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Hi Sam,

My five cents.

Using Fountain codes, which are also a class of EC, one can make all
the blocks equivalent in role (no separation into data and parity
blocks).
http://en.wikipedia.org/wiki/Fountain_code

They resolve a few of the issues that you raised, however they may
raise others - e.g. it's more difficult to determine how many blocks
you need to fetch to reconstruct the data.

On Wed, Oct 17, 2012 at 4:24 PM, Samuel Merritt <sam at swiftstack.com> wrote:
> On 10/15/12 5:36 PM, Duan, Jiangang wrote:
>>
>> Some of our customers are interested in Erasure code than tri-replicate to
>> save disk space.
>> We propose a BP "Light weight Erasure code framework for swift", which can
>> be found here https://blueprints.launchpad.net/swift/+spec/swift-ec
>> The general idea is to have some daemon on storage node to do offline scan
>> - select code object with big enough size to do EC.
>>
>> Will glad to hear any feedback on this.
>
>
> Here, in no particular order, are some thoughts I have.
>
> - Object blocks (both data blocks and parity blocks) will need to be marked
> somehow so that 3 replicas of each block aren't kept. This is a pretty
> fundamental change to Swift; up until now, all objects are treated the same.
> It's essentially introducing the notion of tiered storage into Swift.
>
> - Who's responsible for ensuring the presence of all the blocks? That is,
> assume you have an object that's been split into ten data blocks (D1, D2,
> ..., D10) and 2 parity blocks (P1, P2). The drive with D7 on it dies. Which
> replicator(s) is(are) responsible for rebuilding D7 and storing it on a
> handoff node?
>
> If you have the replicators on each block's machine checking for failures,
> then you'll wind up with more people checking each replica. Here, it would
> be 11 replicators ensuring that each block is present. Compare that to the
> full-replication case, where there are 2 replicators checking on it. That's
> going to result in more traffic on the internal network.
>
> - There will need to be throttles on the transformation daemons (replica ->
> EC and vice versa), as that's very IO intensive. If a big bunch of data is
> uploaded at one time and then not accessed (think large backups), then that
> could be a ticking time bomb for my cluster performance. After those objects
> become "cold", the transformation daemons will thrash my disks and network
> turning them into EC-type objects.
>
> - Does this open up a Swift cluster to a DoS attack? If my objects are
> stored w/EC, then can someone go through and request a few bytes from each
> object in my cluster a few times and cause all my objects to get "hot"?
> Under the proposed scheme, this would turn my objects from EC-storage to
> replica-storage, filling up my disks and killing my cluster. To mitigate
> that, I'd have to keep enough disk around to hold 3 replicas of everything,
> and at that point, I may as well just keep the 3 replicas.
>
> - Another thought for a resource-consumption attack: can someone slowly walk
> my objects and make a large fraction (say, 5%) of them hot each day? That
> seems like it would make the transformation daemons run at maximum capacity
> all the time trying to keep up.
>
> - Retrieval of EC-stored objects becomes more failure-prone. With
> replica-stored objects, 1 out of 3 object servers has to be available for a
> GET request to work. With EC-stored objects and a 10:2 coding, 10 out of 12
> object servers have to be available. That makes network partitions much
> worse for data availability.
>
> - EC-storage is at odds with geographic replication. Of course, Swift
> supports neither one today. However, with geographic replication, one wants
> to have a local replica of each each object in each geographic region, which
> results in more copies for lower latency. With EC-storage, less data is
> stored. When they're combined, the result is a whole lot of traffic across
> slow, expensive WAN links.
>
> - Recombining EC-stored object chunks is going to chew up a ton more CPU on
> either the object or proxy servers, depending on which one does it. If the
> proxy, then it'll add more to an already CPU-heavy workload. If the object
> server, then it'll make using big storage boxes less practical (like one of
> the 48-drives-in-4U servers one can buy).
>
> - Can one change the EC-coding level? That is, if I'm using 10:2 coding (so
> each object turns into 10 data blocks and 2 parity blocks), can I change
> that later? Will that have massive performance impacts on my cluster as more
> data blocks are computed?
>
> It may be that this is like changing the replica count, and the answer is
> "yes, but your cluster will thrash for a long time after you do it".
>
> - Where's the original checksum stored? Clearly, each block will have its
> own checksum for the auditors to use. However, if a client issues a request
> like "HEAD /a/c/o", that'll contain the checksum of the original file. Does
> that live somewhere, or will the proxy have to read all the bytes and
> determine the checksum?
>
> - I wonder what effect this will have on internal-network traffic. With a
> replica-stored object, the proxy opens one connection to an object server,
> sends a request, gets a response, and streams the bytes out to the client.
>
> With an EC-stored object, the proxy has to open connections to, say, 10
> different object servers. Further, if one of the data blocks is unavailable
> (say data block 5), then the proxy has to go ahead and re-request all the
> data blocks plus a parity block so that it can fill in the gaps. That may be
> a significant increase in traffic on Swift's internal network. Further, by
> using such a large number of connections, it considerably increases the
> probability of a connection failure, which would mean more client requests
> would fail with truncated downloads.
>
>
> Those are all the thoughts I have right now that are coherent enough to put
> into text. Clearly, adding erasure coding (or any other form of tiered
> storage) to Swift is not something undertaken lightly.
>
> Hope this helps.
>
>
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