Unfortunately my suspicious is confirmed. For this reason, having the data committed in the internal system or hard disk cache is NOT durable for obvious reasons unless there is a guarantee that no such power outage could happen — if a battery is used on a RAID array, for example.
But you can change the kernel settings to change this defaults if needed. When you need to achieve durability, the answer is obviously yes. In our case it was MySQL - and we solved the problem by switching to cursor fetch mode for the affected queries, see e.
You probably know more or less how databases or things that look like one store their data on disk in a permanent and safe way. This is an important one — at a given point, you must ensure that the data is actually written on disk physically, preventing any loss of data in case of a sudden power outage, for example.
But we may assume that on Linux with ext4 and OSX with proper flags? Both syscalls are benchmarked in order to check if when a slow fsync is in progress the write will also block for the same time. This is not possible with the current standard hint: Therefore, in theory, you may create a file, write some data, synchronize it, close the file, and see your precious file lost forever because of a power outage.
When this operation is started the kernel could allocate new buffers that will be used by new write 2 calls, so my guess is, this is a Linux limitation, not something that must be this way. Slow means a few seconds when the disk is busy and there is some serious amount of data to flush.
Oh, a last funny note: As I like numbers, slow is, for instance, 55 milliseconds against a small file with not so much writes, while the disk is idle. You can not call FlushFileBuffers on a directory handle as far as I can see. Write in 11 microseconds Write in 12 microseconds Write in 12 microseconds Write in 12 microseconds Sync in microseconds 0 Write in microseconds Write in 11 microseconds Write in 11 microseconds Write in 11 microseconds Write in 11 microseconds Subscribe to the RSS feed of this blog or use the newsletter service in order to receive a notification every time there is something of new to read here.
These concepts apply on databases see ACIDbut also on the underlying filesystem. Oh, and by the way: But things are sometimes a bit obscure on the implementation side: The fsync implementations in older kernels and lesser used filesystems does not know how to flush disk caches.
Next up was to limit the heap available to the XD container so that a memory dump created with jmap would fit into our developer machines and use a profiler YourKit in our case in order to track down the memory problem. I guess that some of you out there might have a similar setup and I hope to save you some hours for tracking down your specific problem.
But I started to have the feeling that this would be totally useless, as the write 2 call would block anyway if there was a slow fsync going on against the same file, so I wrote the following test program: With some application this is not a problem. Write in microseconds Write in microseconds Write in microseconds Write in microseconds Write in microseconds Write in microseconds Write in microseconds Write in microseconds Write in microseconds Write in microseconds First we had to somehow bring the system into trouble by firing a lot of events on it.Data can make what is impossible today, possible tomorrow.
We empower people to transform complex data into clear and actionable insights. log says[myid:1] - WARN [[email protected]] - fsync-ing the write ahead log in SyncThread:1 took ms which will adversely effect operation latency.
Name: kernel-default Version: Release: Architecture: x86_64 Install Date: (not installed) Group: System/Kernel Size: License: GPL v2.
On POSIX systems, durability is achieved through sync operations (fsync(), fdatasync(), aio_fsync()): “The fsync() function is intended to force a physical write of data from the buffer cache, and to assure that after a system crash or other failure that all data up to.
Does this warning mean a: the log is too big, b: zk is competing with another process for disk access, c: other? That's timing how long the fsync takes on the txnlog, typically it runs log because the disk is busy, or the OS has a large number of.
Jul 03, · Does fsync() commits rename() effects on a given file? Showing of 31 messages. Does fsync() commits rename() effects on a given file? Write ahead logging, for example, works only if a log record reaches persistent storage before the updated data record it describes.
If this fsync'ing a directory opened in read-only to commit.Download