SQLite: Wal2 Mode
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SQLite: Wal2 Mode
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Re: SQLite: Wal2 Mode
#2> In wal2 mode, the system uses two wal files instead of one. The files are named "-wal" and "-wal2", where "" is of course the name of the database file. When data is written to the database, the writer begins by appending the new data to the first wal file. Once the first wal file has grown large enough, writers switch to appending data to the second wal file. At this point the first wal file can be checkpointed (after which it can be overwritten). Then, once the second wal file has grown large enough and the first wal file has been checkpointed, writers switch back to the first wal file. And so on.
Looks so logical that I don't understand why WAL mode was not implemented like this from the get go. Probably an optimization wrongly dismissed as premature?
Anyways, looking forward to this mode reaching general availability.
Re: SQLite: Wal2 Mode
#3>In wal2 mode, the system uses two wal files instead of one. The files are named "-wal" and "-wal2",
Heh, I wonder how many people will delete the "wal" file thinking that, since they switched to wal2, the wal file must be a leftover.
Re: SQLite: Wal2 Mode
#4> In wal2 mode, the system uses two wal files instead of one. The files are named " -wal" and " -wal2", where " " is of course the name of the database file. When data is written to the database, the writer begins by appending the new data to the first wal file. Once the first wal file has grown large enough, writers switch to appending data to the second wal file. At this point the first wal file can be checkpointed…
I'm just speculating here, but in a normal database you would have different processes writing the wal files to the database or archives.
You don't have that with sqlite, so I don't see an obvious advantage for this, except if they now spawn a process or thread to do this concurrently.
Edit: so I read the doc (shame on me) and it has nothing to do with speed. Its purpose is to prevent a wal file from growing too large.
Re: SQLite: Wal2 Mode
#5> In wal2 mode, the system uses two wal files instead of one. The files are named " -wal" and " -wal2", where " " is of course the name of the database file. When data is written to the database, the writer begins by appending the new data to the first wal file. Once the first wal file has grown large enough, writers switch to appending data to the second wal file. At this point the first wal file can be checkpointed…
Now I can't help but wonder if there should be a `waln` mode where the WAL files would round robin instead of alternate between just two potentially allowing for much more intense write cadence.
Re: SQLite: Wal2 Mode
#6> In wal2 mode, the system uses two wal files instead of one. The files are named " -wal" and " -wal2", where " " is of course the name of the database file. When data is written to the database, the writer begins by appending the new data to the first wal file. Once the first wal file has grown large enough, writers switch to appending data to the second wal file. At this point the first wal file can be checkpointed…
I'm just speculating here, but in a normal database you would have different processes writing the wal files to the database or archives. You don't have that with sqlite, so I don't see an obvious advantage for this, except if they now spawn a process or thread to do this concurrently. Edit: so I read the doc (shame on me) and it has nothing to do with speed. Its purpose is to prevent a wal file from growing too larg…
I believe SQLite supports accessing a single database file from multiple processes or threads at once
Re: SQLite: Wal2 Mode
#7>In wal2 mode, the system uses two wal files instead of one. The files are named " -wal" and " -wal2", Heh, I wonder how many people will delete the "wal" file thinking that, since they switched to wal2, the wal file must be a leftover.
Would it be a problem since the wal you delete, its inode, will still be open and processed at the DB closing normally? Just guessing, never tried that.
Re: SQLite: Wal2 Mode
#8> In wal2 mode, the system uses two wal files instead of one. The files are named " -wal" and " -wal2", where " " is of course the name of the database file. When data is written to the database, the writer begins by appending the new data to the first wal file. Once the first wal file has grown large enough, writers switch to appending data to the second wal file. At this point the first wal file can be checkpointed…
I'm not much into the details but I'm wondering why is it not always writing to "-wal" once it is full, it is renamed to "-wal-0" and it starts writing again into "-wal", once it is full again it is renamed to "-wal-1" ?
Re: SQLite: Wal2 Mode
#9> In wal2 mode, the system uses two wal files instead of one. The files are named " -wal" and " -wal2", where " " is of course the name of the database file. When data is written to the database, the writer begins by appending the new data to the first wal file. Once the first wal file has grown large enough, writers switch to appending data to the second wal file. At this point the first wal file can be checkpointed…
I'm just speculating here, but in a normal database you would have different processes writing the wal files to the database or archives. You don't have that with sqlite, so I don't see an obvious advantage for this, except if they now spawn a process or thread to do this concurrently. Edit: so I read the doc (shame on me) and it has nothing to do with speed. Its purpose is to prevent a wal file from growing too larg…
I guess it is to make it crash save, e.g. in case the server dies in the middle of a write, the database would be corrupt after restart, with the wal file it can recover.
Re: SQLite: Wal2 Mode
#10> In wal2 mode, the system uses two wal files instead of one. The files are named " -wal" and " -wal2", where " " is of course the name of the database file. When data is written to the database, the writer begins by appending the new data to the first wal file. Once the first wal file has grown large enough, writers switch to appending data to the second wal file. At this point the first wal file can be checkpointed…
Now I can't help but wonder if there should be a `waln` mode where the WAL files would round robin instead of alternate between just two potentially allowing for much more intense write cadence.
That would make reading slower, since readers need to read from WAL as well.