From: Brian Warner Date: Tue, 6 May 2008 20:49:01 +0000 (-0700) Subject: docs/helper.txt: explain more about the helper X-Git-Tag: allmydata-tahoe-1.1.0~161 X-Git-Url: https://git.rkrishnan.org/specifications/banana.xhtml?a=commitdiff_plain;h=783fbb12e608795bb2000417c3f523aa84415f72;p=tahoe-lafs%2Ftahoe-lafs.git docs/helper.txt: explain more about the helper --- diff --git a/docs/architecture.txt b/docs/architecture.txt index 0138796d..dac1e212 100644 --- a/docs/architecture.txt +++ b/docs/architecture.txt @@ -259,6 +259,8 @@ commercially-run grid for which all of the storage servers are in a colo facility with high interconnect bandwidth. In this case, the helper is placed in the same facility, so the helper-to-storage-server bandwidth is huge. +See "helper.txt" for details about the upload helper. + VDRIVE and DIRNODES: THE VIRTUAL DRIVE LAYER diff --git a/docs/helper.txt b/docs/helper.txt new file mode 100644 index 00000000..0e71396a --- /dev/null +++ b/docs/helper.txt @@ -0,0 +1,165 @@ + += The Tahoe Upload Helper = + +As described in the "SWARMING DOWNLOAD, TRICKLING UPLOAD" section of +architecture.txt, Tahoe uploads require more bandwidth than downloads: you +must push the redundant shares during upload, but you do not need to retrieve +them during download. With the default 3-of-10 encoding parameters, this +means that an upload will require about 3.3x the traffic as a download of the +same file. + +Unfortunately, this "expansion penalty" occurs in the same upstream direction +that most consumer DSL lines are slow anyways. Typical ADSL lines get 8 times +as much download capacity as upload capacity. When the ADSL upstream penalty +is combined with the expansion penalty, the result is uploads that can take +up to 32 times longer than downloads. + +The "Helper" is a service that can mitigate the expansion penalty by +arranging for the client node to send data to a central Helper node instead +of sending it directly to the storage servers. It sends ciphertext to the +Helper, so the security properties remain the same as with non-Helper +uploads. The Helper is responsible for applying the erasure encoding +algorithm and placing the resulting shares on the storage servers. + +Of course, the helper cannot mitigate the ADSL upstream penalty. + +The second benefit of using an upload helper is that clients who lose their +network connections while uploading a file (because of a network flap, or +because they shut down their laptop while an upload was in progress) can +resume their upload rather than needing to start again from scratch. The +helper holds the partially-uploaded ciphertext on disk, and when the client +tries to upload the same file a second time, it discovers that the partial +ciphertext is already present. The client then only needs to upload the +remaining ciphertext. This reduces the "interrupted upload penalty" to a +minimum. + +This also serves to reduce the number of active connections between the +client and the outside world: most of their traffic flows over a single TCP +connection to the helper. This can improve TCP fairness, and should allow +other applications that are sharing the same uplink to compete more evenly +for the limited bandwidth. + + + +== Setting Up A Helper == + +Who should consider running a helper? + + * Benevolent entities which wish to provide better upload speed for clients + that have slow uplinks + * Folks which have machines with upload bandwidth to spare. + * Server grid operators who want clients to connect to a small number of + helpers rather than a large number of storage servers (a "multi-tier" + architecture) + +What sorts of machines are good candidates for running a helper? + + * The Helper needs to have good bandwidth to the storage servers. In + particular, it needs to have at least 3.3x better upload bandwidth than + the client does, or the client might as well upload directly to the + storage servers. In a commercial grid, the helper should be in the same + colo (and preferably in the same rack) as the storage servers. + * The Helper will take on most of the CPU load involved in uploading a file. + So having a dedicated machine will give better results. + * The Helper buffers ciphertext on disk, so the host will need at least as + much free disk space as there will be simultaneous uploads. When an upload + is interrupted, that space will be used for a longer period of time. + +To turn a Tahoe client node into a helper (i.e. to run a helper service in +addition to whatever else that node is doing), just create a file in your +node's base directory named "run_helper", and put any non-empty string in it. +Then restart the node: + + echo "yes" >$BASEDIR/run_helper + tahoe restart $BASEDIR + +Then restart the node. This will signal the node to create a Helper service +and listen for incoming requests. Once the node has started, there will be a +file named private/helper.furl which contains the contact information for the +helper: you will need to give this FURL to any clients that wish to use your +helper. + + cat $BASEDIR/private/helper.furl |mail -s "helper furl" friend@example.com + +You can tell if your node is running a helper by looking at its web status +page. Assuming that you've set up the 'webport' to use port 8123, point your +browser at http://localhost:8123/ . The welcome page will say "Helper: 0 +active uploads" or "Not running helper" as appropriate. The +http://localhost:8123/helper_status page will also provide details on what +the helper is currently doing. + +The helper will store the ciphertext that is is fetching from clients in +$BASEDIR/helper/CHK_incoming/ . Once all the ciphertext has been fetched, it +will be moved to $BASEDIR/helper/CHK_encoding/ and erasure-coding will +commence. Once the file is fully encoded and the shares are pushed to the +storage servers, the ciphertext file will be deleted. + +If a client disconnects while the ciphertext is being fetched, the partial +ciphertext will remain in CHK_incoming/ until they reconnect and finish +sending it. If a client disconnects while the ciphertext is being encoded, +the data will remain in CHK_encoding/ until they reconnect and encoding is +finished. For long-running and busy helpers, it may be a good idea to delete +files in these directories that have not been modified for a week or two. +Future versions of tahoe will try to self-manage these files a bit better. + +== Using a Helper == + +Who should consider using a Helper? + + * clients with limited upstream bandwidth, such as a consumer ADSL line + * clients who believe that the helper will give them faster uploads than + they could achieve with a direct upload + * clients who experience problems with TCP connection fairness: if other + programs or machines in the same home are getting less than their fair + share of upload bandwidth. If the connection is being shared fairly, then + a Tahoe upload that is happening at the same time as a single FTP upload + should get half the bandwidth. + * clients who have been given the helper.furl by someone who is running a + Helper and is willing to let them use it + +To take advantage of somebody else's Helper, take the helper.furl file that +they give you, and copy it into your node's base directory, then restart the +node: + + cat email >$BASEDIR/helper.furl + tahoe restart $BASEDIR + +This will signal the client to try and connect to the helper. Subsequent +uploads will use the helper rather than using direct connections to the +storage server. + +If the node has been configured to use a helper, that node's HTTP welcome +page (http://localhost:8123/) will say "Helper: $HELPERFURL" instead of +"Helper: None". If the helper is actually running and reachable, the next +line will say "Connected to helper?: yes" instead of "no". + +The helper is optional. If a helper is connected when an upload begins, the +upload will use the helper. If there is no helper connection present when an +upload begins, that upload will connect directly to the storage servers. The +client will automatically attempt to reconnect to the helper if the +connection is lost, using the same exponential-backoff algorithm as all other +tahoe/foolscap connections. + +The upload/download status page (http://localhost:8123/status) will announce +the using-helper-or-not state of each upload, in the "Helper?" column. + +== Other Helper Modes == + +The Tahoe Helper only currently helps with one kind of operation: uploading +immutable files. There are three other things it might be able to help with +in the future: + + * downloading immutable files + * uploading mutable files (such as directories) + * downloading mutable files (like directories) + +Since mutable files are currently limited in size, the ADSL upstream penalty +is not so severe for them. There is no ADSL penalty to downloads, but there +may still be benefit to extending the helper interface to assist with them: +fewer connections to the storage servers, and better TCP fairness. + +A future version of the Tahoe helper might provide assistance with these +other modes. If it were to help with all four modes, then the clients would +not need direct connections to the storage servers at all: clients would +connect to helpers, and helpers would connect to servers. For a large grid +with tens of thousands of clients, this might make the grid more scalable.