Em 22/04/2013 10:20, Greg Freemyer escreveu: > > Mulyadi Santosa <[email protected]> wrote: > >> Hi :) >> >> On Sat, Apr 20, 2013 at 4:42 AM, Daniel Hilst Selli >> <[email protected]> wrote: >>> AFAIK, major page faults are generated when data that is not yet >> present >>> on RAM is loaded from disk, but in this case data is being write do >>> disk, I can't >>> see how writes can generate faults, but still, it seems that is >>> happening, ... !? >>> >>> Is that possible? >> I never pay close attention on these kind of stuffs, but IMHO there is >> a chance that this "write" is actually "update" > I missed the original email, but the kernel filesystem logic typically works > with 4 KB pages. If a write lands on page boundaries then the file system > will do pure writes, but if it doesn't then the filesystem will typically > read the original page from disk then update it and write it back out. > > Small writes of 4 kb as an example that are not page aligned will cause 2 > pages to read, both to be updated and both to be written. > > Adding raid 5/6 below the file system layer greatly accelerates the problem. > Now you need to ensure your writes are the full size of a stripe and stripe > aligned to avoid the read/update/write sequence. Some filesystems build > knowledge of stripes into them to try and optimize this process, some don't. > > Greg > > > Thanks for your answer Greg, I have never been presented to this "read/update/write" logic, now makes a lot of sens, Thanks!
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