Changes between Version 21 and Version 22 of FAQ


Ignore:
Timestamp:
2010-07-01T14:45:02Z (14 years ago)
Author:
terrell
Comment:

boldify the questions

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  • FAQ

    v21 v22  
    1 Q: What is special about Tahoe-LAFS? Why should anyone care about it instead of [http://tahoe-lafs.org/trac/tahoe/wiki/RelatedProjects#OtherProjects other distributed storage systems]?
     1'''Q: What is special about Tahoe-LAFS? Why should anyone care about it instead of [http://tahoe-lafs.org/trac/tahoe/wiki/RelatedProjects#OtherProjects other distributed storage systems]?'''
    22
    33A1: Tahoe-LAFS is the first Free !Software/Open Source storage technology which offers ''provider-independent security''.  ''Provider-independent security'' means that the integrity and confidentiality of your files is guaranteed by mathematics computed on the client side, and is independent of the servers, which may be owned and operated by someone else.  To learn more, read [http://tahoe-lafs .org/source/tahoe/trunk/docs/about.html our one-page explanation].
    … …  
    55A2: Tahoe-LAFS is an extremely reliable, distributed, fault-tolerant storage system. Even if you do not need its security properties, you might want to use Tahoe-LAFS as a very robust and flexible storage platform. (Tahoe-LAFS's security features do an excellent job of staying out of your way when you don't need them.)
    66
    7 Q: "Erasure-coding"?  What's that?
     7'''Q: "Erasure-coding"?  What's that?'''
    88
    99A: You know how with RAID-5 you can lose any one drive and still recover?  And there is also something called RAID-6 where you can lose any two drives and still recover.  Erasure coding is the generalization of this pattern: you get to configure it for how many drives you could lose and still recover.  Tahoe-LAFS is typically configured to upload each file to 10 different drives, where you can lose any 7 of them and still recover the entire file.  This gives radically better reliability than typical RAID setups, at a cost of only 3.3 times the storage space that a single copy takes. (This technique is also known as "forward error correction" and as an "information dispersal algorithm".)
    1010
    11 Q: Is there a way to disable the encryption phase and just use the encoding on the actual content? Won't that save a lot of CPU cycles?
     11'''Q: Is there a way to disable the encryption phase and just use the encoding on the actual content? Won't that save a lot of CPU cycles?'''
    1212
    1313A: There isn't currently a way to disable or skip the encryption phase, but if you watch the status page on your local tahoe-lafs node for uploads, you'll see that the encryption time is orders (yes, plural) of magnitude smaller than the upload time, so there isn't much performance to be gained by skipping the encryption. We prefer 'secure by default', so without a compelling reason to allow insecure operation, our plan is to leave encryption turned on all the time.
    1414
    15 Q: Where should I look for current documentation about Tahoe's protocols?
     15'''Q: Where should I look for current documentation about Tahoe's protocols?'''
    1616
    1717A: http://tahoe-lafs.org/source/tahoe/trunk/docs/architecture.txt
    1818
    19 Q: Does Tahoe-LAFS work on embedded devices such as a [http://www.pogoplug.com PogoPlug] or an [http://openwrt.org OpenWRT] router?
     19'''Q: Does Tahoe-LAFS work on embedded devices such as a [http://www.pogoplug.com PogoPlug] or an [http://openwrt.org OpenWRT] router?'''
    2020
    2121A: Probably! François Deppierraz contributes [http://tahoe-lafs.org/buildbot/builders/FranXois%20lenny-armv5tel a buildbot] which shows that Tahoe-LAFS builds and all the unit tests pass on his QNAP TS-109 NAS box running under Debian Lenny.  There seems to be a lot of interest in this sort of deployment -- someone joins the mailing list and asks about this approximately every month (see [http://tahoe-lafs.org/pipermail/tahoe-dev/ the archives]).  Zandr Milewski [http://tahoe-lafs.org/pipermail/tahoe-dev/2009-November/003157.html reported] that it took him only an hour to build, install, and test Tahoe-LAFS on a !PogoPlug.
    2222
    23 Q: Does Tahoe-LAFS work on Windows?
     23'''Q: Does Tahoe-LAFS work on Windows?'''
    2424
    2525A: Yes.  There are two ways to do it:
    … …  
    3030 2. Follow [http://tahoe-lafs.org/source/tahoe-lafs/trunk/docs/quickstart.html the standard quickstart instructions] to get Tahoe-LAFS running on Windows.
    3131
    32 Q: Does Tahoe-LAFS work on Mac OS X?
     32'''Q: Does Tahoe-LAFS work on Mac OS X?'''
    3333
    3434A: Yes.  Follow [http://tahoe-lafs.org/source/tahoe-lafs/trunk/docs/quickstart.html the standard quickstart instructions] on Mac OS X and it will result in a working command-line tool on Mac OS X just as it does on other Unixes. (This includes the Web User Interface, or "WUI". See the instructions for details.) In addition there is code to generate executables and .dmg packages, but this code is not currently working (as of Tahoe-LAFS v1.6.1). See the "mac" targets [source:Makefile@#L440 in the Makefile].
    3535
    36 Q: Can there be more than one storage folder on a storage node? So if a storage server contains 3 drives without RAID, can it use all 3 for storage?
     36'''Q: Can there be more than one storage folder on a storage node? So if a storage server contains 3 drives without RAID, can it use all 3 for storage?'''
    3737
    3838A: Not directly. Each storage server has a single "base directory" which we abbreviate as $BASEDIR. The server keeps all of its shares in a subdirectory named $BASEDIR/storage/shares/ . (Note that you can symlink  this to whatever you want: you can run most of the node from one place, and store all the shares somewhere else). Since there's only one such subdirectory, you can only use one filesystem per node.On the other hand, shares are stored in a set of 1024 subdirectories of that one, named $BASEDIR/storage/shares/aa/, $BASEDIR/storage/shares/ab/, etc. If you were to symlink the first third of these to one filesystem, the next third to a second filesystem, etc, (hopefully with a script!), then you'd get about 1/3rd of the shares stored on each disk. The "how much space is available" and space-reservation tools would be confused, but basically everything else should work normally.
    3939
    40 Q: Would it make sense to just use RAID-0 and let Tahoe-LAFS deal with the redundancy?
     40'''Q: Would it make sense to just use RAID-0 and let Tahoe-LAFS deal with the redundancy?'''
    4141
    4242A: The Allmydata grid didn't bother with RAID at all: each Tahoe storage server node used a single spindle.
    … …  
    6666easily get shares doubled up and not distributed as evenly as if you'd done a single upload. This is being tracked in ticket #610.
    6767
    68 Q: Suppose I have a file of 100GB and 2 storage nodes each with 75GB available, will I be able to store the file or does it have to fit
    69 within the realms of a single node?
     68'''Q: Suppose I have a file of 100GB and 2 storage nodes each with 75GB available, will I be able to store the file or does it have to fit
     69within the realms of a single node?'''
    7070
    7171A: The ability to store the file will depend upon how you set the encoding parameters: you get to choose the tradeoff between expansion (how much
    … …  
    8383only a single file, so there's some benefit to this even if you're still vulnerable to a full disk/server failure.
    8484
    85 Q: Do I need to shutdown all clients/servers to add a storage node?
     85'''Q: Do I need to shutdown all clients/servers to add a storage node?'''
    8686
    8787A: No, You can add or remove clients or servers anytime you like. The central "Introducer" is responsible for telling clients and servers
    … …  
    9696implement such a feature without a restart requirement too.
    9797
    98 Q: If I had 3 locations each with 5 storage nodes, could I configure the grid to ensure a file is written to each location so that I could handle all
    99 servers at a particular location going down?
     98'''Q: If I had 3 locations each with 5 storage nodes, could I configure the grid to ensure a file is written to each location so that I could handle all
     99servers at a particular location going down?'''
    100100
    101101A: Not directly. We have a ticket about that one (#467, #302), but it's deeper than it looks and we haven't come to a conclusion on how to
    … …  
    126126So you've got a 50% chance of the ideal distribution, and a 1/1000 chance of the worst-case distribution.
    127127
    128 Q: Is it possible to modify a mutable file by "patching" it?
    129 So, if I have a file stored and I want to update a section of the file in the middle, is that possible or would be file need to be downloaded,
    130 patched and re-uploaded?
     128'''Q: Is it possible to modify a mutable file by "patching" it? Also... if I have a file stored and I want to update a section of the file in the middle, is that possible or would be file need to be downloaded, patched and re-uploaded?'''
    131129
    132130A: Not at present. We've only implemented "Small Distributed Mutable Files" (SDMF) so far, which have the property that the whole file must be