From: Ben Peart <firstname.lastname@example.org> To: email@example.com Cc: firstname.lastname@example.org Subject: [RFC] Add support for downloading blobs on demand Date: Fri, 13 Jan 2017 10:52:53 -0500 Message-ID: <email@example.com> (raw) [-- Warning: decoded text below may be mangled --] [-- Attachment #0: Type: text/plain, Size: 4344 bytes --] Goal ~~~~ To be able to better handle repos with many files that any individual developer doesnt need it would be nice if clone/fetch only brought down those files that were actually needed. To enable that, we are proposing adding a flag to clone/fetch that will instruct the server to limit the objects it sends to commits and trees and to not send any blobs. When git performs an operation that requires a blob that isnt currently available locally, it will download the missing blob and add it to the local object store. Design ~~~~~~ Clone and fetch will pass a --lazy-clone flag (open to a better name here) similar to --depth that instructs the server to only return commits and trees and to ignore blobs. Later during git operations like checkout, when a blob cannot be found after checking all the regular places (loose, pack, alternates, etc), git will download the missing object and place it into the local object store (currently as a loose object) then resume the operation. To prevent git from accidentally downloading all missing blobs, some git operations are updated to be aware of the potential for missing blobs. The most obvious being check_connected which will return success as if everything in the requested commits is available locally. To minimize the impact on the server, the existing dumb HTTP protocol endpoint objects/<sha> can be used to retrieve the individual missing blobs when needed. Performance considerations ~~~~~~~~~~~~~~~~~~~~~~~~~~ We found that downloading commits and trees on demand had a significant negative performance impact. In addition, many git commands assume all commits and trees are available locally so they quickly got pulled down anyway. Even in very large repos the commits and trees are relatively small so bringing them down with the initial commit and subsequent fetch commands was reasonable. After cloning, the developer can use sparse-checkout to limit the set of files to the subset they need (typically only 1-10% in these large repos). This allows the initial checkout to only download the set of files actually needed to complete their task. At any point, the sparse-checkout file can be updated to include additional files which will be fetched transparently on demand. Typical source files are relatively small so the overhead of connecting and authenticating to the server for a single file at a time is substantial. As a result, having a long running process that is started with the first request and can cache connection information between requests is a significant performance win. Now some numbers ~~~~~~~~~~~~~~~~ One repo has 3+ million files at tip across 500K folders with 5-6K active developers. They have done a lot of work to remove large files from the repo so it is down to < 100GB. Before changes: clone took hours to transfer the 87GB .pack + 119MB .idx After changes: clone took 4 minutes to transfer 305MB .pack + 37MB .idx After hydrating 35K files (the typical number any individual developer needs to do their work), there was an additional 460 MB of loose files downloaded. Total savings: 86.24 GB * 6000 developers = 517 Terabytes saved! We have another repo (3.1 M files, 618 GB at tip with no history with 3K+ active developers) where the savings are even greater. Future Work ~~~~~~~~~~~ The current prototype calls a new hook proc in sha1_object_info_extended and read_object, to download each missing blob. A better solution would be to implement this via a long running process that is spawned on the first download and listens for requests to download additional objects until it terminates when the parent git operation exits (similar to the recent long running smudge and clean filter work). Need to do more investigation into possible code paths that can trigger unnecessary blobs to be downloaded. For example, we have determined that the rename detection logic in status can also trigger unnecessary blobs to be downloaded making status slow. Need to investigate an alternate batching scheme where we can make a single request for a set of "related" blobs and receive single a packfile (especially during checkout). Need to investigate adding a new endpoint in the smart protocol that can download both individual blobs as well as a batch of blobs.
next reply index Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-01-13 15:52 Ben Peart [this message] 2017-01-13 21:07 ` Shawn Pearce 2017-01-17 21:50 ` Ben Peart 2017-01-17 22:05 ` Martin Fick 2017-01-17 22:23 ` Stefan Beller 2017-01-18 18:27 ` Ben Peart 2017-01-17 18:42 ` Jeff King 2017-01-17 21:50 ` Ben Peart 2017-02-05 14:03 ` Christian Couder 2017-02-07 18:21 ` Ben Peart 2017-02-07 21:56 ` Jakub Narębski 2017-02-08 2:18 ` Ben Peart 2017-02-23 15:39 ` Ben Peart
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