I was thinking other way round. What if we write to WAL pages only to those portions which we need to modify and let kernel do the job the way it sees fit? What will happen if it fails?
Bruce Momjian wrote:
Our current WAL implementation writes copies of full pages to WAL before
modifying the page on disk. This is done to prevent partial pages from
being corrupted in case the operating system crashes during a page
write.
Assuming a WAL page is zero at start and later written say a 128 bytes block. Then how exactly writing 128 bytes is different than writing entire 8K page, especially when we control neither kernel/buffer cache nor disk?
What is partial? Postgresql will always flush entire data block to WAL page isn't it? If write returns, we can assume it is written.
For example, suppose an 8k block is being written to a heap file. First the backend issues a write(), which copies the page into the
kernel buffer cache. Later, the kernel sends the write request to the
drive. Even if the file system uses 8k blocks, the disk is typically
made up of 512-byte sectors, so the OS translates the 8k block into a
contiguous number of disk sectors, in this case 16. There is no
guarantee that all 16 sectors will be written --- perhaps 8 could be
written, then the system crashes, or perhaps part of an 512-byte sector
is written, but the remainder left unchanged. In all these cases,
restarting the system will yield corrupt heap blocks.
We are hoping to prevent WAL page corruption which is part of file system corruption. Do we propose to tacle file system corruption in order to guarantee WAL integrity?
The WAL writes copies of full pages so that on restore, it can check each page to make sure it hasn't been corrupted. The system records an LSN (log serial number) on every page. When a pages is modified, its pre-change image is written to WAL, but not fsync'ed. Later, if a backend wants to write a page, it must make sure the LSN of page page is between the LSN of the last checkpoint and the LSN of the last fsync by a committed transactions. Only in those cases can the page be written because we are sure that a copy of the page is in the WAL in case there is a partial write.
Do we have per page checksum? It could be in control log, not necessarily in WAL. But just asking since I don't know.
Now, as you can image, these WAL page writes take up a considerable amount of space in the WAL, and cause slowness, but no one has come up with a way to recover from partial pages write with it. The only way to minimze page writes is to increase checkpoint_segments and checkpoint_timeout so that checkpoints are less frequent, and pages have to be written fewer times to the WAL because old copies of the pages remain in WAL longer.
If I am not mistaken, we rely upon WAL being consistent to ensure transaction recovery. We write() WAL and fsync/open/close it to make sure it goes on disk before data pages. What else we can do?
I can not see why writing an 8K block is any more safe than writing just the changes.
I may be dead wrong but just putting my thoughts together..
Shridhar
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