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There is page cache before we write data to disk.

So if I have two operations.

write(fileA)
write(fileB)

Then if the system is suddenly shutdown. We don't initiative call the sync() call.

I want to know if it is possible that the data we wrote to fileB has flush to the disk, while the data we wrote to fileA haven't been flush to the disk?

baotiao
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2 Answers2

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I believe that it is possible for fileB to be written to disk before fileA, as the writes will be bundled into block I/O requests and can be reordered at the block device layer by the I/O scheduler in an attempt to minimise disk seeking.

See the kernel documentation for more info about the I/O scheduler (elevator): http://lxr.free-electrons.com/source/Documentation/block/biodoc.txt#L885

austinmarton
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To answer you question in short you may want to consider calling sync() or fsync() system call in your application after the write() to make sure that data is sync-ed to the disk immediately.

flush (or pdflush) kernel threads are responsible for syncing dirty pages to the disk. When the system is being shutdown properly, all the dirty buffers get synced / written to disk. However, this is not the same in case of abrupt power failures as data which is not yet flushed / sync-ed to the disk is obviously lost.

If you don’t call sync() in your application, then dirty buffers get written to the disk upon certain kernel tunables. You could control how the application data gets synced (inactive dirty pages) through sysctl kernel tunable. You may want to consider reading more about the following:

vm.dirty_expire_centisecs - how old (in 1/100th of a sec) the dirty pages must be before writing them to the disk

vm.dirty_writeback_centisecs - how often the kernel will wake up the BDI-flush thread to sync the dirty pages onto the disk

vm.dirty_background_ratio - percentage of system memory which when dirty then system can start writing data to the disks

vm.dirty_ratio - percentage of system memory which when dirty the the process doing writes should block to write out dirty pages to the disks

askb
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  • Yes, I don't call sync() in my application. then I want to know how the pdflush kernel thread flush the data? Is there a arrry to record the dirty pages, and new dirty page append to this array by the insert time. then the pdflush flush the data from the head to then tail? – baotiao Nov 03 '14 at 09:34
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    [enum pageflags](http://www.cs.fsu.edu/~baker/devices/lxr/http/source/linux/include/linux/page-flags.h) has a field `PG_dirty`. – askb Nov 03 '14 at 12:08