implement mutable-file recovery: update can't recover from <k new shares #272
Labels
No labels
c/code
c/code-dirnodes
c/code-encoding
c/code-frontend
c/code-frontend-cli
c/code-frontend-ftp-sftp
c/code-frontend-magic-folder
c/code-frontend-web
c/code-mutable
c/code-network
c/code-nodeadmin
c/code-peerselection
c/code-storage
c/contrib
c/dev-infrastructure
c/docs
c/operational
c/packaging
c/unknown
c/website
kw:2pc
kw:410
kw:9p
kw:ActivePerl
kw:AttributeError
kw:DataUnavailable
kw:DeadReferenceError
kw:DoS
kw:FileZilla
kw:GetLastError
kw:IFinishableConsumer
kw:K
kw:LeastAuthority
kw:Makefile
kw:RIStorageServer
kw:StringIO
kw:UncoordinatedWriteError
kw:about
kw:access
kw:access-control
kw:accessibility
kw:accounting
kw:accounting-crawler
kw:add-only
kw:aes
kw:aesthetics
kw:alias
kw:aliases
kw:aliens
kw:allmydata
kw:amazon
kw:ambient
kw:annotations
kw:anonymity
kw:anonymous
kw:anti-censorship
kw:api_auth_token
kw:appearance
kw:appname
kw:apport
kw:archive
kw:archlinux
kw:argparse
kw:arm
kw:assertion
kw:attachment
kw:auth
kw:authentication
kw:automation
kw:avahi
kw:availability
kw:aws
kw:azure
kw:backend
kw:backoff
kw:backup
kw:backupdb
kw:backward-compatibility
kw:bandwidth
kw:basedir
kw:bayes
kw:bbfreeze
kw:beta
kw:binaries
kw:binutils
kw:bitcoin
kw:bitrot
kw:blacklist
kw:blocker
kw:blocks-cloud-deployment
kw:blocks-cloud-merge
kw:blocks-magic-folder-merge
kw:blocks-merge
kw:blocks-raic
kw:blocks-release
kw:blog
kw:bom
kw:bonjour
kw:branch
kw:branding
kw:breadcrumbs
kw:brians-opinion-needed
kw:browser
kw:bsd
kw:build
kw:build-helpers
kw:buildbot
kw:builders
kw:buildslave
kw:buildslaves
kw:cache
kw:cap
kw:capleak
kw:captcha
kw:cast
kw:centos
kw:cffi
kw:chacha
kw:charset
kw:check
kw:checker
kw:chroot
kw:ci
kw:clean
kw:cleanup
kw:cli
kw:cloud
kw:cloud-backend
kw:cmdline
kw:code
kw:code-checks
kw:coding-standards
kw:coding-tools
kw:coding_tools
kw:collection
kw:compatibility
kw:completion
kw:compression
kw:confidentiality
kw:config
kw:configuration
kw:configuration.txt
kw:conflict
kw:connection
kw:connectivity
kw:consistency
kw:content
kw:control
kw:control.furl
kw:convergence
kw:coordination
kw:copyright
kw:corruption
kw:cors
kw:cost
kw:coverage
kw:coveralls
kw:coveralls.io
kw:cpu-watcher
kw:cpyext
kw:crash
kw:crawler
kw:crawlers
kw:create-container
kw:cruft
kw:crypto
kw:cryptography
kw:cryptography-lib
kw:cryptopp
kw:csp
kw:curl
kw:cutoff-date
kw:cycle
kw:cygwin
kw:d3
kw:daemon
kw:darcs
kw:darcsver
kw:database
kw:dataloss
kw:db
kw:dead-code
kw:deb
kw:debian
kw:debug
kw:deep-check
kw:defaults
kw:deferred
kw:delete
kw:deletion
kw:denial-of-service
kw:dependency
kw:deployment
kw:deprecation
kw:desert-island
kw:desert-island-build
kw:design
kw:design-review-needed
kw:detection
kw:dev-infrastructure
kw:devpay
kw:directory
kw:directory-page
kw:dirnode
kw:dirnodes
kw:disconnect
kw:discovery
kw:disk
kw:disk-backend
kw:distribute
kw:distutils
kw:dns
kw:do_http
kw:doc-needed
kw:docker
kw:docs
kw:docs-needed
kw:dokan
kw:dos
kw:download
kw:downloader
kw:dragonfly
kw:drop-upload
kw:duplicity
kw:dusty
kw:earth-dragon
kw:easy
kw:ec2
kw:ecdsa
kw:ed25519
kw:egg-needed
kw:eggs
kw:eliot
kw:email
kw:empty
kw:encoding
kw:endpoint
kw:enterprise
kw:enum34
kw:environment
kw:erasure
kw:erasure-coding
kw:error
kw:escaping
kw:etag
kw:etch
kw:evangelism
kw:eventual
kw:example
kw:excess-authority
kw:exec
kw:exocet
kw:expiration
kw:extensibility
kw:extension
kw:failure
kw:fedora
kw:ffp
kw:fhs
kw:figleaf
kw:file
kw:file-descriptor
kw:filename
kw:filesystem
kw:fileutil
kw:fips
kw:firewall
kw:first
kw:floatingpoint
kw:flog
kw:foolscap
kw:forward-compatibility
kw:forward-secrecy
kw:forwarding
kw:free
kw:freebsd
kw:frontend
kw:fsevents
kw:ftp
kw:ftpd
kw:full
kw:furl
kw:fuse
kw:garbage
kw:garbage-collection
kw:gateway
kw:gatherer
kw:gc
kw:gcc
kw:gentoo
kw:get
kw:git
kw:git-annex
kw:github
kw:glacier
kw:globalcaps
kw:glossary
kw:google-cloud-storage
kw:google-drive-backend
kw:gossip
kw:governance
kw:grid
kw:grid-manager
kw:gridid
kw:gridsync
kw:grsec
kw:gsoc
kw:gvfs
kw:hackfest
kw:hacktahoe
kw:hang
kw:hardlink
kw:heartbleed
kw:heisenbug
kw:help
kw:helper
kw:hint
kw:hooks
kw:how
kw:how-to
kw:howto
kw:hp
kw:hp-cloud
kw:html
kw:http
kw:https
kw:i18n
kw:i2p
kw:i2p-collab
kw:illustration
kw:image
kw:immutable
kw:impressions
kw:incentives
kw:incident
kw:init
kw:inlineCallbacks
kw:inotify
kw:install
kw:installer
kw:integration
kw:integration-test
kw:integrity
kw:interactive
kw:interface
kw:interfaces
kw:interoperability
kw:interstellar-exploration
kw:introducer
kw:introduction
kw:iphone
kw:ipkg
kw:iputil
kw:ipv6
kw:irc
kw:jail
kw:javascript
kw:joke
kw:jquery
kw:json
kw:jsui
kw:junk
kw:key-value-store
kw:kfreebsd
kw:known-issue
kw:konqueror
kw:kpreid
kw:kvm
kw:l10n
kw:lae
kw:large
kw:latency
kw:leak
kw:leasedb
kw:leases
kw:libgmp
kw:license
kw:licenss
kw:linecount
kw:link
kw:linux
kw:lit
kw:localhost
kw:location
kw:locking
kw:logging
kw:logo
kw:loopback
kw:lucid
kw:mac
kw:macintosh
kw:magic-folder
kw:manhole
kw:manifest
kw:manual-test-needed
kw:map
kw:mapupdate
kw:max_space
kw:mdmf
kw:memcheck
kw:memory
kw:memory-leak
kw:mesh
kw:metadata
kw:meter
kw:migration
kw:mime
kw:mingw
kw:minimal
kw:misc
kw:miscapture
kw:mlp
kw:mock
kw:more-info-needed
kw:mountain-lion
kw:move
kw:multi-users
kw:multiple
kw:multiuser-gateway
kw:munin
kw:music
kw:mutability
kw:mutable
kw:mystery
kw:names
kw:naming
kw:nas
kw:navigation
kw:needs-review
kw:needs-spawn
kw:netbsd
kw:network
kw:nevow
kw:new-user
kw:newcaps
kw:news
kw:news-done
kw:news-needed
kw:newsletter
kw:newurls
kw:nfc
kw:nginx
kw:nixos
kw:no-clobber
kw:node
kw:node-url
kw:notification
kw:notifyOnDisconnect
kw:nsa310
kw:nsa320
kw:nsa325
kw:numpy
kw:objects
kw:old
kw:openbsd
kw:openitp-packaging
kw:openssl
kw:openstack
kw:opensuse
kw:operation-helpers
kw:operational
kw:operations
kw:ophandle
kw:ophandles
kw:ops
kw:optimization
kw:optional
kw:options
kw:organization
kw:os
kw:os.abort
kw:ostrom
kw:osx
kw:osxfuse
kw:otf-magic-folder-objective1
kw:otf-magic-folder-objective2
kw:otf-magic-folder-objective3
kw:otf-magic-folder-objective4
kw:otf-magic-folder-objective5
kw:otf-magic-folder-objective6
kw:p2p
kw:packaging
kw:partial
kw:password
kw:path
kw:paths
kw:pause
kw:peer-selection
kw:performance
kw:permalink
kw:permissions
kw:persistence
kw:phone
kw:pickle
kw:pip
kw:pipermail
kw:pkg_resources
kw:placement
kw:planning
kw:policy
kw:port
kw:portability
kw:portal
kw:posthook
kw:pratchett
kw:preformance
kw:preservation
kw:privacy
kw:process
kw:profile
kw:profiling
kw:progress
kw:proxy
kw:publish
kw:pyOpenSSL
kw:pyasn1
kw:pycparser
kw:pycrypto
kw:pycrypto-lib
kw:pycryptopp
kw:pyfilesystem
kw:pyflakes
kw:pylint
kw:pypi
kw:pypy
kw:pysqlite
kw:python
kw:python3
kw:pythonpath
kw:pyutil
kw:pywin32
kw:quickstart
kw:quiet
kw:quotas
kw:quoting
kw:raic
kw:rainhill
kw:random
kw:random-access
kw:range
kw:raspberry-pi
kw:reactor
kw:readonly
kw:rebalancing
kw:recovery
kw:recursive
kw:redhat
kw:redirect
kw:redressing
kw:refactor
kw:referer
kw:referrer
kw:regression
kw:rekey
kw:relay
kw:release
kw:release-blocker
kw:reliability
kw:relnotes
kw:remote
kw:removable
kw:removable-disk
kw:rename
kw:renew
kw:repair
kw:replace
kw:report
kw:repository
kw:research
kw:reserved_space
kw:response-needed
kw:response-time
kw:restore
kw:retrieve
kw:retry
kw:review
kw:review-needed
kw:reviewed
kw:revocation
kw:roadmap
kw:rollback
kw:rpm
kw:rsa
kw:rss
kw:rst
kw:rsync
kw:rusty
kw:s3
kw:s3-backend
kw:s3-frontend
kw:s4
kw:same-origin
kw:sandbox
kw:scalability
kw:scaling
kw:scheduling
kw:schema
kw:scheme
kw:scp
kw:scripts
kw:sdist
kw:sdmf
kw:security
kw:self-contained
kw:server
kw:servermap
kw:servers-of-happiness
kw:service
kw:setup
kw:setup.py
kw:setup_requires
kw:setuptools
kw:setuptools_darcs
kw:sftp
kw:shared
kw:shareset
kw:shell
kw:signals
kw:simultaneous
kw:six
kw:size
kw:slackware
kw:slashes
kw:smb
kw:sneakernet
kw:snowleopard
kw:socket
kw:solaris
kw:space
kw:space-efficiency
kw:spam
kw:spec
kw:speed
kw:sqlite
kw:ssh
kw:ssh-keygen
kw:sshfs
kw:ssl
kw:stability
kw:standards
kw:start
kw:startup
kw:static
kw:static-analysis
kw:statistics
kw:stats
kw:stats_gatherer
kw:status
kw:stdeb
kw:storage
kw:streaming
kw:strports
kw:style
kw:stylesheet
kw:subprocess
kw:sumo
kw:survey
kw:svg
kw:symlink
kw:synchronous
kw:tac
kw:tahoe-*
kw:tahoe-add-alias
kw:tahoe-admin
kw:tahoe-archive
kw:tahoe-backup
kw:tahoe-check
kw:tahoe-cp
kw:tahoe-create-alias
kw:tahoe-create-introducer
kw:tahoe-debug
kw:tahoe-deep-check
kw:tahoe-deepcheck
kw:tahoe-lafs-trac-stream
kw:tahoe-list-aliases
kw:tahoe-ls
kw:tahoe-magic-folder
kw:tahoe-manifest
kw:tahoe-mkdir
kw:tahoe-mount
kw:tahoe-mv
kw:tahoe-put
kw:tahoe-restart
kw:tahoe-rm
kw:tahoe-run
kw:tahoe-start
kw:tahoe-stats
kw:tahoe-unlink
kw:tahoe-webopen
kw:tahoe.css
kw:tahoe_files
kw:tahoewapi
kw:tarball
kw:tarballs
kw:tempfile
kw:templates
kw:terminology
kw:test
kw:test-and-set
kw:test-from-egg
kw:test-needed
kw:testgrid
kw:testing
kw:tests
kw:throttling
kw:ticket999-s3-backend
kw:tiddly
kw:time
kw:timeout
kw:timing
kw:to
kw:to-be-closed-on-2011-08-01
kw:tor
kw:tor-protocol
kw:torsocks
kw:tox
kw:trac
kw:transparency
kw:travis
kw:travis-ci
kw:trial
kw:trickle
kw:trivial
kw:truckee
kw:tub
kw:tub.location
kw:twine
kw:twistd
kw:twistd.log
kw:twisted
kw:twisted-14
kw:twisted-trial
kw:twitter
kw:twn
kw:txaws
kw:type
kw:typeerror
kw:ubuntu
kw:ucwe
kw:ueb
kw:ui
kw:unclean
kw:uncoordinated-writes
kw:undeletable
kw:unfinished-business
kw:unhandled-error
kw:unhappy
kw:unicode
kw:unit
kw:unix
kw:unlink
kw:update
kw:upgrade
kw:upload
kw:upload-helper
kw:uri
kw:url
kw:usability
kw:use-case
kw:utf-8
kw:util
kw:uwsgi
kw:ux
kw:validation
kw:variables
kw:vdrive
kw:verify
kw:verlib
kw:version
kw:versioning
kw:versions
kw:video
kw:virtualbox
kw:virtualenv
kw:vista
kw:visualization
kw:visualizer
kw:vm
kw:volunteergrid2
kw:volunteers
kw:vpn
kw:wapi
kw:warners-opinion-needed
kw:warning
kw:weapi
kw:web
kw:web.port
kw:webapi
kw:webdav
kw:webdrive
kw:webport
kw:websec
kw:website
kw:websocket
kw:welcome
kw:welcome-page
kw:welcomepage
kw:wiki
kw:win32
kw:win64
kw:windows
kw:windows-related
kw:winscp
kw:workaround
kw:world-domination
kw:wrapper
kw:write-enabler
kw:wui
kw:x86
kw:x86-64
kw:xhtml
kw:xml
kw:xss
kw:zbase32
kw:zetuptoolz
kw:zfec
kw:zookos-opinion-needed
kw:zope
kw:zope.interface
p/blocker
p/critical
p/major
p/minor
p/normal
p/supercritical
p/trivial
r/cannot reproduce
r/duplicate
r/fixed
r/invalid
r/somebody else's problem
r/was already fixed
r/wontfix
r/worksforme
t/defect
t/enhancement
t/task
v/0.2.0
v/0.3.0
v/0.4.0
v/0.5.0
v/0.5.1
v/0.6.0
v/0.6.1
v/0.7.0
v/0.8.0
v/0.9.0
v/1.0.0
v/1.1.0
v/1.10.0
v/1.10.1
v/1.10.2
v/1.10a2
v/1.11.0
v/1.12.0
v/1.12.1
v/1.13.0
v/1.14.0
v/1.15.0
v/1.15.1
v/1.2.0
v/1.3.0
v/1.4.1
v/1.5.0
v/1.6.0
v/1.6.1
v/1.7.0
v/1.7.1
v/1.7β
v/1.8.0
v/1.8.1
v/1.8.2
v/1.8.3
v/1.8β
v/1.9.0
v/1.9.0-s3branch
v/1.9.0a1
v/1.9.0a2
v/1.9.0b1
v/1.9.1
v/1.9.2
v/1.9.2a1
v/cloud-branch
v/unknown
No project
No assignees
2 participants
Notifications
Due date
No due date set.
Dependencies
No dependencies set.
Reference: tahoe-lafs/trac#272
Loading…
Add table
Reference in a new issue
No description provided.
Delete branch "%!s()"
Deleting a branch is permanent. Although the deleted branch may continue to exist for a short time before it actually gets removed, it CANNOT be undone in most cases. Continue?
If a mutable slot has one or two shares at a newer version than the rest, our
current SMDF update code is unable to recover: all attempts to write a newer
version will fail with an UncoordinatedWriteError. The necessary fix is
twofold: the first part is to implement mutable-slot recovery (to replace all
shares with a version that's newer than the loner, by promoting the
older-but-more-popular-and-also-recoverable version to a later seqnum), the
second part is to jump directly to recovery when the post-query pre-write
sharemap determines that someone has a newer version than the one we want to
write (in Publish._got_all_query_results), then try the read-and-replace
again or defer to the application.
Oh how I want the thorough unit tests envisioned in #270! They would have
caught this earlier.
This problem happened to Peter's directory because of the accidental
nodeid-change described in #269. But it could occur without that problem with
the right sort of network partition at the right time.
Ok, on to the bug:
Suppose that share1 is placed on peer1, share2 on peer2, etc, and that we're
doing 3-of-10 encoding. Now suppose that an update is interrupted in such a
way that peer9 is updated but peers0 through 8 are not. Now most peers are at
version 4, but that one loner peer is at version 5. (in the case of Peter's
directory, most of the servers were upgraded incorrectly and changed their
nodeids, thus changing the write_enablers, and the loner peer was the one who
was either upgraded correctly or who wasn't upgraded at all).
If we read this slot, we read 'k' plus epsilon shares (four or five, I
think). We see everyone is at version 4 (rather, everyone we see is at
version 4), so we conclude that 4 is the most recent version. This is fine,
because in the face of uncoordinated writes, we're allowed to return any
version that was ever written.
Now, the next time we want to update this slot, mutable.Publish gets control.
This does read-before-replace, and it is written to assume that we can use a
seqnum one larger than the value that was last read. So we prepare to send
out shares at version "5".
We start by querying N+epsilon peers, to build up a sharemap of the
seqnum+roothash of every server we're thinking of sending a share to, so we
can decide which shares to send to whom, and to build up the testv list. This
sharemap is processed in Publish._got_all_query_results . It has a check that
raises UncoordinatedWriteError if one of the queries reports the existence
of a share that is newer than the seqnum that we already knew about. In this
case, the response from peer9 shows seqnum=5, which is equal-or-greater than
the "5" that we wanted to send out. This is evidence of an uncoordinated
write, because our read pass managed to extract version 4 from the grid, but
our query pass shows evidence of a version 5. We can tolerate lots of 5s and
one or two 4s (because then the read pass would have been unable to
reconstruct a version 4, and would have kept searching, and would have
eventually reconstructed a version 5), but we can't tolerate lots of 4s and
one or two 5s.
So this is the problem: we're spooked into an UncoordinatedWriteError by the
loner new share, and since we don't yet have any recovery code, we can't fix
the situation. If we tried to write out the new shares anyways, we could
probably get a quorum of our new seqnum=5 shares, and if next update time we
managed to reconstruct version 5, then we'd push out seqnum=6 shares to
everybody, and then the problem would go away.
We need recovery to handle this. When the UncoordinatedWriteError is
detected during the query phase, we should pass the sharemap to the recovery
code, which should pick a version to reinforce (according to the rules we
came up with in mutable.txt), and then send out shares as necessary to make
that version the dominant one. (If I recall correctly, I think that means we
would take the data from version 4, re-encode it into a version 6, then push
6 out to everyone).
Once recovery is done, the Publish attempt should still fail with an
UncoordinatedWriteError, as a signal to the application that their write
attempt might have failed. The application should perform a new read and
possibly attempt to make its modification again.
The presence of the #269 bug would interact with recovery code in a bad way:
it is likely that many of these shares are now immutable, and thus recovery
would be unable to get a quorum of the new version. But since we could easily
get into this state without #269 (by terminating a client just after it sends
out the first share update, before it manages to get 'k' share-update
messages out), this remains a serious problem.
mutable update can't recover from <k new sharesto implement mutable-file recovery: update can't recover from <k new sharesSee also related issue #232 -- "http://allmydata.org/trac/tahoe/ticket/232".
I mean #232 -- "Peer selection doesn't rebalance shares on overwrite of mutable file."
Rob hit a bad case today which might have been caused by this issue. He was doing parallel, uncoordinated writes to directories (by accident), and he reported:
For Milestone 1.0.0, let's see if we can figure out what happened with Rob's client there, and see if it is likely to cause any problems for clients in Tahoe v1.0.0, and if not then bump this ticket.
that "I was surprised" is certainly an indication of detected collision. Not
being able to find the encrypted privkey.. oh, I think I understand that one.
The encrypted private key is stored at the end of the share, since it's kind
of large, and the dominant use case (reading) doesn't need to retrieve it. On
the first pass (the "reconnaissance phase"), we find out what the data size
is, and therefore which offset the encprivkey is at. On the second pass, we
use that offset to retrieve the encprivkey. But if someone sneaks in between
our first and second passes (and writes new data, of a different size), then
the key will have moved, and our attempt to read the encprivkey will grab the
wrong data. I'm not sure what exactly will happen (i.e. how we validate the
encprivkey), but somewhere in there it ought to throw a hash check exception
and drop that alleged copy of the privkey, causing the code to try and find a
better one. If this process discards all potential copies, the upload will
fail for lack of a privkey.
So, if Rob could identify the exact time of the collision, we could probably
look at the server logs and confirm the server-side collision sequence.
The rule remains: Don't Do That. :)
Brian:
Thanks for the analysis. The collision sequence that you describe should result in this error message once, but then subsequent attempts to overwrite should succeed. But I thought Rob described a problem where the directory could not be written again. Perhaps I imagined that last part.
If it is a transient problem then I am satisfied with this for allmydata.org "Tahoe" 1.0.
What? No it doesn't! We've decided that it is okay for users of Tahoe to do this, as long as they don't do it with too many simultaneous writers at too fast a rate, and they don't mind all but one of their colliding writes disappearing with a trace.
Right?
You're right. The rule is: Don't Do That Very Much. :)
Okay, we talked about this and concluded that what Rob saw could be explained by the uncoordinated writes that his script accidentally generated. We updated the error messages to be clearer about what was surprising about the sequence numbers. Bumping this ticket out of Milestone 1.0.0.
Brian: should this ticket be updated to include anything from your recent refactoring of the mutable file upload/download code?
It's almost closed. I believe the new servermap scheme should fix this case.
I'm working on a test case to specifically exercise it now.
Closed, by the test added in changeset:ff0b9e25499c7e5f.