Re: Resolved...ish. was: Re: spurious I/O errors from btrfs...at the caching layer?

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

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On 2015-01-25 23:22, Zygo Blaxell wrote:
> It seems that the rate of spurious I/O errors varies most according to
> the vm.vfs_cache_pressure sysctl.  At '10' the I/O errors occur so often
> that building a kernel is impossible.  At '100' I can't reproduce even
> a single I/O error.
>
> I guess this is own my fault for using non-default sysctl parameters,
> although I wouldn't expect any value of this sysctl to cause these
> symptoms... :-P
>
>
Setting that to anything less than about 75 is just asking for trouble. 
  What's I think is happening is that you are eating up memory with the 
vfs cache (because that caches inodes and dentries, which in turn means 
that reflinks don't help decrease it's usage at all) faster than it can 
be reclaimed, and therefore have no space for the file data to be read 
into memory.  This will probably happen with most other filesystems as 
well if you set vfs_cache_pressure really low.

Personally, I wouldn't recommend ever touching that particular sysctl, 
with the exception of increasing it some when you have really fast 
storage (NVMe or UFS-SCSI based SSD's for example) and a relatively 
small set of files that you actually access frequently.



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