2011-10-29 01:48:40 +04:00
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/*
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* Copyright (c) 2011, Google Inc.
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*/
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#ifndef BULK_CHECKIN_H
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#define BULK_CHECKIN_H
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2023-03-21 09:25:53 +03:00
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#include "object.h"
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2018-08-15 20:54:05 +03:00
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core.fsyncmethod: batched disk flushes for loose-objects
When adding many objects to a repo with `core.fsync=loose-object`,
the cost of fsync'ing each object file can become prohibitive.
One major source of the cost of fsync is the implied flush of the
hardware writeback cache within the disk drive. This commit introduces
a new `core.fsyncMethod=batch` option that batches up hardware flushes.
It hooks into the bulk-checkin odb-transaction functionality, takes
advantage of tmp-objdir, and uses the writeout-only support code.
When the new mode is enabled, we do the following for each new object:
1a. Create the object in a tmp-objdir.
2a. Issue a pagecache writeback request and wait for it to complete.
At the end of the entire transaction when unplugging bulk checkin:
1b. Issue an fsync against a dummy file to flush the log and hardware
writeback cache, which should by now have seen the tmp-objdir writes.
2b. Rename all of the tmp-objdir files to their final names.
3b. When updating the index and/or refs, we assume that Git will issue
another fsync internal to that operation. This is not the default
today, but the user now has the option of syncing the index and there
is a separate patch series to implement syncing of refs.
On a filesystem with a singular journal that is updated during name
operations (e.g. create, link, rename, etc), such as NTFS, HFS+, or XFS
we would expect the fsync to trigger a journal writeout so that this
sequence is enough to ensure that the user's data is durable by the time
the git command returns. This sequence also ensures that no object files
appear in the main object store unless they are fsync-durable.
Batch mode is only enabled if core.fsync includes loose-objects. If
the legacy core.fsyncObjectFiles setting is enabled, but core.fsync does
not include loose-objects, we will use file-by-file fsyncing.
In step (1a) of the sequence, the tmp-objdir is created lazily to avoid
work if no loose objects are ever added to the ODB. We use a tmp-objdir
to maintain the invariant that no loose-objects are visible in the main
ODB unless they are properly fsync-durable. This is important since
future ODB operations that try to create an object with specific
contents will silently drop the new data if an object with the target
hash exists without checking that the loose-object contents match the
hash. Only a full git-fsck would restore the ODB to a functional state
where dataloss doesn't occur.
In step (1b) of the sequence, we issue a fsync against a dummy file
created specifically for the purpose. This method has a little higher
cost than using one of the input object files, but makes adding new
callers of this mechanism easier, since we don't need to figure out
which object file is "last" or risk sharing violations by caching the fd
of the last object file.
_Performance numbers_:
Linux - Hyper-V VM running Kernel 5.11 (Ubuntu 20.04) on a fast SSD.
Mac - macOS 11.5.1 running on a Mac mini on a 1TB Apple SSD.
Windows - Same host as Linux, a preview version of Windows 11.
Adding 500 files to the repo with 'git add' Times reported in seconds.
object file syncing | Linux | Mac | Windows
--------------------|-------|-------|--------
disabled | 0.06 | 0.35 | 0.61
fsync | 1.88 | 11.18 | 2.47
batch | 0.15 | 0.41 | 1.53
Signed-off-by: Neeraj Singh <neerajsi@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2022-04-05 08:20:09 +03:00
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void prepare_loose_object_bulk_checkin(void);
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void fsync_loose_object_bulk_checkin(int fd, const char *filename);
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bulk-checkin: only support blobs in index_bulk_checkin
As the code is written today index_bulk_checkin only accepts blobs.
Remove the enum object_type parameter and rename index_bulk_checkin to
index_blob_bulk_checkin, index_stream to index_blob_stream,
deflate_to_pack to deflate_blob_to_pack, stream_to_pack to
stream_blob_to_pack, to make this explicit.
Not supporting commits, tags, or trees has no downside as it is not
currently supported now, and commits, tags, and trees being smaller by
design do not have the problem that the problem that index_bulk_checkin
was built to solve.
Before we start adding code to support the hash function transition
supporting additional objects types in index_bulk_checkin has no real
additional cost, just an extra function parameter to know what the
object type is. Once we begin the hash function transition this is not
the case.
The hash function transition document specifies that a repository with
compatObjectFormat enabled will compute and store both the SHA-1 and
SHA-256 hash of every object in the repository.
What makes this a challenge is that it is not just an additional hash
over the same object. Instead the hash function transition document
specifies that the compatibility hash (specified with
compatObjectFormat) be computed over the equivalent object that another
git repository whose storage hash (specified with objectFormat) would
store. When comparing equivalent repositories built with different
storage hash functions, the oids embedded in objects used to refer to
other objects differ and the location of signatures within objects
differ.
As blob objects have neither oids referring to other objects nor stored
signatures their storage hash and their compatibility hash are computed
over the same object.
The other kinds of objects: trees, commits, and tags, all store oids
referring to other objects. Signatures are stored in commit and tag
objects. As oids and the tags to store signatures are not the same size
in repositories built with different storage hashes the size of the
equivalent objects are also different.
A version of index_bulk_checkin that supports more than just blobs when
computing both the SHA-1 and the SHA-256 of every object added would
need a different, and more expensive structure. The structure is more
expensive because it would be required to temporarily buffering the
equivalent object the compatibility hash needs to be computed over.
A temporary object is needed, because before a hash over an object can
computed it's object header needs to be computed. One of the members of
the object header is the entire size of the object. To know the size of
an equivalent object an entire pass over the original object needs to be
made, as trees, commits, and tags are composed of a variable number of
variable sized pieces. Unfortunately there is no formula to compute the
size of an equivalent object from just the size of the original object.
Avoid all of those future complications by limiting index_bulk_checkin
to only work on blobs.
Inspired-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2023-09-26 18:58:43 +03:00
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int index_blob_bulk_checkin(struct object_id *oid,
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int fd, size_t size,
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const char *path, unsigned flags);
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2011-10-29 01:48:40 +04:00
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2022-04-05 08:20:08 +03:00
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/*
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* Tell the object database to optimize for adding
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* multiple objects. end_odb_transaction must be called
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* to make new objects visible. Transactions can be nested,
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* and objects are only visible after the outermost transaction
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* is complete or the transaction is flushed.
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*/
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void begin_odb_transaction(void);
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/*
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* Make any objects that are currently part of a pending object
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* database transaction visible. It is valid to call this function
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* even if no transaction is active.
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*/
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void flush_odb_transaction(void);
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/*
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* Tell the object database to make any objects from the
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* current transaction visible if this is the final nested
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* transaction.
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*/
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void end_odb_transaction(void);
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2011-10-29 01:48:40 +04:00
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#endif
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