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 <s...@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. > > > _______________________________________________ > Mailing list: https://launchpad.net/~openstack > Post to : openstack@lists.launchpad.net > Unsubscribe : https://launchpad.net/~openstack > More help : https://help.launchpad.net/ListHelp -- Eugene Kirpichov http://www.linkedin.com/in/eugenekirpichov We're hiring! http://tinyurl.com/mirantis-openstack-engineer _______________________________________________ Mailing list: https://launchpad.net/~openstack Post to : openstack@lists.launchpad.net Unsubscribe : https://launchpad.net/~openstack More help : https://help.launchpad.net/ListHelp
