xfs preallocation timeout

xfs preallocation timeout

[Michael Monnerie]

Does /proc/sys/vm/dirty_expire_centisecs influence the maximum time XFS 
takes before the "in-memory but not on disk" preallocation actually 
starts writing to disk? If not, which parameter does influence how long 
files are grouped together in memory before writing them really to disk? 
The longer the better for performance, but more risk for data loss of 
course. 

I ask this because I want to know when copying files over the net to an 
XFS drive, how I can prevent fragmentation to occur. Example: If a 
packet of 1024 Bytes arrives every 1/100th second, it needs 0.64s to get 
64KB of data, 1.28s to get 128KB. If the prealloc timeout is one second, 
it's effectively not used in this case. So if you got a server where you 
know files arrive at a certain speed, fine tuning this prealloc timeout 
could help prevent fragmentation.

mfg zmi
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Re: xfs preallocation timeout

[Dave Chinner]

On Wed, Feb 11, 2009 at 03:22:11PM +0100, Michael Monnerie wrote:
> Does /proc/sys/vm/dirty_expire_centisecs influence the maximum time XFS 
> takes before the "in-memory but not on disk" preallocation actually 
> starts writing to disk?

Yes.

BTW, you are referring to "delayed allocation", not
"pre-allocation". Pre-allocation is the act of immediately
allocating space on disk without writing data; delayed allocation is
avoiding allocation until the data is to be written to disk...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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Re: xfs preallocation timeout

[Michael Monnerie]

On Donnerstag 12 Februar 2009 Dave Chinner wrote:
> Yes.
>
> BTW, you are referring to "delayed allocation", not
> "pre-allocation". Pre-allocation is the act of immediately
> allocating space on disk without writing data; delayed allocation is
> avoiding allocation until the data is to be written to disk...

Thank you Dave. I didn't remember the correct wording. So 
/proc/sys/vm/dirty_expire_centisecs for XFS says how much data, but not 
metadata, I could loose on kernel crash/power outage etc. No need to 
worry, right?

mfg zmi
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