understanding speculative preallocation

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jbr
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understanding speculative preallocation

jbr
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Hello,

I'm looking for general documentation/help with the speculative preallocation feature in xfs.  So far, I haven't really been able to find any definitive, up to date documentation on it.

I'm wondering how I can find out definitively which version of xfs I am using, and what the preallocation scheme in use is.

We are running apache kafka on our servers, and kafka uses sequential io to write data log files.  Kafka uses, by default, a maximum log file size of 1Gb.  However, most of the log files end up being 2Gb, and thus the disk fills up twice as fast as it should.

We are using xfs on CentOS 2.6.32-358.  Is there a way I can know which version of xfs is built into this version of the kernel?  What preallocation schedule does it use?  If I do a xfs_info -V, it reports 3.1.1.

We are using xfs (mounted with no allocsize specified).  I've seen varying info suggesting this means it either defaults to an allocsize of 64K (which doesn't seem to match my observations), or that it will use dynamic preallocation.

I've also seen hints (but no actual canonical documentation) suggesting that the dynamic preallocation works by progressively doubling the current file size (which does match my observations).

What I'm not clear on, is the scheduling for the preallocation. At what point does it decide to preallocate the next doubling of space.  Is it when the current preallocated space is used up, or does it happen when the current space is used up within some threshold.  What I'd like to do, is keep the doubling behavior in tact, but have it capped so it never increases the file beyond 1Gb.  Is there a way to do that?  Can I trick the preallocation to not do a final doubling, if I cap my kafka log files at say, 900Mb (or some percentage under 1Gb)?

There are numerous references to an allocation schedule like this:

freespace       max prealloc size
  >5%             full extent (8GB)
  4-5%             2GB (8GB >> 2)
  3-4%             1GB (8GB >> 3)
  2-3%           512MB (8GB >> 4)
  1-2%           256MB (8GB >> 5)
  <1%            128MB (8GB >> 6)

I'm just not sure I understand what this is telling me.  It seems to tell me what the max prealloc size is, with being reduced if the disk is nearly full.  But it doesn't tell me about the progressive doubling in preallocation (I assume up to a max of 8Gb).  Is any of this configurable?  Can we specify a max prealloc size somewhere?

The other issue seems to be that after the files are closed (from within the java jvm), they still don't seem to have their pre-allocated space reclaimed.  Are there known issues with closing the files in java not properly causing a flush of the preallocated space?

Any help pointing me to any documentation/user guides which accurately describes this would be appreciated!

Thanks,

Jason