As such, it's a good candidate for using a bitmap index.
Working in the support department as I do is a good way to hear lots of horror stories. From time to time, we get reports of what can only be described as database corruption: Attempting to decode a corrupted tuple can cause PostgreSQL to attempt to allocate a gigantic amount of memory and fail with an ERROR or even to die with a segmentation fault, aborting all in-progress transactions and restarting the server.
Such occurrences are painful, to say the least, and users are often left wondering how it happens. How exactly does the database get corrupted?
One obvious possibility is bad hardware. If your hardware is defective, then the data we read back from the disk may be different from the data we previously wrote to the disk, and at that point, all bets are off.
More rarely, it's the memory rather than the disk that goes bad; typically, this causes actual database crashes rather than subtler symptoms of corruption, but not always. Kernel or filesystem bugs can masquerade as hardware corruption; surprisingly, there really are kernels and filesystems out there that don't always return the data that PostgreSQL last wrote when it's read back, often due to locking bugs that only manifest at high levels of concurrency.
Operating system crashes such as a reboot due to unexpected power loss can cause database corruption, if the server is misconfigured or if the kernel and storage hardware report success before they've actually written the data to stable storage.
PostgreSQL relies on write-ahead logging to provide crash recovery; to operate properly, PostgreSQL must be able to guarantee that write-ahead log entries reach stable storage before the corresponding data file changes. There are a variety of ways that this can fail.
First, if the configuration parameter fsync is set to off, then PostgreSQL will not even attempt to flush data to disk at the appropriate times.
This can results in a large performance gains, but the price is that crash recovery is no longer reliable. Second, on some systems, the operating system will report that fsync has succeeded even though the data hasn't truly been written to stable storage.
On Linux, it's sometimes necessary to disable write-caching at various levels, change mount options or filesystem selection, or simply run with a newer Linux distribution where the code is less buggy.
New versions of ext4 seem to be smarter about this than older ext3 code. Greg Smith has written extensively on this topic; see http: Mistakes during backup and restore are a surprisingly frequent cause of database corruption. If running your backups from a script, it is important to include error checks in the script, or at the very least to save the output to a log file.
Otherwise, it is difficult to know, later on, whether an unexpected error might have occurred, resulting in an incomplete or corrupt backup.
Especially on Windows, it's important to make sure that the program used to do the actual file copy can copy files even if they are open and being concurrently modified by other processes.Write IOPS Volume queue length is the number of pending I/O requests for a device.
Latency is the true end-to-end client time of an I/O operation, in other words, the time elapsed between sending an I/O and receiving an acknowledgement that the I/O read or write is complete. Sep 26, · The problem with hash indexes wasn't simply that nobody had bothered to write the code for write-ahead logging, but that the code was not structured in a way that made it possible to add write-ahead logging that would actually work correctly.
TLDR: Write-ahead logs are used to increase resilience and restore data after crashes. You can turn it off for some tables to achieve up to 15x performance boost. In 4xxi we often choose PostgreSQL over other vendors for its versatility.
(6 replies) Is there a way to disable write ahead logging (WAL) in ?
Thanks in advance. Anuradha. However, using fsync results in a performance penalty: when a transaction is committed, PostgreSQL must wait for the operating system to flush the write-ahead log to disk.
When fsync is disabled, the operating system is allowed to . Description. pg_xlogdump displays the write-ahead log (WAL) and prints timestamps for WAL records.
This utility is mainly useful for debugging or educational purposes. This utility can only be run by the user who installed the server, because it requires read-only access to the data directory.