Mercurial > hg
view tests/test-convert-hg-source.t @ 26623:5a95fe44121d
clonebundles: support for seeding clones from pre-generated bundles
Cloning can be an expensive operation for servers because the server
generates a bundle from existing repository data at request time. For
a large repository like mozilla-central, this consumes 4+ minutes
of CPU time on the server. It also results in significant network
utilization. Multiplied by hundreds or even thousands of clients and
the ensuing load can result in difficulties scaling the Mercurial server.
Despite generation of bundles being deterministic until the next
changeset is added, the generation of bundles to service a clone request
is not cached. Each clone thus performs redundant work. This is
wasteful.
This patch introduces the "clonebundles" extension and related
client-side functionality to help alleviate this deficiency. The
client-side feature is behind an experimental flag and is not enabled by
default.
It works as follows:
1) Server operator generates a bundle and makes it available on a
server (likely HTTP).
2) Server operator defines the URL of a bundle file in a
.hg/clonebundles.manifest file.
3) Client `hg clone`ing sees the server is advertising bundle URLs.
4) Client fetches and applies the advertised bundle.
5) Client performs equivalent of `hg pull` to fetch changes made since
the bundle was created.
Essentially, the server performs the expensive work of generating a
bundle once and all subsequent clones fetch a static file from
somewhere. Scaling static file serving is a much more manageable
problem than scaling a Python application like Mercurial. Assuming your
repository grows less than 1% per day, the end result is 99+% of CPU
and network load from clones is eliminated, allowing Mercurial servers
to scale more easily. Serving static files also means data can be
transferred to clients as fast as they can consume it, rather than as
fast as servers can generate it. This makes clones faster.
Mozilla has implemented similar functionality of this patch on
hg.mozilla.org using a custom extension. We are hosting bundle files in
Amazon S3 and CloudFront (a CDN) and have successfully offloaded
>1 TB/day in data transfer from hg.mozilla.org, freeing up significant
bandwidth and CPU resources. The positive impact has been stellar and
I believe it has proved its value to be included in Mercurial core. I
feel it is important for the client-side support to be enabled in core
by default because it means that clients will get faster, more reliable
clones and will enable server operators to reduce load without
requiring any client-side configuration changes (assuming clients are
up to date, of course).
The scope of this feature is narrowly and specifically tailored to
cloning, despite "serve pulls from pre-generated bundles" being a valid
and useful feature. I would eventually like for Mercurial servers to
support transferring *all* repository data via statically hosted files.
You could imagine a server that siphons all pushed data to bundle files
and instructs clients to apply a stream of bundles to reconstruct all
repository data. This feature, while useful and powerful, is
significantly more work to implement because it requires the server
component have awareness of discovery and a mapping of which changesets
are in which files. Full, clone bundles, by contrast, are much simpler.
The wire protocol command is named "clonebundles" instead of something
more generic like "staticbundles" to leave the door open for a new, more
powerful and more generic server-side component with minimal backwards
compatibility implications. The name "bundleclone" is used by Mozilla's
extension and would cause problems since there are subtle differences
in Mozilla's extension.
Mozilla's experience with this idea has taught us that some form of
"content negotiation" is required. Not all clients will support all
bundle formats or even URLs (advanced TLS requirements, etc). To ensure
the highest uptake possible, a server needs to advertise multiple
versions of bundles and clients need to be able to choose the most
appropriate from that list one. The "attributes" in each
server-advertised entry facilitate this filtering and sorting. Their
use will become apparent in subsequent patches.
Initial inspiration and credit for the idea of cloning from static files
belongs to Augie Fackler and his "lookaside clone" extension proof of
concept.
author | Gregory Szorc <gregory.szorc@gmail.com> |
---|---|
date | Fri, 09 Oct 2015 11:22:01 -0700 |
parents | 1538e72209fd |
children | 75be14993fda |
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$ cat >> $HGRCPATH <<EOF > [extensions] > convert= > [convert] > hg.saverev=False > EOF $ hg init orig $ cd orig $ echo foo > foo $ echo bar > bar $ hg ci -qAm 'add foo bar' -d '0 0' $ echo >> foo $ hg ci -m 'change foo' -d '1 0' $ hg up -qC 0 $ hg copy --after --force foo bar $ hg copy foo baz $ hg ci -m 'make bar and baz copies of foo' -d '2 0' created new head Test that template can print all file copies (issue4362) $ hg log -r . --template "{file_copies % ' File: {file_copy}\n'}" File: bar (foo) File: baz (foo) $ hg bookmark premerge1 $ hg merge -r 1 merging baz and foo to baz 1 files updated, 1 files merged, 0 files removed, 0 files unresolved (branch merge, don't forget to commit) $ hg ci -m 'merge local copy' -d '3 0' $ hg up -C 1 1 files updated, 0 files merged, 1 files removed, 0 files unresolved (leaving bookmark premerge1) $ hg bookmark premerge2 $ hg merge 2 merging foo and baz to baz 1 files updated, 1 files merged, 0 files removed, 0 files unresolved (branch merge, don't forget to commit) $ hg ci -m 'merge remote copy' -d '4 0' created new head Make and delete some tags $ hg tag that $ hg tag --remove that $ hg tag this #if execbit $ chmod +x baz #else $ echo some other change to make sure we get a rev 5 > baz #endif $ hg ci -m 'mark baz executable' -d '5 0' $ cd .. $ hg convert --datesort orig new 2>&1 | grep -v 'subversion python bindings could not be loaded' initializing destination new repository scanning source... sorting... converting... 8 add foo bar 7 change foo 6 make bar and baz copies of foo 5 merge local copy 4 merge remote copy 3 Added tag that for changeset 88586c4e9f02 2 Removed tag that 1 Added tag this for changeset c56a7f387039 0 mark baz executable updating bookmarks $ cd new $ hg out ../orig comparing with ../orig searching for changes no changes found [1] #if execbit $ hg bookmarks premerge1 3:973ef48a98a4 premerge2 8:91d107c423ba #else Different hash because no x bit $ hg bookmarks premerge1 3:973ef48a98a4 premerge2 8:3537b15eaaca #endif Test that redoing a convert results in an identical graph $ cd ../ $ rm new/.hg/shamap $ hg convert --datesort orig new 2>&1 | grep -v 'subversion python bindings could not be loaded' scanning source... sorting... converting... 8 add foo bar 7 change foo 6 make bar and baz copies of foo 5 merge local copy 4 merge remote copy 3 Added tag that for changeset 88586c4e9f02 2 Removed tag that 1 Added tag this for changeset c56a7f387039 0 mark baz executable updating bookmarks $ hg -R new log -G -T '{rev} {desc}' o 8 mark baz executable | o 7 Added tag this for changeset c56a7f387039 | o 6 Removed tag that | o 5 Added tag that for changeset 88586c4e9f02 | o 4 merge remote copy |\ +---o 3 merge local copy | |/ | o 2 make bar and baz copies of foo | | o | 1 change foo |/ o 0 add foo bar check shamap LF and CRLF handling $ cat > rewrite.py <<EOF > import sys > # Interlace LF and CRLF > lines = [(l.rstrip() + ((i % 2) and '\n' or '\r\n')) > for i, l in enumerate(file(sys.argv[1]))] > file(sys.argv[1], 'wb').write(''.join(lines)) > EOF $ python rewrite.py new/.hg/shamap $ cd orig $ hg up -qC 1 $ echo foo >> foo $ hg ci -qm 'change foo again' $ hg up -qC 2 $ echo foo >> foo $ hg ci -qm 'change foo again again' $ cd .. $ hg convert --datesort orig new 2>&1 | grep -v 'subversion python bindings could not be loaded' scanning source... sorting... converting... 1 change foo again again 0 change foo again updating bookmarks init broken repository $ hg init broken $ cd broken $ echo a >> a $ echo b >> b $ hg ci -qAm init $ echo a >> a $ echo b >> b $ hg copy b c $ hg ci -qAm changeall $ hg up -qC 0 $ echo bc >> b $ hg ci -m changebagain created new head $ HGMERGE=internal:local hg -q merge $ hg ci -m merge $ hg mv b d $ hg ci -m moveb break it $ rm .hg/store/data/b.* $ cd .. $ hg --config convert.hg.ignoreerrors=True convert broken fixed initializing destination fixed repository scanning source... sorting... converting... 4 init ignoring: data/b.i@1e88685f5dde: no match found 3 changeall 2 changebagain 1 merge 0 moveb $ hg -R fixed verify checking changesets checking manifests crosschecking files in changesets and manifests checking files 3 files, 5 changesets, 5 total revisions manifest -r 0 $ hg -R fixed manifest -r 0 a manifest -r tip $ hg -R fixed manifest -r tip a c d