protocol requires data to be sent in length-prefixed
203 packets, something which wouldn't be very efficient to do in Python for
204 large amounts of data (instance imports/exports can be hundreds of
205 gigabytes). For this reason the proposal is to use a wrapper program
206 written in C (e.g. `fcgiwrap
207 <http://nginx.localdomain.pl/wiki/FcgiWrap>`_) and to write the handler
208 like an old-style CGI program with standard input/output. If data should
209 be copied from a file, ``cat``, ``dd`` or ``socat`` can be used (see
210 note about :ref:`sendfile(2)/splice(2) with Python <python-sendfile>`).
212 The bootstrap cost associated with starting a Python interpreter for
213 a disk export is expected to be negligible.
215 The `spawn-fcgi <http://cgit.stbuehler.de/gitosis/spawn-fcgi/about/>`_
216 program will be used to start the CGI wrapper as "root".
218 FastCGI is, in the author's opinion, the better choice as it allows user
219 separation. As a first implementation step the export handler can be run
220 as a standard CGI program. User separation can be implemented as a
227 The destination can be the same physical machine as the source, another
228 node in the same cluster, or a node in another cluster. While not
229 considered in this design document, instances could be exported from the
230 cluster by implementing an external client for exports.
232 For traditional exports the destination is usually a file on the
233 destination machine. For imports and remote exports, the destination is
234 an instance's disks. All transported data is opaque to Ganeti.
236 Before an import can be started, an RSA key and corresponding
237 Certificate Signing Request (CSR) must be generated using the new opcode
238 ``OpInstanceImportPrepare``. The returned information is signed using
239 the cluster domain secret. The RSA key backing the CSR must not leave
240 the destination cluster. After being passed through a third party, the
241 source cluster will generate signed certificates from the CSR.
243 Once the request for creating the instance arrives at the master daemon,
244 it'll create the instance and call an RPC method on the instance's
245 primary node to download all data. The RPC method does not return until
246 the transfer is complete or failed (see :ref:`EXP_SIZE_FD <exp-size-fd>`
247 and :ref:`RPC feedback <rpc-feedback>`).
249 The node will use pycURL to connect to the source machine and identify
250 itself with the signed certificate received. pycURL will be configured
251 to write directly to a file descriptor pointing to either a regular file
252 or block device. The file descriptor needs to point to the correct
253 offset for resuming downloads.
255 Using cURL's multi interface, more than one transfer can be made at the
256 same time. While parallel transfers are used by the version 1
257 import/export, it can be decided at a later time whether to use them in
258 version 2 too. More investigation is necessary to determine whether
259 ``CURLOPT_MAXCONNECTS`` is enough to limit the number of connections or
260 whether more logic is necessary.
262 If a transfer fails before it's finished (e.g. timeout or network
263 issues) it should be retried using an exponential backoff delay. The
264 opcode submitter can specify for how long the transfer should be
267 At the end of a transfer, succssful or not, the source cluster must be
268 notified. A the same time the RSA key needs to be destroyed.
270 Support for HTTP proxies can be implemented by setting
271 ``CURLOPT_PROXY``. Proxies could be used for moving instances in/out of
272 restricted network environments or across protocol borders (e.g. IPv4
273 networks unable to talk to IPv6 networks).
276 The big picture for instance moves
277 ----------------------------------
279 #. ``OpInstanceImportPrepare`` (destination cluster)
281 Create RSA key and CSR (certificate signing request), return signed
282 with cluster domain secret.
284 #. ``OpBackupPrepare`` (source cluster)
286 Becomes a no-op in version 2, but see :ref:`backwards-compat`.
288 #. ``OpBackupExport`` (source cluster)
290 - Receives destination cluster's CSR, verifies signature using
291 cluster domain secret.
292 - Creates certificates using CSR and :doc:`cluster CA
293 <design-x509-ca>`, one for each disk
294 - Stop instance, create snapshots, start instance
295 - Prepare HTTP resources on node
296 - Send certificates, URLs and CA certificate to job submitter using
298 - Wait for all transfers to finish or fail (with timeout)
301 #. ``OpInstanceCreate`` (destination cluster)
303 - Receives certificates signed by destination cluster, verifies
304 certificates and URLs using cluster domain secret
306 Note that the parameters should be implemented in a generic way
307 allowing future extensions, e.g. to download disk images from a
308 public, remote server. The cluster domain secret allows Ganeti to
309 check data received from a third party, but since this won't work
310 with such extensions, other checks will have to be designed.
312 - Create block devices
313 - Download every disk from source, verified using remote's CA and
314 authenticated using signed certificates
315 - Destroy RSA key and certificates
318 .. TODO: separate create from import?
321 .. _impexp2-http-resources:
323 HTTP resources on source
324 ------------------------
326 The HTTP resources listed below will be made available by the source
327 machine. The transfer ID is generated while preparing the export and is
328 unique per disk and instance. No caching should be used and the
329 ``Pragma`` (HTTP/1.0) and ``Cache-Control`` (HTTP/1.1) headers set
330 accordingly by the server.
332 ``GET /transfers/[transfer_id]/contents``
333 Dump disk contents. Important request headers:
335 ``Accept`` (:rfc:`2616`, section 14.1)
336 Specify preferred media types. Only one type is supported in the
337 initial implementation:
339 ``application/octet-stream``
340 Request raw disk content.
342 If support for more media types were to be implemented in the
343 future, the "q" parameter used for "indicating a relative quality
344 factor" needs to be used. In the meantime parameters need to be
345 expected, but can be ignored.
347 If support for OS scripts were to be re-added in the future, the
348 MIME type ``application/x-ganeti-instance-export`` is hereby
349 reserved for disk dumps using an export script.
351 If the source can not satisfy the request the response status code
352 will be 406 (Not Acceptable). Successful requests will specify the
353 used media type using the ``Content-Type`` header. Unless only
354 exactly one media type is requested, the client must handle the
355 different response types.
357 ``Accept-Encoding`` (:rfc:`2616`, section 14.3)
358 Specify desired content coding. Supported are ``identity`` for
359 uncompressed data, ``gzip`` for compressed data and ``*`` for any.
360 The response will include a ``Content-Encoding`` header with the
361 actual coding used. If the client specifies an unknown coding, the
362 response status code will be 406 (Not Acceptable).
364 If the client specifically needs compressed data (see
365 :ref:`impexp2-compression`) but only gets ``identity``, it can
366 either compress locally or abort the request.
368 ``Range`` (:rfc:`2616`, section 14.35)
369 Raw disk dumps can be resumed using this header (e.g. after a
372 If this header was given in the request and the source supports
373 resuming, the status code of the response will be 206 (Partial
374 Content) and it'll include the ``Content-Range`` header as per
375 :rfc:`2616`. If it does not support resuming or the request was not
376 specifying a range, the status code will be 200 (OK).
378 Only a single byte range is supported. cURL does not support
379 ``multipart/byteranges`` responses by itself. Even if they could be
380 somehow implemented, doing so would be of doubtful benefit for
383 For raw data dumps handling ranges is pretty straightforward by just
384 dumping the requested range.
386 cURL will fail with the error code ``CURLE_RANGE_ERROR`` if a
387 request included a range but the server can't handle it. The request
388 must be retried without a range.
390 ``POST /transfers/[transfer_id]/done``
391 Use this resource to notify the source when transfer is finished (even
392 if not successful). The status code will be 204 (No Content).
401 .. highlight:: python
403 The following code sample shows how to write downloaded data directly to
404 a file without pumping it through Python::
407 curl.setopt(pycurl.URL, "http://www.google.com/")
408 curl.setopt(pycurl.WRITEDATA, open("googlecom.html", "w"))
411 This works equally well if the file descriptor is a pipe to another
415 .. _backwards-compat:
417 Backwards compatibility
418 -----------------------
420 .. _backwards-compat-v1:
425 The old inter-cluster import/export implementation described in the
426 :doc:`Ganeti 2.2 design document <design-2.2>` will be supported for at
427 least one minor (2.x) release. Intra-cluster imports/exports will use
428 the new version right away.
436 Together with the improved import/export infrastructure Ganeti 2.2
437 allowed instance export scripts to report the expected data size. This
438 was then used to provide the user with an estimated remaining time.
439 Version 2 no longer supports OS import/export scripts and therefore
440 ``EXP_SIZE_FD`` is no longer needed.
443 .. _impexp2-compression:
448 Version 1 used explicit compression using ``gzip`` for transporting
449 data, but the dumped files didn't use any compression. Version 2 will
450 allow the destination to specify which encoding should be used. This way
451 the transported data is already compressed and can be directly used by
452 the client (see :ref:`impexp2-http-resources`). The cURL option
453 ``CURLOPT_ENCODING`` can be used to set the ``Accept-Encoding`` header.
454 cURL will not decompress received data when
455 ``CURLOPT_HTTP_CONTENT_DECODING`` is set to zero (if another HTTP client
456 library were used which doesn't support disabling transparent
457 compression, a custom content-coding type could be defined, e.g.
464 The HTTP/1.1 protocol (:rfc:`2616`) defines trailing headers for chunked
465 transfers in section 3.6.1. This could be used to transfer a checksum at
466 the end of an import/export. cURL supports trailing headers since
467 version 7.14.1. Lighttpd doesn't seem to support them for FastCGI, but
468 they appear to be usable in combination with an NPH CGI (No Parsed
471 .. _lighttp-sendfile:
473 Lighttpd allows FastCGI applications to send the special headers
474 ``X-Sendfile`` and ``X-Sendfile2`` (the latter with a range). Using
475 these headers applications can send response headers and tell the
476 webserver to serve regular file stored on the file system as a response
477 body. The webserver will then take care of sending that file.
478 Unfortunately this mechanism is restricted to regular files and can not
479 be used for data from programs, neither direct nor via named pipes,
480 without writing to a file first. The latter is not an option as instance
481 data can be very large. Theoretically ``X-Sendfile`` could be used for
482 sending the input for a file-based instance import, but that'd require
483 the webserver to run as "root".
487 Python does not include interfaces for the ``sendfile(2)`` or
488 ``splice(2)`` system calls. The latter can be useful for faster copying
489 of data between file descriptors. There are some 3rd-party modules (e.g.
490 http://pypi.python.org/pypi/py-sendfile/) and discussions
491 (http://bugs.python.org/issue10882) for including support for
492 ``sendfile(2)``, but the later is certainly not going to happen for the
493 Python versions supported by Ganeti. Calling the function using the
494 ``ctypes`` module might be possible.
497 Performance considerations
498 --------------------------
500 The design described above was confirmed to be one of the better choices
501 in terms of download performance with bigger block sizes. All numbers
502 were gathered on the same physical machine with a single CPU and 1 GB of
503 RAM while downloading 2 GB of zeros read from ``/dev/zero``. ``wget``
504 (version 1.10.2) was used as the client, ``lighttpd`` (version 1.4.28)
505 as the server. The numbers in the first line are in megabytes per
506 second. The second line in each row is the CPU time spent in userland
507 respective system (measured for the CGI/FastCGI program using ``time
512 ----------------------------------------------------------------------
513 Block size 4 KB 64 KB 128 KB 1 MB 4 MB
514 ======================================================================
515 Plain CGI script reading 83 174 180 122 120
517 0.6/3.9 0.1/2.4 0.1/2.2 0.0/1.9 0.0/2.1
518 ----------------------------------------------------------------------
519 FastCGI with ``fcgiwrap``, 86 167 170 177 174
521 ``/dev/zero`` 1.1/5 0.5/2.9 0.5/2.7 0.7/3.1 0.7/2.8
522 ----------------------------------------------------------------------
523 FastCGI with ``fcgiwrap``, 68 146 150 170 170
524 Python script copying from
525 ``/dev/zero`` to stdout
526 1.3/5.1 0.8/3.7 0.7/3.3 0.9/2.9 0.8/3
527 ----------------------------------------------------------------------
528 FastCGI, Python script using 31 48 47 5 1
529 ``flup`` library (version
532 23.5/9.8 14.3/8.5 16.1/8 - -
533 ----------------------------------------------------------------------
536 It should be mentioned that the ``flup`` library is not implemented in
537 the most efficient way, but even with some changes it doesn't get much
538 faster. It is fine for small amounts of data, but not for huge
542 Other considered solutions
543 --------------------------
545 Another possible solution considered was to use ``socat`` like version 1
546 did. Due to the changing model, a large part of the code would've
547 required a rewrite anyway, while still not fixing all shortcomings. For
548 example, ``socat`` could still listen on only one protocol, IPv4 or
549 IPv6. Running two separate instances might have fixed that, but it'd get
550 more complicated. Using an existing HTTP server will provide us with a
551 number of other benefits as well, such as easier user separation between
555 .. vim: set textwidth=72 :
projects / ganeti-local / blob