1. Data file writes are notoriously random, and writing the log is sequential. Ironically, the sectors mapped by the OS to the disk are likely not at all sequential, but they likely are more sequential than the random data writes.
2. Log writing allows use of small, super fast drives (e.g. Solid State Disks) to speed up total database performance. You can have slower drives for the large files in the database and still get acceptable performance.
3. WAL allows for syncing only the pages changed. For example, suppose 14 transactions are in flight and each one modifies 40 pages of a data file. When one transaction commits, 560 pages are dirty, but only 40 need to be written. Without very close control of which buffers get dirtied to the OS (and Pg may have this, I am not sure), then all 560 pages may get written in place of the 40 that actually need to be written.
My only complaint is about larger systems which have a single (or mirrored) large arrays. If I have a very fast array of some sort that has proper caching, and my data files are on the array, look at my options for log files:
1. Put them on the array.
Pros:
* Fastest "drive" available
* RAID, so most reliable "drive" available
Cons:
* All changes get dumped twice: once for WAL, once at checkpoint.
* The array is no slower on random writes then sequential ones, which means that the benefits of writing to WAL vs. the data files are lost.
2. Put them on an actual (or mirrored actual) spindle Pros: * Keeps WAL and data file I/O separate Cons: * All of the non array drives are still slower than the array
3. Put them on mirrored solid state disks or another array Pros: * Very fast * WAL and data file I/O is separate Cons: * Big $. Extremely large $/GB ratio. * If an array, hordes of unused space.
I suspect (but cannot prove) that performance would jump for systems like ours if WAL was done away with entirely and the individual data files were synchronized on commit.
Is there a simple way to turn off WAL in the config files so that I may do some benchmarking?
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.
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.
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.
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.
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