On Mon, Oct 7, 2019 at 8:48 AM Asif Rehman <asifr.rehman@gmail.com> wrote: > Sure. Though the backup manifest patch calculates and includes the checksum of backup files and is done > while the file is being transferred to the frontend-end. The manifest file itself is copied at the > very end of the backup. In parallel backup, I need the list of filenames before file contents are transferred, in > order to divide them into multiple workers. For that, the manifest file has to be available when START_BACKUP > is called. > > That means, backup manifest should support its creation while excluding the checksum during START_BACKUP(). > I also need the directory information as well for two reasons: > > - In plain format, base path has to exist before we can write the file. we can extract the base path from the file > but doing that for all files does not seem a good idea. > - base backup does not include the content of some directories but those directories although empty, are still > expected in PGDATA. > > I can make these changes part of parallel backup (which would be on top of backup manifest patch) or > these changes can be done as part of manifest patch and then parallel can use them. > > Robert what do you suggest?
I think we should probably not use backup manifests here, actually. I initially thought that would be a good idea, but after further thought it seems like it just complicates the code to no real benefit.
Okay.
I suggest that the START_BACKUP command just return a result set, like a query, with perhaps four columns: file name, file type ('d' for directory or 'f' for file), file size, file mtime. pg_basebackup will ignore the mtime, but some other tools might find that useful information.
yes current patch already returns the result set. will add the additional information.
I wonder if we should also split START_BACKUP (which should enter non-exclusive backup mode) from GET_FILE_LIST, in case some other client program wants to use one of those but not the other. I think that's probably a good idea, but not sure.
Currently pg_basebackup does not enter in exclusive backup mode and other tools have to
use pg_start_backup() and pg_stop_backup() functions to achieve that. Since we are breaking
backup into multiple command, I believe it would be a good idea to have this option. I will include
it in next revision of this patch.
I still think that the files should be requested one at a time, not a huge long list in a single command.
sure, will make the change.
I have refactored the functionality into multiple smaller patches in order to make the review process easier. I have divided the code into backend changes and pg_basebackup changes. The
backend replication system now supports the following commands:
- START_BACKUP
- SEND_FILE_LIST
- SEND_FILES_CONTENT
- STOP_BACKUP
The START_BACKUP will not return the list of files, instead SEND_FILE_LIST is used for that. The START_BACKUP now calls pg_start_backup and returns starting WAL position, tablespace header information and content of backup label file. Initially I was using tmp files to store the backup_label content but that turns out to be bad idea, because there can be multiple non-exclusive backups running. The backup label information is needed by stop_backup so pg_basebackup will send it as part
of STOP_BACKUP.
The SEND_FILE_LIST will return the list of files. It will be returned as resultset having four columns (filename, type, size, mtime).
The SEND_FILES_CONTENT can now return the single file or multiple files as required. There is not much change required to
support both, so I believe it will be much useable this way if other tools want to utilise it.
As per suggestion from Robert, I am currently working on making changes in pg_basebackup to fetch files one by one. However that's not complete and the attach patch
is still using the old method of multi-file fetching to test the backend commands. I will send an updated patch which will contain the changes on fetching file one by one.
I wanted to share the backend patch to get some feedback in the mean time.